Final report Literature review: Digestate use in … Literature Review.pdf · Final report Literature review: Digestate use ... cost or expense incurred or arising from reliance on

Final report

Literature review: Digestate use

in protected horticulture

A desk study that collates information relevant to the UK on the use of digestates as a fertiliser or growing media ingredient for protected ornamental and food crop production.

Project code: OMK005-005 Date: 4 March 2015 Research date: January-March 2014

WRAP‘s vision is a world where resources are used sustainably. Our mission is to accelerate the move to a sustainable resource-efficient economy through re-inventing how we design, produce and sell products; re-thinking how we use and consume products; and re-defining what is possible through re-use and recycling. Find out more at www.wrap.org.uk

Document reference: WRAP, 2014, Banbury, Literature review: Digestate use in protected horticulture, Prepared by M E Dimambro, J Steiner and F Rayns

Document reference: [e.g. WRAP, 2006, Report Name (WRAP Project TYR009-19. Report prepared by…..Banbury, WRAP]

Written by: Mary Dimambro and Joachim Steiner (Cambridge Eco Ltd) and Francis Rayns (Garden Organic)

Front cover photography: Strawberries growing in a nutrient solution containing whole food waste digestate at Warwick Crop Centre (Cambridge Eco Ltd)

While we have tried to make sure this report is accurate, WRAP does not accept liability for any loss, damage, cost or expense incurred or arising from reliance on this

report. Readers are responsible for assessing the accuracy and conclusions of the content of this report. Quotations and case studies have been drawn from the public

domain, with permissions sought where practicable. This report does not represent endorsement of the examples used and has not been endorsed by the organisations

and individuals featured within it. This material is subject to copyright. You can copy it free of charge and may use excerpts from it provided they are not used in a

misleading context and you must identify the source of the material and acknowledge WRAP‘s copyright. You must not use this report or material from it to endorse or

suggest WRAP has endorsed a commercial product or service. For more details please see WRAP‘s terms and conditions on our website at www.wrap.org.uk

http://www.wrap.org.uk/

Literature review: Digestate use in protected horticulture 3

Executive summary

The aim of this review is to describe the current state of knowledge worldwide regarding the use of whole digestate, separated liquid digestate (liquor) and separated solid digestate (fibre) from anaerobic digestion (AD) in protected horticulture. Overall, published studies indicate that there is significant potential for digestates (whole, liquor and fibre) to be used in protected horticulture for both ornamental and edible crop production. A number of studies have examined the use of whole and separated liquor as a liquid organic fertiliser in soil-grown crops and also in soil-less production. Evidence was also found of whole, liquor and fibre digestate as growing media ingredients. Several studies have been identified, investigating either composting the digestate fibre alone or co-composting with other ingredients. The composted end-products were then assessed for their potential as growing media or as growing media ingredients. The evidence suggests that whole, liquor and fibre digestate, in combination with other standard industry ingredients, can generally achieve similar or better yields compared to standard growing practices, provided the recommended nutrient and electrical conductivity (EC) levels are considered. Moreover, in some instances improved crop quality and/or taste was reported when using digestates. As the trials reported generally focussed on a limited number of digestate treatments, further optimisation of the treatments may have given better results, and would be recommended, should digestate use be considered for commercial use in protected horticulture. Large scale adoption of digestates within the protected sector could result in a reduction in the use of peat and inorganic fertilisers in this sector. There is evidence that growing media incorporating digestate fibre (from biosolids) are used in commercial growing media products endorsed by US local administration, and winning national awards by the US Environmental Protection Agency. Perceptions around food safety could present a barrier to the uptake of digestates in some markets. Very few studies were found to consider microbiological aspects of food safety with regards to the use of digestates in protected horticulture. No evidence was found to suggest that the use of digestates posed a greater microbiological risk than commercial growing media and fertilisers, although one UK study did recommend that further research was required. Moreover, where overseas studies included an analysis of food safety in terms of microbial contamination or levels of potentially toxic elements, these were not deemed to be greatly different to those in conventional growing media and fertilisers. In addition, PAS110 compliance ensures PTEs, pathogens and organic contaminants are monitored and kept within safe levels. In regulatory terms, the current market limitations imposed by the anaerobic digestate quality protocol (ADQP) present a significant barrier to the uptake of digestates in some markets. The UK regulatory approach should be kept under review, particularly given the apparently successful deployment of digestates in a range of markets elsewhere.


Contents

1.0 Introduction ................................................................................................. 7 2.0 Methodology ................................................................................................. 7 3.0 Findings ........................................................................................................ 8 4.0 Whole digestate and digestate liquor studies with soil-grown crops ........... 9

4.1 UK research: Use of digestates as a liquid fertiliser in strawberry production ... 9 4.2 Research outside the UK: Using whole digestate and digestate liquor as a fertiliser for soil-grown crops ............................................................................... 10

5.0 Whole digestate and digestate liquor studies with soil-less production .... 13 5.1 UK research: Hydroponic production of tomato and lettuce with three whole digestates .......................................................................................................... 13 5.2 Research outside the UK regarding whole digestate and digestate liquor with soil-less production ............................................................................................. 15

6.0 Whole digestate and digestate liquor as a growing media ingredient ........ 18 6.1 UK research: Bark admixtures: Formulation and testing of novel organic growing media using quality digestates for the production of containerised plants ... 18 6.2 Research outside the UK regarding whole digestate and digestate liquor as a growing media ingredient ................................................................................... 20

7.0 Digestate fibre use in horticulture .............................................................. 21 7.1 Digestate fibre as a growing medium ingredient ......................................... 22

7.1.1 UK research: The use of cattle slurry digestate fibre in mixtures with coir and pine bark as plant growth substrates in the intensive production of glasshouse tomato crops .......................................................................... 22 7.1.2 Research outside the UK: Digestate fibre as a growing medium ingredient ................................................................................................ 24

7.2 Using composted digestate fibre as a growing media ingredient ................... 31 7.2.1 UK research: The potential of using composted digestate fibre for horticulture for ornamental production ....................................................... 31 7.2.2 Research outside the UK: Using composted digestate fibre as a growing media ingredient ...................................................................................... 33

7.3 Using digestate fibre co-composted with other materials as a growing media ingredient .......................................................................................................... 34

7.3.1 Digestate fibre co-composted with vine prunings .............................. 35 7.3.2 Digestate fibre co-composted with wood chip and flax straw ............. 35 7.3.3 Digestate fibre co-composted with sulphur and almond shell powder .. 38

8.0 Bioassays using digestates as a growing media ingredient ........................ 38 9.0 Commercial studies .................................................................................... 39 10.0 Conclusions ................................................................................................ 40 11.0 References .................................................................................................. 42

Acknowledgements

The authors would like to thank Rob Lillywhite and Catherine Keeling of the University of Warwick for assistance with the literature searching and interpretation, in addition to the UK and European academics who provided information on their research.


Glossary

AD Anaerobic digestion

Admixture The substance that results from mixing all the ingredients in a growing media

‗recipe‘; more generally a mixture which results when two different materials

are combined without occurrence of chemical reactions

ADQP Anaerobic Digestate Quality Protocol – End of waste criteria for the production

and use of quality outputs from the anaerobic digestion of source segregated

wastes

Biofertiliser

Biofertiliser is the name adopted for digestates certified as compliant with UK

end of waste criteria

Biogas Mixture of gases produced by anaerobic digestion

Biosolids

BMW

Treated sewage sludge

(Source segregated) biodegradable municipal waste

CAT

COD

cv

DECC

Calcium chloride/DTPA

Chemical oxygen demand

Cultivar

The Department of Energy & Climate Change

Digestate fibre Fibrous fraction of material derived by separating the coarse fibres from the

whole digestate

Digestate liquor Liquid fraction of material remaining after separating coarse fibres from whole

digestate

DM

EA

EC

Dry matter

Environment Agency

Electrical conductivity. Units: 106 μS/cm = 103 mS/cm = 1 S/cm

Fertigation

FM

GRH

The application of water-soluble fertilisers through an irrigation system which

is principally used in trickle and tape systems

Fresh matter

Ground rice hulls

GWC Green waste compost


HONS

‗Hardy Ornamental Nursery Stock‘. Plants grown on nurseries for use in

gardens and managed landscapes. Hardy refers to them being able to survive

the winter without significant damage from frosts.

Hygromull

Lecaton

MBT

Open-pore hydrophilic PU foam improves WHC and substrate aeration, and is

able to adsorb nutrients.

Thermally expanded burnt clay granules improve the physical characteristics of

substrates, are able to absorb cations, and may release Ca

Mechanical Biological Treatment – combination of mechanical and biological

treatments for extracting recyclables from mixed household waste

MSW Municipal Solid Waste

ORG

PAS110

Organic Recycling Group, part of the Renewable Energy Association

The British Standards Institution‘s publicly-available specification BSI PAS 110

is a specification for digestate quality (BSI, 2014). It is also a core requirement

for the UK‘s end of waste positions for digestate. PAS110 specifies:

Controls on input materials and the management system for the process of

anaerobic digestion and associated technologies

Minimum quality of whole digestate, separated fibre and separated liquor

Information that is required to be supplied to the digestate recipient

Perlite

PTEs

Expanded volcanic Al-silicate increases the aeration and water-holding capacity

of substrates

Potentially toxic elements (heavy metals)

Styromull

TKN

TOC

TS

WHC

Whole digestate

Closed-pore polystyrene foam which improves substrate aeration

Total Kjeldahl nitrogen

Total organic carbon

Total solids

Water holding capacity

Material resulting from an anaerobic digestion process that has not undergone

post-digestion separation

WRAP Waste & Resources Action Programme


1.0 Introduction In 2012, an estimated 1.44 million tonnes of digestate was produced from anaerobic digestion (AD) in the UK, with approximately 99% of this being used in agriculture and field horticulture, with the very small remainder used for land restoration (WRAP, 2013b).

Whilst there are undoubted benefits associated with the use of digestate in agriculture1, the reliance on a single market is risky, and AD industry resilience would be improved if a wider range of markets could be developed. Indeed, the Defra/DECC Anaerobic Digestion Strategy and Action Plan identified the need to find appropriate markets for quality digestates, and one of WRAP‘s aim is to facilitate the increased uptake of digestate use by the horticultural industry (Defra and DECC, 2011). Within this context WRAP has supported a number of field projects to investigate potential non-agricultural markets for digestate. These include landscaping and regeneration, energy crops on previously developed land, sports and amenity turf, and soil manufacture (under projects OMK001 and OMK004). The use of digestates in protected plant horticulture has also been highlighted as an area for potential development (WRAP, 2011b, Zero Waste

Scotland, 2010), and this has been progressed via WRAP programme OMK006. This includes

four research projects which considered a range of uses of digestates for protected horticultural crops, including edibles and ornamentals (WRAP 2015a, 2015b, 2015c, 2015d). At present, the Anaerobic Digestate Quality Protocol (ADQP) does not cover digestates used in these emerging markets (WRAP and Environment Agency, 2010). WRAP‘s OMK006 trials recommended that a brief review of existing research into the use of digestates in protected horticulture could be of benefit to the industry, highlighting less mainstream potential uses for digestate as well as informing future regulatory approaches to such emerging digestate markets. This desk study aims to address these recommendations. 2.0 Methodology Search terms were established for the literature searching, including synonyms, spelling variations and different combinations of terms as listed below. Academic searching was largely carried out using the ISIS Web of Science and Coventry University ‗Locate‘. These covered not only the principal agricultural and horticultural databases, but also food-related databases, environmental science, water, pollution, toxicology and general science databases. A number of these databases also index grey literature. The titles retrieved from these searches were screened, and then abstracts were downloaded. These abstracts were subjected to a second round of screening where useful papers were selected. Full text was obtained for these items. In addition, relevant publications cited in reports obtained in the searching were also obtained. In addition to the standardised interrogations of the academic databases mentioned above, unstructured searches were also carried out using internet search engines (Google, Google Scholar, Yahoo, Google.de and Defra Science search). For grey literature searches using internet search engines there are always an almost infinite number of hits, and so only relevant web pages were investigated. Some literature was available from WRAP, EA and ORG websites and sources, as well as the authors‘ databases and libraries.

1 For example, see www.wrap.org.uk/dc-agri

http://www.wrap.org.uk/dc-agri


The searches focussed on publications containing information about the use of digestate in horticultural systems. The search terms used included digestate, anaerobic digestion residue, anaerobically digested, anaerobic fermentation residue, fermented residue, biogas residue, biogas slurry, biogas effluent plus a combination of one or more of the following:

Whole, liquor, liquid, separated, fibre, fiber, solid

Horticulture, protected horticulture, glasshouse production

Hydroponics, growing media, soil-less production, glasshouse production, nutrient

solution, horticulture, peat, pot trial

Food crop plants: Including tomato, pepper, cucumber, lettuce, strawberries, pak-choi,

raspberries

Ornamentals: Including ferns, cyclamens, chrysanthemums, begonias, primroses,

polyanthus, hedera, osteospermum, poinsettias, fuchsias, geraniums, pansies, impatiens,

cineraria, hydrangea and various species of trees including pine.

This list is not exhaustive but represents the main search terms used, with plant species

listed in the Defra glasshouse survey (Defra, 2008) as the main starting point, where

individual species searches were undertaken.

In addition to literature published in English, articles written in German were also investigated, since AD is well-established in German speaking countries, with the above search terms in German language equivalents. Some literature was also obtained published in Italian, although no extensive searching in the Italian language was undertaken. European digestate experts were contacted to ascertain whether further information could be found regarding work which is not available in English, for an insight into any additional European studies. 3.0 Findings The review of the academic literature revealed that a range of information was available for a number of crops. The idea of using digestate, either whole or separated, in growing media had been suggested in a number of reports and reviews (for example: (Hogg et al., 2007, WRAP, 2011b, Arvanitoyannis, 2008)) but it is not always clear if this is based on experience or purely recommendations for research. Sometimes the terminology was unclear, particularly in publications from overseas, so it was difficult to be sure if the authors were referring to digestate of the consistency, feedstock type, quality and AD technology of that presently available in the UK. Discussions with European experts revealed a few additional studies which generally focussed on the use of digestates from maize and silage-based systems which tend to have high dry matter content. These findings are collated and discussed below, according to digestate use and type. Unfortunately not all publications clarify whether the digestate has been separated into fibre and liquor fractions or remains whole, but where this is defined this information is included. Some studies were not written in English, German or Italian, and in such cases where only the abstract was in English with insufficient information available for the purposes of this review, this has been highlighted. A range of relevant publications in Chinese were identified, with insufficient information in the brief English abstracts to understand the trial design or specific use of the digestate. These publications included the use of digestates in the production of tomatoes (Tongguo, 2011, Chang, 2010, Li and Zhang, 2001, Xie, 2010), cucumber (Tongguo, 2011), mini


cucumbers (Hao, 2007), peppers/capsicum (Zhao, 2007, Jianxiang, 2007, Wei, 2009, Tongguo, 2011), aubergine (Li and Zhang, 2012), lettuce (AI, 2006), spinach (Li, 2003), Chinese cabbage (Dong-Dong, 2010, Zhang et al., 2010), celery (Yi, 2011) and strawberries (Zhao, 2010, Chen, 2007). 4.0 Whole digestate and digestate liquor studies with soil-grown crops Crops grown in soil or growing media (which may include peat or coir), are often fertilised with a nutrient solution which may be delivered as part of the irrigation regime (fertigation), or as a separate fertiliser once, or at set time points. Firstly, recent UK work on the use of digestate as a fertiliser for strawberries is discussed. This is followed by a review of studies of whole and liquor digestate use as a fertiliser, with the majority of work in this area focussing on tomato production. 4.1 UK research: Use of digestates as a liquid fertiliser in strawberry production A feasibility study undertaken in 2013 had the aim of establishing whether six digestates produced in the UK could be used successfully as fertiliser for strawberry production (WRAP, 2015d). The feedstock materials included potato waste, dairy cattle slurry, food waste, maize, and a mixture of maize, manure and milk waste. Some digestates were whole digestates, whilst others were the separated liquor fraction. Overall, the results showed that it is possible to produce strawberries using digestate-based nutrient solutions with yields and quality similar to strawberries grown with traditional mineral fertilisers (see front cover photograph). Table 1. Final nutrient concentrations in the digestate solutions following dilution for the strawberry

trial (WRAP, 2015d)

The digestate amendments column on the right indicates the additional nutrients added to all six digestates, to ensure a minimum requirement

appropriate to the crop. The figures refer to the mineral elements as specified in the left hand column.

* The control treatment solution was prepared using a 1-1-3 feed with the addition of calcium and potassium nitrate (figures in this column

include all additions)

** No additional iron was added to the Maize/Manure(s) digestate

ǂ The ECs given here are of the prepared digestate (and control) feed solutions as delivered to the plants (digestate + water + amendments)

(l) = digestate liquor; (w) = whole digestate

Glasshouse trials were carried out with strawberry plants, (variety Elsanta), planted in standard commercial peat grow-bags. Digestates were diluted between 25 and 51 times to bring their nitrogen concentrations down to commercial norms (see Table 1). Some minor amendments were then performed to optimise the profile of the nutrient solution. Solutions

Recommendation Control*

Maize/ Maize Potato Slurry Food Food Digestate

amendments Manure (l)

(l) (w) (w) (w) (l)

min N mg/l 120-150 120 71 71 71 71 71 71 49 (as KNO3)

(Req. dilution

factor) (21x) (23x) (27x) (51x) (53x) (36x)

P mg/l 40-55 27 11 8 4 4 4 3 47 (as MKP)

K mg/l 250-300 217 175 150 179 72 43 41 196

(as KNO3 & KP)

Ca mg/l 100-120 79 52 43 29 33 29 33

Mg mg/l 25 8 12 9 6 8 6 6

Fe µg/l 1200-1700 417 1897 767 176 446 138 240 1200**

Mn µg/l 500-800 281 164 107 30 86 25 36 500

Mo µg/l 20-50 31 0 0 0 0 0 0 40

Total solids (%)

0.26 0.22 0.08 0.10 0.05 0.08

EC (dS/m)ǂ 1.5-1.8 1.8 2.2 2.1 2.1 1.9 2.1 1.9


were delivered to the plants via low pressure trickle irrigation. Fruit yield and quality were compared with fruits from control plants fertigated using conventional nutrient solution. There were significant differences in yield and fruit quality between digestate treatments of various feedstock types: Three of the six digestates performed as well as the control in terms of both total fruit yield and Class 1 yield. Two (food and dairy cattle slurry based) treatments produced fruit that generally out-performed the control throughout the season with respect to taste assessments. Improvement in fruit flavour was the most notable difference. The other three digestate treatments had slightly reduced yields compared to the control, but this reduction was large enough to be significant only in one of the treatments (digestate derived from maize, manure and milk waste). All three of these digestate treatments achieved better results in the taste test than the control. Figure 1. Day 18 showing strawberry plants fertigated with digestate and an industry standard

(control). All plants observed to be healthy with no effect of treatment in evidence (WRAP, 2015d).

4.2 Research outside the UK: Using whole digestate and digestate liquor as a fertiliser for soil-grown crops

Tomato trials were carried out over several years at the GBZ Straelen, in Germany, and summarised briefly in a number of research notes (Andreas, 2004a, Andreas, 2005, Andreas, 2006, Andreas, 2003). Tomatoes were grown under glass with white mulch foil as a soil cover. In most cases, no data is available on the digestate type, nutrient breakdown or application timings in these research notes other than what is reported below. In 2002, the trial consisted of two treatments both with an application rate of 300 kg N/ha over the season. The control was two applications of calcium ammonium nitrate, with the second treatment being digestate (from farm residues) applied through drip-feeding. For the three tomato varieties trialled, there were no significant differences in yield between the control and digestate treatments (Andreas, 2003). In 2005, a standard organic treatment of horn meal as the base fertiliser and vinasse (distillation residue) applied as a top dressing (two applications), was compared to a digestate treatment which comprised three applications of digestate after planting (with 1.28 kg N, 0.3 kg P2O5 and 1.28 kg K2O/m³), with both treatments having the same total N applied (340 kg N/ha) (Andreas, 2006). Tomato yield differences between treatments were


insignificant, varying slightly between the five tomato varieties trialled. In 2004, with different tomato varieties and similar treatments, the results were slightly more scattered than 2005, but comparable yields were observed in both the digestate and control treatments for each variety (Andreas, 2005).

Figure 2: Tomatoes grown using a digestate fertiliser solution at GBZ Straelen (Andreas, 2004a).

Figure 3: Left: Cocktail tomato ―Oakley‖, grown using digestate (Reintges 2013, pers. comm.). Right: Round ―organic‖ tomatoes grown using a digestate fertiliser solution at GBZ Straelen (Andreas, 2007).

Figure 4: Peppers grown using a digestate fertiliser solution at GBZ Straelen in 2004 (left) and 2007

(right) (Andreas, 2004b, Andreas, 2007).

Furthermore, unpublished studies at GBZ Straelen investigated pepper grown using maize/cereal/sugar beet digestate. No results are publicly available, although photographs


have been published to show young pepper plants thriving on digestate fertiliser in 2004 and 2007 (Andreas, 2004b). In a study from the USA, digestate produced from pig slurry and wastewater from a pig unit was treated with a trickling nitrification biofilter with polystyrene beads, which converted almost 90% of the ammonium in the digestate into nitrate (Cheng, 2004). The resultant nitrified digestate was subsequently used as both fertiliser and irrigation water for approximately 14,400 tomato plants grown in perlite in greenhouses. All tomatoes were grown using the treated digestate, with no commercial control. Experimental data indicate that the tomato greenhouses used approximately 12 m3 of the effluent and 3.84 kg nitrogen per day. Moreover, the daily yield was 520 kg (37 g/plant) of marketable fruit, which was deemed by the authors to represent a financially viable operation.

Table 2. Average nutrient concentration of the digestate after treatment with the nitrification biofilter, which was subsequently used for tomato production (Cheng, 2004)

Biofilter- treated digestate

TKN mg/l 24.2

NH4-N mg/l 14.0

NO3-N mg/l 109.3

P mg/l 39.8

o-PO4-P mg/l 30.1

K mg/l 398.5

COD mg/l 114.9

TOC mg/l 44.8

pH 7.2

TS % 0.336

VS % of TS 15.9

A study on cucumber production used digestates produced from pig manure, with one digestate type termed ‗biogas slurry‘ (assumed to be whole or liquid digestate due to the 3% organic matter content), and the other termed ‗biogas residue‘ (assumed to be separated fibre due to the 30% organic matter content) (Duan et al., 2011). One treatment (treatment 1) used digestate in three different ways (see Table 3): Digestate fibre was used as a base fertiliser, providing 1/3 of the nutrients, whole/liquor digestate was used as a top dressing, and whole/liquor digestate was used as a foliar application. In both instances the whole/liquor digestate was mixed 1:1 with water. Inorganic fertiliser was used as the control treatment and also applied in three ways: Base fertiliser, top dressing and foliar application (treatment 2, see Table 3). The chlorophyll content in the leaves was higher in the digestate treatment. Compared with the control, the cucumbers grown with digestate were longer with a lower curvature (both properties deemed an advantage), with a significant total yield increase of 16%. Moreover, the cucumbers grown with digestate had higher concentrations of soluble sugars and proteins, indicating an improved nutritional quality. Both the digestate and control cucumbers had concentrations of Pb, Cd, Hg and As which were lower than the minimal detectable limit of the Chinese national standard. The incidences of aphids and mildew on cucumber plants grown with digestate were both significantly fewer compared to the control.


Table 3. Fertiliser treatments for cucumbers (Duan et al., 2011)

Nutrient

application

Treat-

ment

Digestate

fibre (kg)

Digestate

‘slurry’

(kg)

Water

(kg)

Diammonium

phosphate

(kg)

Carbamide

(kg)

Potassium

chloride

(kg)

Potassium

sulphate

compound

fertiliser

(kg)

Base 1 20 0 0 0 0 0 0

fertiliser 2 0 0 0 0.9 0.15 0.60 0

Top 1 0 40 40 0 0 0 0

dressing 2 0 0 79.69 0.18 0.03 0.10 0

Foliar 1 0 5 5 0 0 0 0

application 2 0 0 9.95 0 0 0 0.05

The following study focuses on rye grass, which is not generally grown in the glasshouse, but nonetheless provides additional data on the use of digestates as a fertiliser for pot grown plants. In a German pot trial, perennial rye grass (Lolium perenne) was grown in loamy sand for five months in 21 cm diameter, 25.5 cm high pots (Benzenberg et al., 2011). After 40 days, a one-off fertiliser application was made, with two types of digestates from crops (silage feedstock species not specified in the paper, but most likely grass). One digestate was produced from the separated liquid fraction of silage, and the other digestate from whole crop (unseparated) silage. Inorganic fertiliser was used as the control treatment. The two digestates and the inorganic fertiliser were all applied at five N-rates: 0, 50, 100, 150 and 200 kg N/ha. Increasing the N rate resulted in increasing biomass yield. For each N rate, there was a similar above-ground biomass yield for the two digestates and the mineral N fertiliser. However, for the digestate made from the liquid fraction of silage, the biomass yield response levelled off at 150 kg N/ha. 5.0 Whole digestate and digestate liquor studies with soil-less production Soil-less production via hydroponics includes a range of systems, from plant roots grown purely in nutrient solution through to plants grown with nutrient solution in inert media such as perlite, rockwool or gravel. Further details of hydroponics systems are discussed in a recent WRAP report (WRAP, 2015b). The majority of work on the potential of digestates in such systems has been undertaken outside the UK, with one recent WRAP funded trial conducted in England, as discussed below. 5.1 UK research: Hydroponic production of tomato and lettuce with three whole

digestates The feasibility of using digestates for the hydroponic production of tomato and lettuce in England was investigated in a recent study (WRAP, 2015b). Three whole digestates were used, with two from food waste and a third from cattle manure and potato waste. Overall, the study showed that digestate was suitable as an amendment in hydroponic solutions, but that diluting the digestate to an appropriate ammonium concentration for the crop is an important consideration. Further research was recommended on the potential for pathogens to be present in the digestates.


The digestates were all diluted to ensure that ammonium represented only 10% of the required total nitrogen concentration in the final solution (see Table 4). Inorganic nutrients were then added to ensure that there were no nutrient deficits. Two controls were used; an industry standard and also an industry standard plus ammonium (at 10% of total nitrogen). Plants of tomato (cultivar Supersweet 100) and lettuce (cultivar Little Gem) were established in rockwool blocks that were then placed into an open hydroponic system where the aerated nutrient solutions were changed every seven days. Plant growth and (for tomato) fruiting were monitored and compared with plants grown using conventional nutrient solutions. Table 4. Dilutions required to reduce digestate ammonium to 10% of the total N (WRAP, 2015b). Where there are three sets of numbers, those outside brackets refer to the pre-flowering stage for tomato, those within parenthesis refer to fruit-

set (tomatoes) and those within square brackets refer to lettuce.

Cattle manure &

potato waste Food waste 1 Food waste 2

Total-N (mg/l) 3359 6912 4327

Ammonium-N (mg/l) 2846 6654 4227

Nitrate-N (mg/l) 513 258 100

Excess total-N (x) 30 (23) [17] 61 (48) [31] 38 (30) [21]

Ammonium-N (% of total) 85 92 98

Dilution needed (x) 252 (198) [142] 589 (462) [333] 374 (294) [211]

Nitrate amendment required (mg/l) 100 (127) [176] 102 (129) [179] 101 (129) [180]

Table 5. The final nutrient concentrations of the solutions for the hydroponics trial(WRAP, 2015b)

mg/l N P Mg K Ca Mn Fe Cu Zn Mo Cl B

Tomato: Pre-fruiting 113 62 50 199 122 0.62 2.5 0.05 0.09 0.03 0.85 0.44

Tomato: Post-fruiting 144 62 50 199 165 0.62 2.5 0.05 0.09 0.03 0.85 0.44

Lettuce 200 62 50 154 247 0.62 2.5 0.05 0.09 0.03 0.85 0.44

The tomato fruit yield was similar for all five treatments, with sugar content and the results of taste tests indicating that fruit quality was also equivalent. Tomato fruit grown in one of the food waste digestate treatments contained more cobalt than fruit from other treatments, while fruits grown in the inorganic solution amended with 10% ammonium contained less sodium than those grown in the cattle manure and potato waste digestate. Figure 5. Lettuce and tomato grown using digestate nutrient solution (WRAP, 2015b)

Lettuce yields were unaffected by the digestates, although leaves had higher concentrations of calcium and copper when grown in solutions containing digestates. Salmonella was tentatively identified (using conventional plating techniques) in a number of digestate


samples, as was E. coli. In consequence, the lettuce plants were tested to ascertain whether internalisation of the pathogens had occurred. Although the lettuce was not contaminated by these pathogens, the report highlights that microbiological safety concerns may limit the usefulness of using anaerobic digestates for lettuce production until further data are available, although at the current digestate dilution levels the microbiological risks involved do not appear to be any greater than those associated with conventional production methods. 5.2 Research outside the UK regarding whole digestate and digestate liquor with soil-less

production In three studies from the USA, Liedl et al. (2004b, 2004a, 2006) compared diluted digestate produced from poultry litter with commercial hydroponic feeds in soil-less tomato, cucumber and lettuce production, and found promising results in all three systems, as described below. In a hydroponic tomato production trial, tomato plants were grown in buckets containing a mixture of 85% coarse perlite and 15% coir (v/v). Two nutrient solutions were used for fertigation: A commercial standard fertiliser (150ppm N, 50 ppm P, 200 ppm K, 150 ppm Ca, 50 ppm Mg, 60 ppm S and various trace elements (not specified)), and diluted poultry litter digestate, with the NH3-N content of the digestate diluted to match the N concentration in the commercial fertiliser treatment. Unfortunately no data on the nutrient content or dilution rate of the digestate is presented in the paper. The pH of both nutrient solutions was adjusted to achieve 5.5-6.8 using 85% H3PO4 and EC levels of 2.2-2.8 mS/cm. With digestate alone, plant growth rate was reduced, and fewer, smaller fruits were produced, with signs of ammonia toxicity. The authors highlighted that tomato plants are sensitive to fertilisers where ammonia is the dominant form of N. To reduce ammonia levels the ammonium/nitrate ratio was balanced by firstly heating and air sparging the digestate to reduce ammonia levels, and then by adding Ca(NO3)2, to return the nutrient solution to the same N concentration as the commercial control. When this solution was fed to the tomato plants signs of Mg deficiency were evident. It was found that when the N forms had been balanced and the Mg concentration supplemented using MgSO4 with the same Mg concentration as the commercial control, the poultry litter digestate solution was found to function as well as the commercial control (Liedl, 2004a). A summary of the results is shown in

Figure 6.

Figure 6. Summary of the main findings of a series of hydroponic tomato trials (Liedl, 2004a)

In the second trial, four growth trials were conducted on hydroponically grown lettuce transplants (nutrient film technique). In all trials there were four treatments, including a


commercial hydroponic solution (150ppm N, 54 ppm P, 250 ppm K, 134 ppm Ca, 33 ppm Mg and various trace elements (not specified) and three concentrations of poultry litter digestate (no raw digestate analysis data provided, but for some information on the diluted digestates see Table 6). The first three trials used 100, 200 and 300 ppm ammonium-N and the fourth 50, 100 and 150 ppm ammonium-N in the digestate. The nutrient solution pH was maintained at 5.5-6.1 using H3PO4, with this addition enhancing P levels of the digestate treatments. The dilution of the digestate was found to be critical, as increasing the concentration above the 100-200 ppm N optimum for lettuce production was detrimental to shoot fresh weight, with higher digestate concentrations also changing the taste by enhancing bitter characteristics. Increasing digestate concentration also increased root fresh weight. The 100 ppm N digestate treatment produced shoot fresh weights not significantly different from those produced using the conventional commercial solution. Tissue N content ranged from 4.3-5.2% (assumed to be DM), with highest N observed in the commercial control treatment, with more desirable plant tissue N levels found in the digestate treatments (Liedl, 2004b). Table 6. Analysis of the nutrient solutions used in the fourth lettuce trial, in ppm (Liedl, 2004b).

Nutrient component

Commercial control

Digestate 50 ppm N

Digestate 100 ppm N

Digestate 150 ppm N

Nitrate 348.00 9.35 0.00 0.00

Ammonium 0.34 0.49 43.60 126.00

Phosphorus 129.55 221.97 410.80 632.37

Potassium 110.48 238.76 216.06 263.54

Calcium 223.86 29.53 51.74 63.41

Magnesium 55.41 11.30 15.88 20.51

Iron 3.29 0.28 0.55 0.76

Manganese 0.60 0.01 0.41 0.88

Boron 0.34 0.16 0.26 0.40

Copper 0.13 0.23 0.35 0.54

Zinc 1.26 0.20 1.05 0.84

Molybdenum 0.09 0.03 0.01 0.03

Sodium 27.06 35.44 52.00 70.13

Aluminium 0.00 0.01 0.14 0.12

pH 5.69 5.67 5.25 5.51

Conductivity mS/cm 1.87 0.95 1.31 2.12

In soil-less cucumber production (perlite/coir media), the digestate was diluted to the same mineral nitrogen concentration as that found in the commercial feed, and solution pH maintained at 5.6-6.0. Average fruit weight decreased by 11 % when plants were grown with the digestate, but the percentage of fruits classified as grade 1 increased (33 % grade 1, compared to 26 % with the commercial feed) (Liedl, 2006).

In a Canadian study, digestate (‗process liquid wastewater‘) from the anaerobic digestion of mixed municipal solid waste was used as a material for fertigation of three ornamentals (silverleaf dogwood, common ninebark and Spiraea) over a three month period in a growing medium consisting of 73% bark, 22% peat, and 5% pea gravel by volume (Chong, 2008). Four weeks after planting, four fertigation treatments commenced:

1 Recirculated control fertilizer solution based on a nutrient formula with a targeted EC of 2.2 dS/m;

2 recirculated mushroom farm wastewater (diluted 10x with tap water);

3 recirculated digestate from MSW (diluted 20x with tap water);

4 Nutryon (Polyon) 17-5-12 (17N–2P–10K) 6 month controlled-release fertilizer with micro-nutrients

(Nutrite, Elmira, Ont.) topdressed at a rate of 39 g/container (nutrients not recirculated).


The liquid was recirculated using a computerised recirculating injector system. Chemical monitoring was undertaken with supplementary feeding provided where necessary to ensure a balanced nutrient supply. Dilution ensured that sulphate, chloride and sodium were not oversupplied, with a target EC of 2.2 dS/m for all treatments. Growth, in terms of plant dry weight and height, was as good with the digestate treatment as with standard controlled release fertilisers. A Chinese study investigated the effect of a range of nutrient solutions including animal manure digestate (termed biogas slurry so assumed to be whole or liquor) on growth and leaf nitrate concentration of lettuce, using five treatments (Wenke, 2009):

1 Inorganic nutrient solution (control): NO3--N, 15mmol/l

2 Organic nutrient solution: N from diluted digestate (1:5 v/v): 8.69 mmol N/l

3 Organic nutrient solution: N from diluted digestate (1:5 v/v) + glycerine: 15 mmol N/l

4 Organic nutrient solution: N from diluted digestate (1:4 v/v): 10.43 mmol N/l

5 Organic nutrient solution: N from diluted digestate (1:4 v/v) + glycerine: 15 mmol N/l

The composition of inorganic nutrient solution was as follows (all in mmol/l): 0.75 K2SO4; 0.25 KH2PO4; 0.65 MgSO4; 0.1 KCl; 7.5 Ca(NO3)2; 1.0x10-3 H3BO3; 1.0x10-3 MnSO4; 1.0x10-4 CuSO4; 5.0x10-6 (NH4)6Mo7O24; 1.0x10-3 ZnSO4; 0.1 EDTA-Fe(II). The composition of digestate nutrients was: N 0.73 g/l; P 0.028 g/l; K 0.74 g/l; NH4

+-N 0.7 g/l; NO3--N 8.62

mg/l; calcium 352 mg/l; magnesium 227 mg/l; SO24- 15 mg/l; iron 4.1 mg/l; manganese 0.48

mg/l; copper 0.96 mg/l; zinc 3.35 mg/l; EC 6.36 (no EC units in the paper); pH 7.8. The EC after dilution was 2.05 for the 1:4 v/v treatments and 1.76 in the 1:5 v/v treatments. The initial pH of all solutions was adjusted to 6. Lettuce seeds were pre-germinated in an incubator and then planted into 5cm high pots (base 3 cm –top 5 cm Ф) filled with sterilised sand, with three seeds per pot. Once the seedlings were present all pots were watered with a 5ml standard inorganic solution of 4mmol/l NO3

—N, then 9 and 15 days later the plants were fertigated with 400 ml/pot of the treatment nutrient solutions. On day 20 the plants were harvested. Treatments 2 and 3 significantly increased shoot weight and number of expanded leaves, compared to the control. Compared with the control, all organic treatments (2-5) significantly reduced shoot nitrate content, with the treatments including glycine being lower than the digestate only treatments. The authors concluded that digestate offers potential to replace inorganic fertilisers in the hydroponic production of lettuce, with lower nitrate levels deemed an advantage for consumer health, as excessive intake of nitrate was highlighted as being a potential health hazard. Although not specifically a horticultural crop, but of relevance due to the use of digestate for fertigation, the following Canadian trial is included. The use of digestate for the fertigation of two cool season turfgrass species (green creeping bent grass and Kentucky bluegrass) was investigated (Michitsch et al., 2008). The grasses were grown in plastic containers (8.3cm diameter and 20cm deep) in a 4:1 sand:peat mixture for three months in a growth room, with three cuts being undertaken during the trial period. Digestate from BMW was applied weekly at rates of 25, 50, 100 and 200% of the recommended rate of 25 kg N/ha. This was compared to a standard inorganic fertiliser at the same N rates. It was found that the use of digestate at the recommended N doses performed as well as the inorganic fertiliser at the same N rate. Moreover, the low NO3:NH4 ratio had no observable effect on the growth of the grasses.


6.0 Whole digestate and digestate liquor as a growing media ingredient The studies below focus on the mixing of digestates with other growing media ingredients to support the growth of a range of crops. One recent UK study used whole and liquor digestates for the production of three ornamentals as described below. 6.1 UK research: Bark admixtures: Formulation and testing of novel organic growing

media using quality digestates for the production of containerised plants Four different digestates were mixed at five different rates with a base growing medium comprising bark, wood fibre and topsoil (see Table 7) (WRAP, 2015a). The four digestate feedstocks were food waste (whole digestate), food waste (separated liquor, different source), potato waste (whole digestate) and maize (separated liquor). It was found that a suitable base mix for subsequent addition of digestate and the creation of admixtures with an open structure was: 60% bark, 30% wood fibre and 10% topsoil by volume. This was combined in a ratio of 5l base mix to five different volumes of digestate (0.1l, 0.25l, 0.5l, 0.75l and 1l) to create the final experimental admixtures. In addition to the four digestates, each added at five rates, two industry standards were used as controls – one peat based and the other peat free. The plant species investigated were wavy cyclamen (Cyclamen repandum), fern (Asplenium scolopendrium) and black pine (Pinus nigra). All were planted into the admixtures and assessed regularly for at least 90 days. Table 7. Details of the digestate type and volumes used for the admixtures trial (WRAP, 2015a)

Digestate Digestate Volume of digestate in 5 litres of admixture (60% bark, 30% wood fibre, 10% top soil)

feedstock Type 100ml 250ml 500ml 750ml 1000ml

Food waste Separated liquor FS1 FS2 FS3 FS4 FS5

Food waste Whole FW1 FW2 FW3 FW4 FW5

Potato waste Whole PW1 PW2 PW3 PW4 PW5

Maize Separated liquor MS1 MS2 MS3 MS4 MS5

For black pines there was no significant difference in any growth parameter compared to the controls, including number of stems, plant height and plant quality. A growth vs time analysis showed pines of all treatments grew at comparable rates. Destructive harvest analysis to determine the mean dry weight of the whole plant, of stems and of roots as well as dry matter content and mean root-to-shoot ratio did not generally show statistically significant differences between the digestate-bark admixtures and the controls. For ferns there was mostly no significant difference in any growth parameter compared to the controls, for assessments made of number of fronds, frond length, chlorophyll content and foliage quality. A growth vs time analysis showed ferns of nearly all treatments growing at comparable rates (see Figure 7). Destructive harvest analysis to determine the mean dry weight of the whole plant, of stems and of roots as well as dry matter content and root-to-shoot ratio did not generally show statistically significant differences between the various treatments and the controls. Admixtures with high doses of 1l food waste digestate were the exception to the above findings, with a significant reduction in fern chlorophyll content and leaf quality, which was attributed to comparatively high sodium content and EC originating from the food waste digestates.


Table 8. Control and admixture characteristics (further parameters are listed in the project report)

(WRAP, 2015a)

pH EC Density Ammonia-N Nitrate-N Total N Total P Total K

Admixture

uS/cm kg/m3

FW

mg/l

FW

mg/l

FW

% w/w

DW

mg/kg

DW

mg/kg

DW

BC 6.4 435 468 48.1 6.9 0.72 3850 1330

PC 5.39 311 420 30.3 131 0.91 2427 949

MS1 6 96 405 38.3 18.8 0.31 1695 531

MS2 6.3 150 406 63.2 21.9 0.36 1859 508

MS3 7 213 462 97 17.4 0.43 2780 537

MS4 7.41 317 551 146 19.1 0.51 3044 600

MS5 7.63 431 575 225.8 6.1 0.6 3910 648

FS1 5.97 102 366 40.1 17.6 0.39 1624 489

FS2 6.59 161 414 79.9 20 0.38 1669 609

FS3 7.3 238 505 128.3 22.3 0.45 1989 684

FS4 7.72 411 545 246 2.3 0.47 2065 688

FS5 7.67 536 607 299.5 8.4 0.63 2473 767

PW1 5.92 106 419 46.7 19.9 0.36 1874 552

PW2 6.3 122 427 51.6 18.2 0.4 2375 539

PW3 7.22 155 465 75.1 15.3 0.43 3262 581

PW4 7.63 315 599 172.1 10.7 0.44 4303 596

PW5 7.7 397 571 200.9 42.2 0.47 4622 601

FW1 6.2 174 344 66.2 60 0.42 1644 487

FW2 7.16 424 411 127.1 20.6 0.41 1879 661

FW3 7.77 409 461 224.9 19.2 0.52 2302 722

FW4 7.91 579 507 327.8 17.6 0.61 2763 826

FW5 7.9 898 591 441.9 20.2 0.69 2681 796

For cyclamen, there was no significant difference in any growth parameter measured compared to the controls. A growth vs. time analysis showed cyclamen in all treatments senescing at similar times. Destructive harvest analysis was carried out on the cyclamen corms, as all plants had senesced at the point of harvest. The mean dry weight of the corm and dry matter content did not generally show statistically significant differences between the various treatments and the control. Figure 7. Fern plants after 90 days of growth in peat-free control (left) and a maize digestate-bark

admixture (right) (WRAP, 2015a)


No fertiliser supplementation was required during the trial, indicating that the range of digestates investigated was able to provide an appropriate level of nutrients for the three species under test. All four digestates contained essential plant macro- and micronutrients. Analysis of the digestates also showed that the majority of the available nitrogen was in the form of ammonium, with some nitrates also present. The outcome of the experiments showed that using digestate/bark admixtures as a growing medium for all three plant species generally did not significantly affect plant quality. 6.2 Research outside the UK regarding whole digestate and digestate liquor as a growing

media ingredient A Czech glasshouse study focussed on determining the impact of the application of digestate from pig slurry on the yield and dry matter content of a range of cultivars of both tomato and pepper plants in peat-bark growing media (Kouřimská et al., 2009, Kouřimská et al., 2012). The digestate consisted of (per l) 595 mg NH4

+, 755 mg PO43-, and 1.1–1.25 g K2O.

The four treatments, all in 20 l pots were: 1 Unfertilised control;

2 Inorganic standard 15 g (NH4)2SO4 and 9 g K2HPO4 added to each pot (20 l) prior to planting, and 7.5 g

(NH4)2SO4 added 30 days later;

3 Digestate: 0.8 l added to each pot prior to planting, and another 2 l added 30 days later;

4 50% of inorganic fertiliser (7.5 g (NH4)2SO4 and 4.5 g K2HPO4) and 50% (0.4 l) of digestate added to

each pot prior to planting, and 3.75 g (NH4)2SO4 and 1.5 l of digestate added 30 days after planting.

The amendments were mixed into the growing media at the start of the experiment and then more of the same amendments were added after 30 days as a liquid feed. The trials with tomatoes were repeated with new plants and growing media over five years, and for the peppers over two years. The unfertilised control had the lowest yield. There were no significant differences in yields with the organic and inorganic fertiliser treatments (2, 3 and 4), for all cultivars of both tomato and pepper, although the combined treatment showed a slight trend for being the highest yielding. Interestingly, the dry matter contents of both tomato and pepper were in the following descending order: Digestate (treatment 3) > combined (treatment 4) > control (treatment 1)> inorganic only (treatment 2). The higher dry matter content of the crop in the digestate treatment was deemed by the authors to be a positive effect, indicating that the digestate matches the nutritional needs of the plants in the most suitable way. The higher dry matter content can be an important factor for vegetable products (such as ketchups, dry products, puree etc.), improving the shelf life of fresh vegetables. Lower water content inhibits the growth of undesirable microorganisms, which cause decay of the products. Moreover, the heavy metal content remained below permitted limits, and was not significantly different to conventionally fertilised tomatoes. There was no difference between these plants and those grown in peat-bark media that had had a conventional inorganic fertiliser added. An Indian study mixed digestate slurry (assumed to be whole or separated liquor, as feedstock not defined) 1:1 with wheat bran, and after 21 days mixed this with soil (Kichadi and Sreenivasa, 1998). The study also included a treatment with dried digestate at 10% moisture, mixed with soil. The main result was that the digestate, with or without the addition of two beneficial mycorrhizal fungi, increased tomato plant growth and fruit yield, in addition to supressing the soil borne pathogenic fungus Scerotium rolfsii. The following two studies discussed in this section focus on crops not generally grown in the glasshouse, but nonetheless provide additional data on the use of whole and liquor fraction digestates as a growing media ingredient.


A Norwegian pot study investigated the effect of a range of waste derived amendments including liquid digestate from source separated household waste and nitrified liquid digestate of the same origin, in addition to standard N P K fertiliser, on the growth of spring barley (Hordeum vulgare). Plants were grown in a mixture of sand and peat, with the amendments mixed into the top portion of the soil prior to sowing, at two rates of 80 and 160 kg N ha-1. The liquid digestate was found to be as good as the NPK fertiliser. However, the nitrified liquid digestate treatment caused significantly increased nitrate leaching, and a lower yield (Haraldsen et al., 2010). A one year Czech study (Lošák et al., 2012) investigated the effect of a pig slurry/maize silage digestate on pot grown kohlrabi (a nitrogen demanding crop). All pots contained 6 kg of fluvial soil, with four treatments:

1 untreated control,

2 urea (1.5 g N/pot),

3 digestate (N-P-K-Mg g/pot: 1.5-0.18-0.69-0.08),

4 urea, triple super phosphate, KCl, MgSO4 (N-P-K-Mg g/pot: 1.5-0.18-0.69-0.08).

The N dose was the same in treatments 2-4, being 1.5 g N/pot. In treatment 4 the P, K and Mg doses corresponded to those supplied in the digestate treatment. The treatments were thoroughly mixed and the kohlrabi seeds were sown ten days later. Yields in the digestate treatment were comparable to those achieved with artificial fertilisers (over three times those of the unfertilised control). Both digestate and fertilisers reduced the ascorbic acid (Vitamin C) content of the crop but tissue nitrate concentration (high levels of which can be harmful to health) was much higher in the fertilised treatments 2 and 4. 7.0 Digestate fibre use in horticulture The separated solid fraction of digestate (termed digestate fibre) is produced by separating the whole digestate via a range of different processes including screw, screw press, centrifuge and membrane filtration (Cavinato, 2013). The use of digestate fibre in growing media has been highlighted as a potential emerging application in the UK (WRAP, 2013a), although this end use is not currently permitted by PAS110 and the ADQP, and as such there are no guidelines to date regarding desired characteristics. As a comparison, the existing guidelines for the specification of quality compost for use in growing media can be used as an important resource for recommended targets and limits. The main quality parameters for compost use in growing media include stability, phytotoxicity, PTEs and physical and chemical properties, with upper limits of 50 mg/l for NH4-N, 150 mg/l for sodium, 1000 mg/l for chloride and 1500 μS/cm for EC (WRAP, 2011a). Interestingly, a recent industry survey regarding the potential use of digestate fibre as a growing media ingredient highlighted the following (WRAP, 2013a):

‗The production of growing media for container growing of plants for professional and amateur use is a highly specialised market with stringent requirements for the physical, chemical and biological characteristics of the product. Companies have internal specifications for components to meet requirements such as moisture content, bulk density, air-filled porosity and available water capacity. There is a requirement for constituents to be low in nutrients and soluble salts, and possibly low pH – to allow crop-specific nutrients to be added in the correct proportions. They must have low levels of physical contaminants, toxic elements and organic contaminants, be free from pathogens and also microbially stable to prevent further physical breakdown during product use. There is a continued need to find suitable peat replacement products that are not only technically robust but also sustainable


and of low cost, and this keeps a general interest in digestate as a possible contender for use in growing media by some producers.‘

Published research has investigated digestate fibre mixed with a range of materials (including other wastes and commercial growing medium ingredients) or the use of composted digestate, on the performance of a range of edible and ornamental crops. In other cases digestate has been composted together with other ingredients to produce a product. These scenarios are discussed below. Digestate fibre is also being used in other (non-horticultural) applications. For example, matured or dried digestate fibre is used for cattle bedding in the US (Alexander, 2012). However such uses are beyond the scope of this literature review, and are not discussed further here. When the digestate fibre is subsequently composted either alone or together with other feedstocks, provided the composting process meets the requirements of PAS100 (BSI, 2011), it would be regulated as a compost, rather than as a digestate. This has implications for the waste status of the material, since ‗end of waste composts‘ can be used for a wider range of applications than ‗end of waste digestates‘. Further information on acceptable markets for the different materials can be found in their respective quality protocols (WRAP and Environment Agency, 2010, 2012).

7.1 Digestate fibre as a growing medium ingredient

Solid digestate (fibre) has been trialled as a growing media ingredient with a range of other constituents, and some examples are summarised in Table 9. A recent UK study focussing on the use of digestate fibre for tomato production is discussed below. 7.1.1 UK research: The use of cattle slurry digestate fibre in mixtures with coir and pine

bark as plant growth substrates in the intensive production of glasshouse tomato crops

The use of digestate fibre produced from cattle slurry was assessed as a growing media ingredient for tomato production in a recent feasibility study (Challinor, 2014). Overall, the study concluded that digestate fibre may act as an additional nutrient reservoir for the sustained growth and yield of crops, such as tomato. The digestate fibre was left to stand for six weeks prior to mixing. The three treatments were coir only, 50:50 digestate fibre:coir (v/v) and 50:50 digestate fibre:pine bark (v/v). The higher digestate fibre pH was moderated by the lower pH of the coir and the bark, to create growing media deemed suitable for tomato production (see Table 10). The resultant growing media were filled into polythene sleeves, and then planted with tomatoes (cv Dometica). Water and nutrients were delivered via standard drip irrigation. The feed regime was tailored to best meet the nutrient demands of the plants and optimise nutrient availability. It was possible to obtain similar fruit quality in both the digestate mixtures and the standard coir substrate over a period of eight months.


Table 9. Summary of a range of digestate fibre trials *Studies at the University of Padova with all studies using the following treatments: Increasing rates (0, 33, 67 and 100%) of ground rice hulls

(GRH), with or without 20% digestate, with the remainder being peat.

Species Mixes used Results Author

Lettuce and cress Solid digestate derived from manure or maize and agro-industrial waste mixed with peat

Peat and digestate mixes performed as well as peat

Crippa (2011)

Cherry laurel, Spirea ‗Grefsheim‘ and wild privet

Solid digestate mixed with standard growing media

Very good results using up to 60 % solid digestate in the growing media mix

Wrede pers. comm.

Roses Solid maize silage based digestate mixed with standard growing media

The 20 % digestate treatment improved the quality of roses

Wrede (2012)

Rose cuttings (propagation trial)

Distillery fruit waste solid digestate mixed with rice husks and peat*

Digestate improved rose cuttings and did not influence rooting as compared to treatments without digestate

Tassinato (2011)

Geranium cuttings (propagation trial)


Digestate did not affect number of leaves, leaf weight, cutting height, stem weight or % rooting. Digestate reduced cutting diameter and root weight, hence increasing the root:shoot ratio

(Tassinato, 2011)

Tomato and Salvia splendens


With 20% digestate in the rice hull compost, transplant growth was equivalent to the peat compost control

Bassan et al. (2012)

Tomato Fruit and wine waste solid digestate mixed with rice husks and peat*

Digestate improved cuttings and did not influence rooting

Bassan et al. (2014)

There were no visible symptoms of plant damage caused by pesticides or herbicides during the trial, demonstrating that there was no such contamination in the digestate mixes. The coir treatment had the highest marketable yield at 30.19 kg/m2, with the digestate and bark mix yielding 27.85 kg/m2, compared with 25.89 kg/m2 from the digestate and coir mix. The author highlighted that it is likely that positional effects of the substrate rows and the absence of replication in the trial glasshouse may have influenced the plot yields. Further research on potential crop contaminants and also the presence of human, animal and plant pathogens in the digestate was recommended, especially prior to future use in intensive cropping. Moreover, it was highlighted that the physical, chemical and biological characteristics of the digestate fibre must be fully quantified before use, as an understanding of the analytical profile will help to avoid any of the potential nutritional difficulties, and assist management decisions during the crop growth period.


Table 10. Physical and chemical properties including water extractable nutrients (fresh samples) of

the three growing media used for the tomato trial (Challinor, 2014)

Parameter Units Coir

Coir & digestate

fibre (50:50)

Bark & digestate

fibre (50:50)

pH 6.4 8.8 8.5

Conductivity μS/cm 20°C 267 413 381

Bulk Density g/l 412 423 468

Dry Matter % m/m 12.9 22 27.1

Moisture % m/m 87.1 78 72.9

Water extractable

Ammonium-N mg/l < 1 < 1 < 1

Nitrate-N mg/l < 5 28.7 11.3

Phosphorus mg/l 8.08 63.8 64.2

Potassium mg/l 282 488 447

Calcium mg/l 1.45 19.6 26.6

Magnesium mg/l < 1 12.6 18.7

Sodium mg/l 80.6 106 85.5

Chloride mg/l 334 195 141

Sulphur mg/l 3.15 30.7 41.1

Iron mg/l < 0.5 0.875 0.569

Manganese mg/l < 0.1 < 0.1 < 0.1

Boron mg/l 0.184 0.231 0.229

Zinc mg/l < 0.1 < 0.1 < 0.1

Copper mg/l < 0.5 < 0.5 < 0.5

Molybdenum mg/l < 0.1 < 0.1 < 0.1

Figure 8. UK tomato trial in July 2013 (left) and September 2013 (right). Left: Digestate and bark,

middle row: Digestate and coir, right: Coir only.

7.1.2 Research outside the UK: Digestate fibre as a growing medium ingredient A German study investigated the use of digestate fibre from 80% maize silage and 20% rye silage mixed (v/v) in concentrations of 0, 20, 30 and 40% with peat (Rakers et al., 2010). Petunia surfinia ―White‖ seedlings were potted and grown for five weeks, after which yield, height and number of buds were determined, with no significant effects between the


treatments. At the end of the trial, all plants were deemed suitable for sale in garden centres. A 70 day greenhouse pot trial compared the growth of lettuce over 70 days in two Italian soils (sandy loam and loam), combined with 200 and 400 kg N/ha of the solid fraction of pig manure (pH 7, N 2.9% DM, organic C 43.5% DM), the digestate fibre from pig manure (pH 8, N 3% DM, organic C 35.9% DM), and granular urea as an inorganic fertiliser (Trinchera et al., 2013). There was also an unfertilised control treatment. The lettuce transplants were grown in 2 l pots. The N availability of both the solid digestate and manure were comparable to standard industry practice for fertilising short term horticultural crops. Plant quality was good at the end of the experiment, in both the soils and for both of the organic amendments. Even though the plant dry matter was partially reduced with the organic amendments, as compared to the inorganic fertiliser, the macro and micronutrient uptake was lower. This indicates improved nutrient use efficiency of the organic amendments compared to the urea treatment. Moreover, the digestate treatments generally performed as well as or better than the unfertilised controls. Crippa et al (2011) used digestate fibre (derived from manure or maize and agro-industrial waste) to partly replace peat in growing media. A 30 day lettuce trial showed promising results with good physical characteristics. The pure digestate fibre treatment without peat had a high conductivity that inhibited cress germination during a preliminary bioassay, whereas peat and digestate mixes performed as well as peat in the lettuce trial. Wrede (2012) compared a number of different blends of solid maize silage-based digestate mixed with standard growing media and with standard growing media alone as the control treatment (see Figure 8). The 0% and 20% digestate treatments included 4g/l slow release fertiliser, and the 30% and 40% digestate treatments included 3g/l slow release fertiliser (nutrient contents of the digestate and fertiliser were not defined). Two varieties of roses (with both cuttings with formed roots, and also plants with a grafted rootstock) were grown in 5.5 l pots for seven months. The 20% digestate treatment generally improved the quality of the roses (assessed by plant height, bud weight and sorting the plants visually into two quality categories) compared to those grown in the control media. The 30% and 40% digestate treatments did not result in improved quality. Reduction of bud weight for 30% and 40% digestate treatments was attributed to the lower dose of controlled release fertiliser compared to the 0% and 20% digestate treatments. Figure 9. Digestate amended rose trials at the Gartenbauzentrum der Landwirtschaftskammer

Schleswig-Holstein, Germany (Wrede, 2012).


In a personal communication, Wrede discussed trials in Germany undertaken in 2012 with three ornamental shrubs (cherry laurel, Spiraea grefsheim and wild privet). The results of these trials were not yet published, however there were very good results using up to 60% maize silage digestate fibre in the growing media mix. The digestates used were found to have good structural properties (e.g. cellulose and lignin). Research regarding ornamental production using solid digestates is continuing at the Landwirtschaftskammer Schleswig-Holstein, Germany. An American research group investigated the use of dairy manure digestate fibre (pH 8.4, EC 3.5 dS/m) on ornamentals as a peat replacement in a number of glasshouse trials (MacConnell et al., 2010, MacConnell and Collins, 2009). An initial pot trial using Petunia grandiflora focussed on the following treatments:

1 60% peat moss, 25% compost (sterilized greenhouse cull plant waste), 15% pumice;

2 85% peat moss and 15% pumice;

3 85% digestate fibre and 15% pumice;

4 60% digestate fibre, 25% compost, 15% pumice;

5 40% digestate fibre, 40% peat moss and 20% pumice.

6 All media had dolomite lime (CaMg(CO3)2) incorporated at a rate of 0.59 kg/m3. Plants

were fertigated daily with a mixture of 20-10-20 and 15-0-15 at 150 ppm N. When the digestate fibre was the main growing media ingredient, chlorosis was observed (due to poor availability of Fe and Mn), in addition to lower fresh weight, height and number of flower buds than the peat based treatments. The authors detail a range of trials focussing on improvement of the digestate fibre to address this. It was found that a lowering of the pH through post digestion acidification made Mn and Fe more available. It was also found that nutrient amendment was required to improve results. Thus in later trials the amended digestate fibre was diluted with peat, and the petunia plants performed as well as peat, as shown in

7 Figure 10. The treatments were: 1 80% peat moss, 20% P with 1.78 kg/m3 dolomite lime (CaMg(CO3)2) and 0.89 kg/m3 limestone flour

(CaCO3);

2 70% digestate fibre, 30% pumice;

3 70% digestate fibre, 30% pumice with 0.89 kg/m3 S;

4 70% digestate fibre, 30% pumice with 0.89 kg/m3 S and 4.15 kg/m3 gypsum (CaSO4·2H2O);

5 70% digestate fibre, 30% pumice with 0.89 kg/m3 S, 4.15 kg/m3 gypsum, and 0.89 kg/m3 limestone

flour.

Figure 10. Comparison of 1:1 optimised digestate fibre pre-treated media and peat (left) and peat control (right) (MacConnell et al., 2010)


In all treatments plants were fertigated daily with 125 ppm nitrogen using a 20-20-20 fertiliser. The use of sulphur (S) to acidify the digestate fibre produced plants with fresh weight and greenness equal to peat, but with inadequate root development. Adding gypsum along with S as amendments to digestate fibre produced petunia aerial and root systems that were equal to the peat treatment. In summary, these petunia growth trials indicate that amended digestate fibre can produce a substrate equal to peat moss for container plant growing media. A number of Italian studies were identified where digestate was mixed with rice husks/hulls and a range of other growing media ingredients, including peat and perlite (Brazzale, 2012, Dengo, 2013, Sambo et al., 2010, Vanetto, 2012, Zanta, 2001). The digestate fibre had a dry matter content of 30-40% for these trials (Giorgio Ponchia 2014 pers comm). Four substrates were prepared with increasing rates (0, 33, 67 and 100%) of ground rice hulls (GRH) mixed with peat. These mixes were either amended with 20% digestate produced from fruit and wine waste, or used without digestate. Tomato seedlings were grown under three irrigation regimes. Water holding capacity and easily available water decreased with the increase in GRH content. Differences between substrates were increased when digestate was added. Digestate generally reduced total pore space and air-filled porosity, but not in all mixes. Electrical conductivity was increased with the addition of digestate. Higher nutrient concentrations were observed in substrates containing digestate. The addition of digestate improved the growth of the tomato plants (Bassan et al., 2014).

Figure 11. Cyclamen plants (Brazzale, 2012). Top photo with 0%, 10%, 30% and 50% rice husks (left to right), plus peat without digestate in all four pots. Bottom photo with 0%, 10%, 30% and

50% rice husks (left to right), plus peat with 20% digestate in all four pots.


Another Italian study investigated a range of substrates including peat, rice husks, perlite and digestate from fruit and wine production waste, contained in pots made of plastic or rice husks (Vanetto, 2012). The trial was conducted at two plant nurseries. Euphorbia pulcherrima (poinsettia) plants were cultivated in substrates containing 0, 10, 30 and 50% rice hulls and peat (nursery 1), or 0%, 10% and 30%, rice hulls and peat (nursery 2), with and without the addition of digestate at 20% by volume. In the 0% rice hull treatments, perlite (at a rate of 10%) was used instead. Fertigation was undertaken for all treatments at a rate of 0.8 g/l, commencing with 20:20:20 NPK, and then 25:5:10 for two applications and 17:7:27 for two applications and finally one application of 15:5:25 at nursery 1, with no mention of any fertigation at nursery 2. The mixes containing 20% digestate fibre had an EC which was at least double that of the other mixes in the nursery 1 trial (0.21-0.23 μS/cm without digestate; 0.49-0.83 μS/cm with digestate), but with less of a difference in the nursery 2 trial (295-335 μS/cm without digestate; 415-450 μS/cm with digestate), with the ideal EC range suggested by the author at being 0.2-0.5 μS/cm (which is significantly lower than the UK recommended limit for growing media of 1500 μS/cm (WRAP, 2011a)). The addition of 20% digestate in nursery 1 in all mixes reduced the growth of poinsettia plants, including fresh and dry weight of the separate plant parts and total plant weight. This reduced growth on digestate addition was attributed to the increased EC levels. In nursery 2 this reduction was not observed. Another Italian study investigated a range of substrates including peat, rice husks and perlite with two varieties of Cyclamen persicum (Brazzale, 2012). The same approach (and proportions of rice hull to peat, with or without 20% digestate amendment, but no perlite) was used as described above. The same fertigation regime was provided for all treatments, with N, P, K, Ca and Mg concentrations varying throughout the season. In this instance, digestate increased the growth of leaves and roots and total fresh weight of the plants (see Figure 11). Table 11. Details of six growing media used in a cyclamen trial (Dengo, 2013)

Treatment 1 2 3 4 5 6

% perlite 20 0 0 20 0 0

% GRH 0 20 40 0 16 32

% peat 80 80 60 60 64 48

% digestate fibre 0 0 0 20 20 20

Density (g/cm3) 79.4 92.5 100.9 114.2 117.0 126.6

Total porosity (% v/v) 78.1 81.7 87.5 80.5 87.2 88.7

Porosity in the air (% v/v) 40.9 39.8 42.6 34.4 34.0 40.5

WHC (% v/v) 37.2 41.9 44.9 46.1 53.2 48.2

DM (%) 32.4 31.2 41.8 30.8 33.4 34.0

OM (%) 74.6 88.3 81.8 66.5 77.2 76.5

pH 7.1 7.0 7.1 6.9 7.0 7.0

EC (μS/cm) 230 260 217 670 653 620

N-NO3 (mg/l) 3.41 2.58 1.17 9.00 8.34 5.05

N-NH4 (mg/l) 5.61 7.03 2.06 3.5 6.81 2.51

P2O5 (mg/l) 4.59 4.66 6.82 3.4 7.11 7.09

K (mg/l) 24.1 39.1 35.1 83.1 81.8 70.8

Ca (mg/l) 15.8 20.5 9.2 39.3 31.3 23.6

Mg (mg/l) 3.06 4.24 1.71 5.18 5.41 3.85

SO4 (mg/l) 22.7 17.5 15.2 24.5 22.7 17.5


A four month growth trial was carried out with Cyclamen persicum in a polytunnel (Dengo,

2013), with six treatments (see Table 11). The digestate fibre was from fruit and wine

production waste. As with the other Italian trials discussed above, the mixes containing digestate had a significantly higher EC (620-670 μS/cm) than the non-digestate mixes (217-

260 μS/cm), with a range of characteristics of the growing media shown in Table 11. Plants

were fertigated (0.8g/l with 15-10-15-2 NPKMg) 17 times for the mixes with up to 20% perlite or 20% GRH, and 19 times for the mixes with up to 40% of GRH. The digestate had a limited effect on all parameters measured, including number of leaves and flowers, and the fresh and dry weight of roots and shoots. The author concluded that GRH could be considered an alternative to perlite, while 20% of digestate was seen as a good alternative to peat. A number of trials were undertaken in an in-depth three year study in Germany, using fresh and composted digestate fibre as a growing media ingredient (Meinken et al., 2009). In the first trial, seven types of fresh digestate fibre from crops (crops not specified but assumed to include maize) was mixed with peat to achieve a salt content of either 1g/l or 2g/l (termed digestate salt 1 or digestate salt 2), apart from one digestate fibre which had only 1g/l salt concentration. No information was provided by the authors regarding the make-up of the salt or EC levels. However, these two salt concentrations of 1g/l and 2g/l correspond roughly to a conductivity of 0.6mS/cm and 1.2mS/cm (Kehres and Thelen-Jüngling, 2006). The volume of digestate fibre in the resultant mix was between 9.1% and 33% to achieve a salt content of 1g/l and double that (i.e. up to 66%) for the 2g/l trials. There were two peat-only controls with additional salt added at concentrations of 1g/l and 2g/l. For each combination, 25 seeds were sown in 9l pots. Plants investigated were Chinese cabbage and barley. Plant fresh weight and leaf colour were assessed after 4 weeks. It was found that Chinese cabbage had significantly lower fresh weight when using the digestates, as compared to the two controls. Fresh weight of Chinese cabbage was approximately 87% of the 1g/l control for the digestate salt 1 treatments, and only 73% of the 2 g/l control for the digestate salt 2 treatments. Barley grain fresh weight varied more and on occasion the digestate fibre treatments performed better than the control. For the digestate salt 1 treatment, the variation was between 20% and 140% of the control with the same salt content, while for the digestate salt 2 treatment, the variation was between 7% and 87% with respect to the corresponding control. When looking at the leaf colour, it was found that pronounced chlorosis occurred for three of the seven types of digestate fibre and necrosis occurred for two types of digestate fibre, whereby the highest observed leaf quality was that for the controls. Both Chinese cabbage and barley plants showed a similar tendency for both necrosis and chlorosis. The authors of the study interpreted their findings by the assumption that unidentified growth inhibiting substances were present in the digestate fibre. They further hypothesised that the high pH of 6.9 observed in only one digestate fibre mix could have induced a lack of iron. The authors conclude that the use of fresh digestate fibre in growing media carries high risks for the healthy growth of seedlings (Meinken et al., 2009). Subsequently, trials were carried out where the digestate fibre was composted prior to use, as described in Section 0. The following studies focus on the growth of arable crops (rape and sunflowers) and rye grass in digestate fibre. Although these crops are not produced under glass commercially, these trials demonstrate further evidence regarding the suitability of digestate fibre as a growing media component either alone or in combination with other ingredients.


In an in-depth PhD study, rye grass (Lolium perenne), rape (Brassica napus) and sunflower (Helianthus annuus) were grown in 6 l pots in 30 different substrates (Van, 2012). Green waste compost and digestate fibre from pig manure and maize were used as received and also leached to reduce salt levels. After leaching, the content of nutrients was almost unchanged, whereas the digestate EC was reduced from 3.7 to 2.3 mS/cm. The treatments were:

A peat based control

100% compost both unleached and leached

100% digestate fibre both unleached and leached

A range of additives (perlite, styromull, hygromull, lecaton, peat and coir) mixed 20% (v/v) into 80% unleached and leached compost or digestate

In the first trial, rye grass was sown and grown for four months, with four cuts undertaken at monthly intervals. For the accumulated DM yield of the four cuts, the 100% unleached and leached compost and digestate fibre treatments were comparable or significantly greater than the control. However, the 80% leached compost with 20% additives treatment generally had a significantly lower DM yield than the control. The 80% unleached digestate fibre with 20% hygromull treatment resulted in a higher rye grass yield and total nutrient uptake than the other treatments. This was attributed to the fact that hygromull has a high water-holding capacity and is able to store nutrients and deliver them later in the growing season than the other additives. In the second trial, rape was sown and harvested after 51 days (compost treatments) and 58 days (digestate treatments). Subsequently sunflowers were sown in the same pots as recently harvested rape, and the sunflower plants were harvested after 40 days. Generally, the DM yield of rape and sunflowers grown in compost or digestate fibre was slightly higher or comparable to the control. Moreover, there were no significant differences in DM yield of sunflower between the pure materials (100% compost or digestate fibre) and those mixed with additives. In summary, the author recommended the use of hygromull or peat as an additive to digestate fibre in growing media. A study in Ireland compared the use of digestate with compost on the growth of perennial rye grass (Prasad et al., 2013). All treatments provided 218 kg N/ha. The trial was conducted in 15 cm diameter pots containing 5 l of soil. The treatments included soil only, compost, digestate fibre, compost or digestate fibre with inorganic N as a top dressing five times, digestate liquor applied five times and inorganic fertiliser-only applied five times. The grass was harvested five times over a four month period, with the liquid amendments applied after each harvest. The grass harvest was best in the inorganic fertiliser treatment. Nitrogen uptake was improved when the treatment included both the organic amendment and the inorganic top dressing. Digestate liquor applied at the rate described above caused burning of the foliage. This study also provides an in-depth comparison of the compost and digestate fibre used in the trial. The authors highlight that it is very important to measure the inorganic nitrogen (mostly NH4-N) and express it as a % of total N, to ensure that the grower has an accurate indicator of crop nitrogen availability. Do and Scherer (2012) described the use of ‗solid phase‘ digestate (presumed to be fibre digestate, from pig manure and maize) and green waste compost alone or as 80:20 growing media blends with perlite, styromull, hygromull, lecaton, peat and coir. Rye grass was grown in 6 l pots and cut four times at 30 day intervals. Plants were watered daily. Satisfactory growth of rye grass as a test crop was obtained with relatively little difference between the treatments. The 80:20 (v/v) mix of digestate fibre:hygromull resulted in a higher ryegrass yield and a significantly higher uptake of P and Mg than the control. The higher total nutrient


uptake was mainly caused by a higher uptake of the fourth cut. This is attributed to the fact that hygromull can store nutrients and release them later in the growing season. 7.2 Using composted digestate fibre as a growing media ingredient For the UK, the composting of digestate fibre has been highlighted as a potential means to stabilise the product further and also to reduce odour (WRAP, 2013a). In the US, digestate fibre is already routinely being composted (or dried) to ensure the final product is stable, making it easier to store and transport, in addition to adding value in the growing media ingredient market place (Alexander, 2012, MacConnell et al., 2010). Composting digestate has also been highlighted as a means of reducing nutrient loss of the raw digestate (Warnars and Oppenoorth, 2014). A limited number of studies were found regarding the use of composted digestates as growing media, including recent UK work which is discussed below. 7.2.1 UK research: The potential of using composted digestate fibre for horticulture for

ornamental production Digestate fibre from three feedstocks (potato waste, BMW and maize) was composted for 12 weeks. The composted digestate fibre was then mixed with green waste compost (GWC), with the volume of composted digestate fibre being 10%, 30% and 50% (WRAP, 2015c). Some characteristics of these growing media are shown in Table 12. Table 12. Analysis of the growing media mixes at the start of the ornamentals trial (WRAP, 2015c) CDF = composted digestate fibre, GWC = green waste compost

Bulk density pH

Conductivity P K Mg Available N

as NH4 Total N

g/l

µS/cm mg/l mg/l mg/l mg/l kg/t

Peat based control

277.64 6.18 670 19.57 286.61 51.48 132.68 17.94

Peat free control

223.8 6.12 504 39.83 64.77 4.93 129.2 10.88

50 % potato waste CDF: 50% GWC

508 8.22 2080 68.27 2136.35 64.69 38.56 20.56

50% BMW CDF: 50% GWC

399.44 7.72 1582 14.61 1583.06 77.82 178.12 18.78

50% Maize CDF: 50% GWC

349.68 7.66 1384 15.54 1660.19 44.75 86.12 19.04


540.32 8.18 2128 42.09 2240.2 88.27 420.52 17.42


431.96 7.96 1652 11.09 1771.57 64.64 152.16 17.84


481.6 7.78 1926 5.39 2072.6 104.42 96.24 16.72


529.56 8.06 2108 7.56 2382.01 95.85 163.78 14.8


464.8 7.98 1778 4.54 1938.64 76.32 120.46 14.7


457.64 8.02 1736 5.76 1908.03 60.12 124.08 14.76

The species used in this trial were representative of UK hardy ornamental nursery stock (HONS) and bedding plants, and had a range of nutrient and pH tolerances. A Heuchera hybrid, Euonymus fortunei, Dryopteris erythrosora (fern), Viburnum tinus purpureum and


Pelargonium F1 were grown in two litre containers of these different mixes and their growth compared with plants grown in industry-standard controls comprising peat-free and peat-reduced media. In general, the species grown in the 50:50 composted digestate fibre: GWC mix performed as well as, and sometimes better than, the mixes containing the lower proportions of composted digestate fibre. Four of the five species tested produced generally good quality plants across the range of growing media mixes. The exceptions were the ferns (Dryopteris), where plants failed to grow in the majority of the compost mixes except for the peat-based control, and Heuchera which performed poorly in the two mixes containing 30% and 50% composted potato digestate fibre. This reduction in growth in the Heuchera was attributed to the levels of pH above 8, EC over 2000 µS/cm and/or potassium above 2000 mg/l in the potato fibre mixes. Leaching prior to use as a growing media ingredient was proposed as a way to reduce the high EC levels of the composted digestate fibre. The potato waste digestate was also much denser and wetter than the other two digestates, and harder to handle and mix with the GWC. Figure 12. UK ornamental growth trials with composted digestate fibre as a growing media

ingredient (WRAP, 2015c)

The main conclusions of the project were:

Composted digestate fibre can, in many cases, be successfully used to grow a range of high value ornamental species;

Composted digestate fibre can be reliably incorporated as a growing media component up to at least 50% by volume;

The digestate fibre used in this study did not require pH adjustment, nutrient leaching or nutrient supplementation prior to use in order to produce commercially satisfactory levels of growth for the majority of the plant species tested; and

The physical structure of the digestate fibre compost blend with GWC appeared stable over a three month time line.

The same three CDF digestates were used for a subsequent germination study (WRAP, 2015c). The treatments were:

1 Control: Peat-reduced modular compost (as used in commercial plant raising);

2 85% 3 – 6mm GWC with 15% coir;

3 85% 6mm screened potato waste CDF + 15% coir (control);

4 85% 6mm screened BMW CDF + 15% coir;

5 85% 6mm screened maize CDF + 15% coir.


The germination time, rate and success, as well as the evenness of growth of Chinese cabbage, tomato and lettuce were assessed weekly over a 28 day period. For all three species, the seeds failed to germinate during the 28 days in the potato waste CDF treatment, which the authors attributed to the high EC values for this material. Moreover, germination of the Chinese cabbage seeds was significantly reduced by the GWC treatment, compared to the control, with the maize and BMW CDF treatments performing as well as the control. For lettuce, with the exception of the poor performance of the potato waste CDF treatment, the pattern of germination was similar for the other four treatments. However, germination of the tomato seedlings in the control treatment was significantly more rapid than all the other treatments. In conclusion, over the observed time period, the maize and BMW CDF mixes performed as well as the control in terms of germination of the lettuce and Chinese cabbage seeds, but significantly less well with the tomato seeds. 7.2.2 Research outside the UK: Using composted digestate fibre as a growing media

ingredient In a 2011 study, growing media for potting mixes were formulated from composted anaerobically digested cattle biosolids (0 - 60%), aged pine bark (30 - 90%) and 10% sand. The resultant mixes had suitable physical properties (air-filled porosity, container capacity, total porosity and bulk density). Where the digestate component was 30% or greater, high sodium, chloride, potassium and phosphorus were deemed likely to cause problems for plant growth (Ridout, 2011). Digestate fibre from cow manure was composted in a windrow for 100 days (Inbar, 1985). During composting the pH fell from 7.5 to 6.6 whilst the amount of soluble salts increased. This digestate compost was then compared to the raw digestate fibre, both being used as a growing media ingredient for tomato, cucumber and pepper. The raw digestate fibre was found to inhibit tomato growth (even after additional fertilisation), whereas the composted digestate fibre was as good or superior to peat. Finally, a 2010 study examined the response over nine weeks of rooted Chrysanthemum cuttings grown in a range of substrates local to Washington, USA (Krucker, 2010). The substrates were:

1 100% Groco (a biosolids digestate which was then composted with sawdust); 2 100% Tagro (a biosolids digestate cake mixed 2:1:1 (v/v) with sawdust and sand);

3 100% dairy manure compost;

4 100% digestate fibre from dairy manure; 5 50% Groco:50% douglas-fir bark (v/v);

6 50% Tagro:50% bark (v/v); 7 50% dairy compost:50% bark (v/v);

8 50% digestate fibre from dairy manure:50% bark (v/v); and 9 the control, a commercial peat–perlite mixture (70% to 80% sphagnum peatmoss, in addition

to perlite, dolomitic limestone and gypsum, no nutrients added).

The dairy manure digestate fibre had an initial EC average (8.3 dS/m) above the recommended range so it was leached with a volume of water equivalent to four times the volume of the substrate. After leaching, the EC, nitrate, and pH levels for the two digestate treatments (treatments 4 and 8 below) were in the same range as the other substrates. A micronutrient mix was added to all substrates. Four days after transplanting, all plants were fertilised with a solution containing 200mg/l N, 100 mg/l P and 200mg/l K. Thereafter N


fertiliser was applied as a solution at two N rates, being 200mg/l N every 2 days (high N rate) or every 4 days (low N rate). To ensure that all plants received the same amount of P and K, a solution containing 100 mg/l P and 200 mg/l K was applied to both the high N and low N treatments every 4 days. Two irrigation treatments were also tested (capillary mat or overhead sprinkler). The main results were: 1. Surface irrigation and high N rate treatments produced comparable shoot dry weight,

shoot growth index (SGI), quality, and flower bud counts in all treatments except for 100% Groco (substrate 1), which had similar SGI, but the other parameters were lower;

2. Surface-irrigated, low N rate treatments equalled or exceeded the control with respect to shoot dry weight, SGI, and flower buds. Quality was similar to or better than the controls in all but 50% dairy compost:50% bark (treatment 7);

3. Sub-irrigated, high N rate treatments resulted in similar growth, quality, and flower bud measurements to the control, except for a reduction in performance with 100% Groco (treatment 1);

4. Sub-irrigated, low N rate treatments produced dry weight, SGI, quality, and flower buds similar to or better than the control in treatments except for 100% Groco (treatment 1) and 50% dairy compost:50% bark (treatment 7).

The generally inferior performance in 100% Groco (treatment 1) was attributed to low water-holding capacity. In substrates with higher available N (Groco, Tagro, Tagro:bark, and digestate fibre from dairy manure), plant growth parameters generally did not respond to doubling the applied N; whereas in the other substrates, including the control, growth generally increased in response to additional N. The differences in leaf colour across treatments were not large, with more of a colour difference observed due to the two N treatments rather than due to the different substrates. Root growth of plants in the substrates was similar to the control in both irrigation systems. The authors concluded that biosolids and dairy manure products can be used as substrates under reduced fertilisation (low N rate), with both surface and sub-irrigation systems. Interestingly, the Tagro:bark mix produced for this study was adapted by the City of Tacoma, WA, and since 2004 has been sold in bulk and bags as Tagro Potting Soil. In 2009 the production used 25% of the total biosolids output from the city of Washington, and the product was awarded the U.S. Environmental Protection Agency‘s highest rating for use in landscaping, vegetable gardens and indoor container gardens (City of Tacoma, 2014). 7.3 Using digestate fibre co-composted with other materials as a growing media

ingredient Several papers described the potential of co-composting digestate with other materials (such as vine prunings, wheat straw or grape marc waste) in order to stabilise the digestate and thus, to improve its properties for use as a more suitable growing medium ingredient (Bustamante, 2012, Bustamante et al., 2013). A recent WRAP report suggests co-composting digestate fibre with green waste to improve physical, chemical and biological characteristics (WRAP, 2013a).


7.3.1 Digestate fibre co-composted with vine prunings For example, the solid fraction of digestate (from cattle manure with 4.3% cattle slurry and 11.6% maize-oat silage) was composted either alone or with 10% or 20% vine prunings (on a fresh weight basis) (Bustamante, 2012). In addition 0.2% sulphur was added to reduce pH, and 1% almond shell powder added to increase the C:N ratio. For this study and that discussed below (Restrepo, 2013), the composting method was trapezoidal static piles 1.5m high with a 2x3m base, with aeration from the base. After 95 days the aeration was stopped and the compost left to mature for a further month. Water was added as necessary. The ammonia concentration of the digestate composts was much lower than the fresh digestate fibre, with this being predominantly attributed to volatilisation and immobilisation. The digestate fibre composts showed greater in vitro suppression of Fusarium oxysporum f. sp. melonis, than the uncomposted digestate. The resultant compost was considered to have suitable physical properties for use as growing media, including bulk density, total pore space and shrinkage. For all three composts produced, the composting reduced pH and C:N but increased the EC (see Table 13). The authors imply that the composting of digestate fibre can result in the production of an added-value product due to this much greater degree of stability and maturity (compared with uncomposted digestate), in addition to enhanced physical properties (Bustamante, 2012). Table 13. Characteristics of raw digestate, compost feedstocks and final digestate composts (131 days old) (Bustamante, 2012)

pH EC

(dS/m)

OM

(g/kg) C:N

Raw digestate fibre at the start of the trial 8.8 4.49 797 19.20

Material composted (including 0.2% S and 1% almond shell)

100% digestate fibre Day 0 8.46 4.65 833 19.7

Mature 6.92 7.52 659 10.4

90% digestate fibre, Day 0 8.37 3.51 850 21.9

10% vine shoot prunings Mature 6.88 6.19 702 11.9

80% digestate fibre, Day 0 8.34 3.18 858 26.7

20% vine prunings Mature 7.02 5.07 706 13.4

7.3.2 Digestate fibre co-composted with wood chip and flax straw A number of trials were undertaken in an in-depth three year study using fresh and composted digestate fibre as a growing media ingredient. The original publication includes an in-depth investigation of the composting process in addition to the growth trials outlined below for composted digestate fibre, with preliminary trials on fresh digestate fibre described in Section 7.1.2 (Meinken et al., 2009). The digestate fibres obtained were from three sources with a range of feedstocks (digestate 1: 30% salad waste, 30% fresh maize, 20% cow manure, 20% maize starch, potato starch and fruit juice waste; digestate 2: 50% maize silage, 10% sugar beet tops, 30% cereal dust, 10% processing fat; digestate 3: 12% forage rye silage, 18% crushed cereal grain, 70% pig manure). Digestate fibre was mixed with wood chip or flax straw at 20% and 40% by volume and composted for six weeks. The compost was turned weekly for the first four weeks and then once in the sixth week. The stability of the compost was measured during the composting process and found to increase during the composting process, finally reaching a stability


level which in Germany is deemed suitable for growing media applications. Temperatures were generally higher in the compost piles during the first few weeks of composting and all were lower at the end. A cress bioassay test was undertaken each week on the composted digestate fibre (CDF) during the composting period. During the first few weeks the cress test results indicated poor cress growth, with good growth in the fifth and sixth weeks of composting. Ornamental plant trials Digestate fibre from three sources was composted as above with wood chip and flax straw for seven weeks (as above). The CDF (see table 14) was then mixed 16-40% with peat to achieve a salt concentration of 1g/l of fresh growing media. Chalk was then added to achieve pH 5.5, and ammonium nitrate was added to achieve the same N level as the control (peat-loam mixture with 1.5g of multi-nutrient fertiliser/l). Seeds of Scaevola, Sutera and Pelargonium were sown directly into the 15 CDF growing media mixes and a peat control, and grown for a number of weeks. For all three plants and all treatments, additional fertiliser was added (Meinken et al., 2009). Table 14. Composition of the CDF after seven weeks of composting, before additions, also showing

the % of CDF added to peat to create the final mixes used in the trial (Meinken et al., 2009). F = flax straw, W = wood chip. Chemical extraction methods in brackets.

CDF characteristics before mixing with peat

Additive

Salt N P2O6 K2O Na Cl Cu Zn %

Digestate before pH (H2O) (CAT) (CAL) (CAL) (H2O) (H2O) (agua (agua mixed

composting

g/l mg/l mg/l mg/l mg/l mg/l

regia) mg/kg

regia) mg/kg

with peat

1 0 6.8 5.3 542 8070 2320 347 410 43 196 19

1 20% F 7.1 3.73 228 7142 2157 291 355 40 170 27

1 40% F 7.1 2.92 139 5239 1754 221 267 37 162 34

1 20% W 7.2 3.7 213 6856 2018 292 370 40 186 27

1 40% W 7.1 2.75 65 6106 1643 228 274 34 165 36

2 0 6.9 3.94 254 1539 2819 312 598 39 142 27

2 20% F 7.4 1.97 50 647 2020 164 417 28 180 40

2 40% F 7.4 1.5 48 459 1610 117 291 25 101 40

2 20% W 7.4 1.8 54 643 1911 157 383 28 106 40

2 40% W 7.2 1.26 38 397 1339 109 274 21 103 40

3 0 7.1 6.16 608 5629 3165 654 1356 114 407 16

3 20% F 7.2 4.57 504 3971 2779 455 978 98 298 22

3 40% F 6.9 3.44 455 2554 1949 298 558 73 216 29

3 20% W 6.8 4.45 146 3198 2633 467 904 90 299 22

3 40% W 6.4 3.6 63 2495 1749 367 653 73 246 28

The Scaevola aemula ‗Saphira‗ (fan flower) trial lasted for 13 weeks. The CDF mix treatments all resulted in some leaf discolouration or necrosis, which in some cases spread over the whole leaf, thus reducing plant quality. However, the growth of plants in terms of fresh weight was comparable or greater in some of the CDF growing media mixes than the peat control. Correlation with phosphate content was proposed as the issue with CDF mixes where growth was reduced. The greatest reduction in plant quality correlated with CDF mixes with highest phosphate content (up to 8 mg P2O5/l). The lowest phosphate content CDF mixes resulted in the best plant quality. Scaevola is known to be sensitive to high levels of phosphate (Schmitz, 2009). The authors commented that high concentrations of


phosphate can result in the lock-up of iron by the formation of iron (lll) phosphate (Meinken et al., 2009). Also, this may prevent iron from being transported within the plant. The Sutera ‗Baja‗ growth trial lasted for 14 weeks. Only slight differences in fresh weight between treatments were observed, with all CDF mixes being comparable or greater than the control. However, chlorosis and in fewer cases necrosis appeared at the end of the trial in some treatments, which was attributed to iron deficiency. The Pelargonium zonale ‗Deep Red Robe‗ trial lasted 11 weeks. In this case, the treatments with higher levels of phosphate resulted in higher fresh weight than lower phosphate treatments. Compared to the peat control, some yields were higher and some lower with the CDF mixes. Plants all grew well, with good quality observed. The number of flowers was significantly less than the control in two of the 15 CDF mix treatments. The sunflower Helianthus annuus 'Estate' was grown as an example of a salt tolerant species used for cut flowers. The same CDF mixes as above were used but adjusted with chalk to be pH 6. Seeds were planted directly into the substrates with the trial lasting for 13 weeks. Half drain pipes 4.5m long (20x7cm) were used with seeds planted every 20cm (approx. 45 seeds per treatment). Drip fertigation was implemented from week 4 onwards. After seven weeks there were some significant differences in plant growth, although by the end of the trial (week 13) there were no significant differences in stem length, flower diameter and stem fresh weight in all treatments including the control. There was a large variation in results within each treatment at the end of the trial, which the authors attributed to uneven germination rates. Balcony plant trial The same CDF mixes were used as described for the trial above. The pH of the mixes was adjusted to 5.5-6 using chalk. The peat loam mix control included fertiliser (easily soluble) 1g/l growing media N:P:K 14:16:18 and 2g/l growing media granulated fertiliser N:P:K 20:10:15. The CDF were mixed with fertiliser (easily soluble) N:P:K 14:16:18, to achieve the same available N as the control (140 mg N/litre growing media). The 5 plant species used for the trial were:

Osteospermum ecklonis ‗Astra Violet‗ (Florensis) (common name: Cape Marguerite)

Nemesia fruticosa ‗Lilac‗(Florensis) (common name: Nemesia)

Sutera cultivars ‗Snowflake‗ (common name: Sutera)

Bidens ferulifolia ‗Golden Star‗(common name: Fern-leaved beggarticks)

Angelonia gardneri ‗Angleface Blue‗ (common name: Blue wings)

One of each plant type was transplanted into a 20 l rectangular pot (five plants per pot), with three replicates of each treatment. The trial lasted for 22 weeks, with liquid fertiliser applied weekly from week 6 onwards in all treatments (Meinken et al., 2009). There was no significant difference in growth per pot compared to the control, assessed by total fresh weight at the end of the trial. All plants grew well. For one species (Osteospermum), there was a large variability in the number of flowers, often being higher for the digestate fibre composts compared to the control. The authors suggested that the mixes trialled were suitable for growing ornamental flowering plants in balcony pots, provided they complied with the German standard for growing media (BGK, 2007).


7.3.3 Digestate fibre co-composted with sulphur and almond shell powder In a Spanish seedling production trial, digestate fibre from cattle manure, cattle slurry and maize-oat silage was composted with 0.2% sulphur and 1% almond shell powder for 125 days as described above, and then mixed with peat in a dilution series of 25, 50 and 75% (v/v); the control was 100% peat (Restrepo, 2013). Tomato (Lycopersicon esculentum), muskmelon (Cucumis melo), and pepper (Capsicum annuum) were grown in the mixes for 40 days. Typically, in a commercial system the plants would subsequently be transplanted into new containers or outside in the field. The plants were all fertigated twice a week with an industry standard solution containing (mM) 136 N (NO3

− and NH4+), 49.4 P2O5, 23.4 K2O, 22.8 CaO, 6.25 Fe, 0.24 Cu, 3.18 Mn,

0.54 Zn, and 0.16 Mo. All mixes containing the composted digestate fibre had higher EC values and pH than pure peat. Higher EC was deemed to be the main limiting factor for the use of composted digestate fibre as a growing media ingredient, with values ranging from 0.44 dS/m for 100% peat to 5.25 dS/m for the 75% digestate compost. The pH values (ranging from 5.97 for 100% peat to 6.37 for 75% digestate compost) were deemed to be within the acceptable range for seedling production. As the percentage volume of digestate compost was increased, so were the N, K, Mg and Zn contents in the shoots of the plant seedlings of the three species. Micronutrient contents were generally not affected, with the exception of an increase in Zn in 75% digestate compost. Muskmelon and pepper seed germination were comparable in all treatments. However, for tomato seed germination, only the 25% digestate compost:peat mix was comparable to the control, with higher concentrations of digestate compost reducing emergence. This was attributed to the EC levels. The 25% digestate compost:peat mix was deemed to be the most similar to the peat control in terms of shrinkage, readily available water and water buffering capacity. 8.0 Bioassays using digestates as a growing media ingredient Bioassays are generally used to test germination, seedling growth, weeds and phytotoxins (pesticide and herbicide residues) in amendments such as composts and digestates in a controlled environment. Several of the published examples of digestate use as a growing media ingredient for container grown plants are bioassays focussing on the germination and growth of cereals and cress. Those of most relevance to digestate use in protected horticulture are discussed below. A Spanish study evaluated the quality of BMW as a component for growing media using a bioassay with cress and barley (Moldes, 2006). For the first compost, the BMW was processed through an aerobic bioprocess for 15 days in composting tunnels, followed by 3 months of curing (termed aerobic BMW). For the second compost the BMW was processed through two steps: AD, followed by an aerobic curing step to stabilize the digested residue (length of composting time not specified) (termed anaerobic BMW). Peat and composted pine bark were used as the two controls, and then diluted by 25, 50 and 75% with the aerobic and anaerobically processed products. Both BMW products were found to have high EC. All treatments were fertilised with a nutrient solution to result in 220 mg N/l, 338 mg P/l and 194 mg K/l, plus micronutrients. Interestingly, when barley germination was assessed at 3, 7 and 10 days, both BMW products mixed with the composted bark had higher germination rates than the substrates which included peat. Moreover, the aerobic BMW performed better than the anaerobic product. When cress was grown in the same mixes,


plant growth was observed to be equal to or superior to the peat only treatment when fractions of 50% or less BMW (both anaerobic and aerobic) compost diluted with composted pine bark were used. Products derived from bark mixes of up to 50% BMW were recommended as a result. In Finland a study was undertaken to assess the effects of digestates (whole, solid and liquid fractions of three digestates: 1) 90% pig manure and 10% industrial byproducts, 2) 100% sewage sludge and 3) 20% biowaste, 70% sewage sludge and 10% grease trap sludge) using various bioassays (Maunuksela et al., 2012). Pig manure and inorganic fertiliser (NPK concentrations not specified) were used as controls. The digestates and controls were diluted with artificial soil, sphagnum peat, or a peat-based commercial growing medium. The mixing ratio in each case was determined by the ammonium nitrogen content of the samples, with a target value of 90 kg NH4-N/ha. The mixtures were supplemented with a commercial trace nutrient mixture, and their pH was adjusted with ground limestone to 5.5 – 6.0 for sphagnum peat, and 6.0 – 6.5 for artificial soil. Petri dish cress tests lasting 72 hrs and 18 d pot trials with Chinese cabbage and barley were undertaken. For the cress test, the digestates mixed with peat treatments performed better in terms of both germination and root growth than peat with NPK fertiliser. The digestate mixed with soil treatments inhibited cress growth slightly, compared to the soil with NPK control, except for the digestate 1 fibre (90% pig manure, 10% industrial byproducts), which stimulated cress growth. However, the cress grown in the digestate mixed with soil treatments performed as well as or better than the manure control. The fresh weight and root growth of the barley seedlings was, in the vast majority of digestate treatments, enhanced compared to the controls. However, for the Chinese cabbage, some growth inhibition and chlorophyll content reduction with the digestate treatments was observed compared to the controls, with the exception of the digestate 3 fibre treatment (20% biowaste, 70% sewage sludge and 10% grease trap sludge), which stimulated growth. The authors attributed this generally negative effect to the Chinese cabbage being more salt sensitive than barley and cress. 9.0 Commercial studies It is likely that further investigations into the use of digestate in growing media will have been undertaken privately by manufacturers or plant raisers, and not be freely available in the published literature. Several websites provide additional examples:

Marsh Sea farm in Lincolnshire - working with Bulrush horticulture to develop a range of

growing media including digestate: (http://www.greenergreens.co.uk/our-initiatives/)

Charles Gould (Michigan State University) describes using digestate fibre in high quality

growing media: (http://fyi.uwex.edu/midwestmanure/files/2009/10/Gould-PPT-Fiber.pdf)

http://www.greenergreens.co.uk/our-initiatives/

http://fyi.uwex.edu/midwestmanure/files/2009/10/Gould-PPT-Fiber.pdf


10.0 Conclusions The literature obtained during this review has demonstrated that there have been a wide range of predominantly academic trials focussing on the use of digestate to grow a range of crops. In general, plant growth has not been greatly affected by the use of digestate to replace inorganic fertilisers. Where high concentrations of digestate were used, increases in EC and/or ammonia levels have in some cases negatively impacted on plant response. However, when digestates complied with the range of standard fertiliser or growing media parameters (e.g. EC, ammonia), results tended to be comparable or even slightly better than using standards such as peat or peat free growing media. Thus it is important to ensure that the EC and N levels are within the standard industry ranges used for the specific application. Moreover, it is important to measure the inorganic nitrogen (NH4-N and NO3-N) and express it as a % of total N, to ensure that the grower has an accurate indicator of crop nitrogen availability. Where the whole or liquor digestate was used as a liquid feed, tailoring the digestate nutrient content (through dilution and nutrient addition) to meet the needs of the crop tended to result in yields comparable to the inorganic industry standard. Where digestate fibre or (co-)composted digestate fibre is used as a growing media ingredient, it is important to create a final growing media mix with a suitable EC and pH for the crop. Crop sensitivity to high concentrations of specific nutrients such as phosphate or sodium should be considered, and the growing media mix designed accordingly. A summary table highlighting the studies obtained relating to the use of various digestate fractions on different crop types is summarised in Table 15. As the trials reported generally focussed on a limited number of digestate treatments, further optimisation of the treatments may have given better results, and would be recommended, should digestate use be considered for commercial use in protected horticulture.


Table 15. Summary table of studies found on whole, liquid and fibre digestate according to end use

and crop type. *The production of ornamentals does not tend to take place in soil-less production systems

Digestate type

Main use Edible crops tested

Ornamental crops tested

Liquid and whole

Organic fertiliser: Soil-grown

Tomato, pepper, strawberry, cucumber

Silverleaf dogwood, common ninebark, spirea, green creeping bent grass, Kentucky bluegrass, perennial rye grass

Liquid and whole

Organic fertiliser: Soil-less production

Tomato, cucumber, lettuce

N/A*

Liquid and whole

Growing media ingredient

Tomato, pepper, barley, spinach, Brassica rapa

Cyclamen, fern, European pine

Fibre Growing media ingredient

Tomato, lettuce, cress, cucumber, rape, sunflower, Chinese cabbage

Petunia, cherry laurel, spirea, privet, rose, geranium, scarlet sage, poinsettia, cyclamen, perennial rye grass, barley

Fibre composted alone


Cress, barley Heuchera, euonymus, fern, pelargonium, chrysanthemum

Fibre composted with other ingredients


Tomato, pepper, musk melon

Chrysanthemum, sunflower, Scaevola, Sutera, Pelargonium, Osteospermum ecklonis, Nemesia fruticosa, Bidens ferulifolia, Angelonia gardneri


11.0 References AI, T., LIU, Q. Y., LI, J. Y., & CHEN, D. H. 2006. Effects of anaerobic fermentation residues

on growth characteristics and nutrition quality of lettuce [J]. Renewable Energy, 6, 16.

ALEXANDER, R. 2012. Farmers are finding economic benefit to generating their own cow bedding and quick release fertilizer. Biocycle, 53, 56.

ANDREAS, C. 2004a. Auch die Tomaten mit Biogasrestnährlösung in Dammkultur und im Bioanbau entwickeln sich prächtig. Landwirtschaftskammer NRW, GBZ Straelen / Köln-Auweiler.

ANDREAS, C. 2004b. Geselle Thomas K. bei kritischer Betrachtung des Paprikas mit Biogasrestnährlösung. Landwirtschaftskammer NRW, GBZ Straelen / Köln-Auweiler.

ANDREAS, C. 2007. Die Cocktailtomate ´Oakley´ reift schon, während die runden Biotomaten mit Biogasrestnährlösung noch wachsen müssen. Landwirtschaftskammer NRW, GBZ Straelen / Köln-Auweiler.

ANDREAS, C., REINTGES, T. 2003. Bewässerungsdüngung im Bodenanbau ist mit Biogasrestnährlösung sinnvoll und möglich. Landwirtschaftskammer Rheinland - Gartenbauzentrum Straelen, Versuche im deutschen Gartenbau

ANDREAS, C., REINTGES, T. 2005. Der Einsatz von Restnährlösung aus Biogasanlagen zur Düngung von Tomaten ist auch im Bioanbau interessant. Landwirtschaftskammer NRW, GBZ Straelen / Köln-Auweiler, Versuche im deutschen Gartenbau

ANDREAS, C., REINTGES, T. 2006. Restnährlösung aus Biogasanlagen ist zur Düngung von Tomaten im Bioanbau einsetzbar. Landwirtschaftskammer NRW, GBZ Straelen / Köln-Auweiler, Versuche im deutschen Gartenbau

ARVANITOYANNIS, I. S. V., T. H. 2008. Vegetable waste treatment: comparison and critical presentation of methodologies. Critical Reviews in Food Science and Nutrition, 48, 205-247.

BASSAN, A., SAMBO, P., ZANIN, G. & EVANS, M. R. 2012. Use of fresh rice hulls and anaerobic digestion residues as substrates alternative to peat. Acta Horticulturae, 927, 1003-1010.

BASSAN, A., SAMBO, P., ZANIN, G. & EVANS, M. R. 2014. Rice hull-based substrates amended with anaerobic digested residues for tomato transplant production. Acta Horticulturae, 1018, 573-581.

BENZENBERG, M., FRICKE, T. & WACHENDORF, M. 2011. Effects of biogas digestates on the above and below ground biomass of Lolium perenne L. Grassland farming and land management systems in mountainous regions. Proceedings of the 16th Symposium of the European Grassland Federation. Gumpenstein, Austria.

BGK 2007. Qualitätskriterien und Güterichtlinien Bundesgütegemeinschaft Kompost e.V. BRAZZALE, L. 2012. Impiego di substrati e vasi ecosostenibili per la coltivazione del ciclamino

(Cyclamen persicum Mill.) Tesi di Laurea, Università Degli Studi di Padova. BSI 2011. PAS 100:2011. Specification for composted materials. British Standards Institution

(BSI). Third Edition ed. London. BSI 2014. PAS 110:2014 Specification for whole digestate, separated liquor and separated

fibre derived from the anaerobic digestion of source-segregated biodegradable materials. The British Standards Institution.

BUSTAMANTE, M. A., ALBURQUERQUE, J.A., RESTREPO, A.P., DE LA FUENTE, C., PAREDES C., MORAL, R., BERNAL, M. P. 2012. Co-composting of the solid fraction of anaerobic digestates, to obtain added-value materials for use in agriculture. Biomass and Bioenergy, 45, 1-10.

BUSTAMANTE, M. A., RESTREPO, A. P., ALBURQUERQUE, J. A., PÉREZ-MURCIA, M. D., PAREDES, C., MORAL, R. & BERNAL, M. P. 2013. Recycling of anaerobic digestates by composting: effect of the bulking agent used. Journal of Cleaner Production, 47, 61-69.


CAVINATO, C. 2013. Digestate quality, standard and nutrient recovery potential. Summer School on Biogas Technology Renewable Energy Production and Environmental Benefit. Universita Ca' Foscari Venezia.

CHALLINOR, P. 2014. The use of cattle slurry digestate solids in mixtures with coir and pine bark as plant growth substrates in the intensive production of glasshouse tomato crops. Defra.

CHANG, P., ZHANG, Y., LI, Y. M., ZHANG, X. Q., & QU, Y. H. 2010. Influence of Biogas Residue of Artificial Substrates on Tomato Seedling [J]. Northern Horticulture, 15, 44.

CHEN, D. H., LIU, Q. Y., AI, T., & LI, J. Y. 2007. Effect of Anaerobic Fermentation Residues on Yield and Quality of Strawberry in Solar Greenhouse [J]. Northern Horticulture, 9, 33.

CHENG, J., SHEARIN, T.E., PEET, M.M., WILLITS, D.H. 2004. Utilization of treated swine wastewater for greenhouse tomato production. Water Sci Technol, 50, 77-82.

CHONG, C., PURVISA, P. LUMISA, G., HOLBEINB, B.E., VORONEYC R.P., ZHOUD H., LIUB H.-W., ALAMA M.Z. 2008. Using mushroom farm and anaerobic digestion wastewaters as supplemental fertilizer sources for growing container nursery stock in a closed system. Bioresource Technology, 89, 2050-2060.

CITY OF TACOMA. 2014. TAGRO [Online]. Available: http://www.cityoftacoma.org/government/city_departments/environmentalservices/tagro [Accessed 18/03/2014.

CRIPPA, L., ZACCHEO, P., ORFEO, D. 2011. Utilization of the solid fraction of digestate from anaerobic digestion as container media substrate. In: FARRÉ, M. (ed.) ISHS International Symposium on Growing Media, Composting and Substrate Analysis, Book of Abstracts.

DEFRA 2008. Glasshouse survey 2007, England. DEFRA AND DECC 2011. Anaerobic Digestion Strategy and Action Plan. DENGO, M. 2013. Prove di coltivazione del ciclamino (Cyclamen persicum Mill.) con substrati

e vasi ecosostenibili. Tesi di Laurea, Università Degli Studi di Padova. DONG-DONG, H. 2010. Effect of Application of Biogas Slurry on the Chinese Cabbage Yield

and Quality under Soil-less Cultivation. Journal of Anhui Agricultural Sciences, 4. DUAN, N., LIN, C., GAO, R. Y., WANG, Y., WANG, J. H. & HOU, J. 2011. Ecological and

economic analysis of planting greenhouse cucumbers with anaerobic fermentation residues. Procedia Environmental Sciences, 5, 71-76.

HAO, X. J., HONG, J. P., GAO, W. J. 2007. Effect of biogas slurry and biogas residues on quality of greenhouse mini cucumber [J]. Soil and Fertilizer Sciences in China, 5, 9.

HARALDSEN, T. K., ANDERSEN, U., KROGSTAD, T. & R., S. 2010. Liquid digestate from anaerobic treatment of source separated household waste as fertilizer for barley. Proceedings of the 7th International ORBIT 2010 Conference. Heraklion, Crete.

HOGG, D., J., B., SCHLEISS, K. & FAVOINO, E. 2007. Dealing with food waste in the UK. Banbury: WRAP.

INBAR, Y., CHEN, Y. AND HADAR, Y. 1985. The use of composted slurry produced by methanogenic fermentation of cow manure as a growth media. Acta Horticulturae, 172, 75-82.

JIANXIANG, W. A. N. G. 2007. Study on Anaerobic Processed Liquid Livestock Manure in Organic Media Culture Peppers. Journal of Anhui Agricultural Sciences, 35, 1730.

KEHRES, B. & THELEN-JÜNGLING, M. 2006. Methodenbuch zur Analyse organischer Düngemittel, Bodenverbesserungsmittel und Substrate. Köln: Bundesgütegemeinschaft Kompost e.V.

KICHADI, S. N. & SREENIVASA, M. N. 1998. Interaction effects of Glomus fasciculatum and Trichoderma harzianum on Sclerotium rolfsii in presence of biogas spent slurry in tomato. Karnataka Journal of Agricultural Sciences, 11.

KOUŘIMSKÁ, L., BABIČKA, L., VÁCLAVÍKOVÁ, K., D., M., Z., P. & M., K. 2009. The effect of fertilisation with fermented pig slurry on the quantitative and qualitative

http://www.cityoftacoma.org/government/city_departments/environmentalservices/tagro

http://www.cityoftacoma.org/government/city_departments/environmentalservices/tagro


parameters of tomatoes (Solanum lycopersicum). Soil and Water Resources, 4, 116-121.

KOUŘIMSKÁ, L., POUSTKOVÁ, I. & BABIČKA, L. 2012. The use of digestate as a replacement of mineral fertilizers for vegetables growing. Scientia Agriculturae Bohemica, 43, 121-126.

KRUCKER, M. H., R.L.; COGGER, C. 2010. Chrysanthemum production in composted and non composted organic waste substrates fertilized with nitrogen at two rates using surface and subirrigation. Hortscience, 45, 1695-1701.

LI, H. H., YE, X. J., & WANG, Z. Y. 2003. Effect of biogas slurry on nitrate and nutrient quality of Malabar spinach grown in medium culture. China Biogas, 21, 37-39.

LI, Y. & ZHANG, H. Y. 2012. Effects of Substrate with Biogas Residue on Photosynthetic and Physiological Characteristics of Eggplant Seedlings in Poor Light [J]. Northern Horticulture, 14, 3.

LI, Y. & ZHANG, Z. 2001. Effect of Biogas slurry on Tomato Quality. China Biogas, 1, 10. LIEDL, B. E., CUMMINS, M., YOUNG, A., WILLIAMS, M. L., CHATFIELD, J. M. 2004a. Liquid

effluent from poultry waste bioremediation as a potential nutrient source for hydroponic tomato production. Acta Horticulturae, 659, 647-652.

LIEDL, B. E., CUMMINS, M., YOUNG, A., WILLIAMS, M. L., CHATFIELD, J. M. 2004b. Hydroponic lettuce production using liquid effluent from poultry waste bioremediation as a nutrient source. Acta Horticulturae, 659, 721-728.

LIEDL, B. E., WILFONG, K., TAYLOR, C., MAZZAFERRO, K. 2006. Liquid effluent from poultry waste bioremediation as a nutrient source for hydroponic cucumber production. HortScience, poster abstract, 41, 997.

LOŠÁK, T., MUSILOVÁ, L., ZATLOUKALOVÁ, A., SZOSTKOVÁ, M., HLUŠEK, J., FRYČ, J., VÍTĚZ, T., HAITL, M., BENNEWITZ, E. & MARTENSSON, A. 2012. Digestate is equal or a better alternative to mineral fertilization of kohlrabi. Acta Univ. Agric. et Silvic. Mendel. Brun, 1, 91-96.

MACCONNELL, C., FREAR, C. & LIAO, W. 2010. Pretreatment of AD-Treated Fibrous Solids for Value-Added Container Media Market. CSANR Research Report.

MACCONNELL, C. B. & COLLINS, H. P. 2009. Utilization of re-processed anaerobically digested fibre from dairy manure as a container media substrate. Acta Horticulturae, 819, 279-286.

MAUNUKSELA, L., HERRANEN, M. & TORNIAINEN, M. 2012. Quality Assessment of Biogas Plant End Products by Plant Bioassays. International Journal of Environmental Science and Development, 3, 305-310.

MEINKEN, E., SCHUMACHER, H., J., SCHMITZ, H. J. & SPRAU, G. 2009. Umweltverträgliche Restnährstoffverwertung aus Biogasanlagen als Torfersatzstoffe im Gartenbau. Deutschen Bundesstiftung Umwelt.

MICHITSCH, R. C., CHONG, C., HOLBEIN, B. E., VORONEY, R. P. & LIU, H.-W. 2008. Fertigation of cool season turfgrass species with anaerobic digestate wastewater. Floriculture and Ornamental Biotechnology, 2, 32-38.

MOLDES, A., CENDN, Y., LPEZ, E., BARRAL, M. T. 2006. Biological Quality of Potting Media Based on MSW Composts: A Comparative Study. Compost Science & Utilization, 14, 296-302.

PRASAD, M., LEE, A. & GAFFNEY, M. T. 2013. A detailed chemical and nutrient characterisation of compost and anaerobic digestate fibre including a comparative release of Nitrogen and Phosphorus. Dublin: rx3.

RAKERS, L., SCHLÜTER, E., DIECKMANN, S., DINKLAGE, S. & DAUM, D. 2010. Eignung von getrockneten pflanzlichen Gärresten als Zuschlagstoff in gärtnerischen Kultursubstraten. XXXIX. Osnabrücker Kontaktstudientage, 32.

RESTREPO, A. P., E. MEDINA, A. PÉREZ-ESPINOSA, E. AGULLÓ, M.A. BUSTAMANTE, C. MININNI, M.P. BERNAL, R. MORAL. 2013. Substitution of Peat in Horticultural Seedlings: Suitability of Digestate-Derived Compost from Cattle Manure and Maize Silage Codigestion. Communications in Soil Science and Plant Analysis, 44, 668–677.


RIDOUT, M. E., TRIPEPI, R.R. 2011. Initial chemical and physical properties of potting mixes amended with anaerobically digested cattle biosolids. Acta Horticulturae 891.

SAMBO, P., FREZZA, A. & MORESCO, D. 2010. Digestato da borlanda di frutta: efficacia fertilizzante su lattuga. Informatore Agrario, 29, 41-43.

SCHMITZ, H. J. M. E. 2009. Composts from residues of anaerobically treated renewable resources and their suitability in growing media. Acta Horticulturae, 819.

TASSINATO, S. 2011. Prove di taleaggio di rosa (Rosa ×hybrida 'La Sevillana") e Geranio (Pelargonium peltatum 'Ville de Paris') su substrati contenenti lolla di riso e residui di digestato anaerobico Tesi di Laurea, Università degli Studi di Padova.

TONGGUO, G., NAN, C., WEIQUN, L., BAOZHEN, L., HONGLI, Y. 2011. Effect of highly efficient nutrient solution of biogas slurry on yield and quality of vegetables. Agricultural Science & Technology-Hunan, 12, 567-570.

TRINCHERA, A., RIVERA, C. M., MARCUCCI, A. & REA, E. 2013. Biomass Digestion to Produce Organic Fertilizers: A Case-Study on Digested Livestock Manure In: MATOVIC, D. M. D. (ed.) Biomass Now - Cultivation and Utilization.

VAN, D. T. C. 2012. Compost and residues from biogas plant as potting substrates for salt-tolerant and salt-sensitive plants. PhD, Rheinischen Friedrich-Wilhelms-Universität zu Bonn.

VANETTO, T. 2012. Prove di coltivazione di Poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) con substrati e vasi ecosostenibili. Tesi di laurea, Università degli Studi di Padova.

WARNARS, L. & OPPENOORTH, H. 2014. Bioslurry: a supreme fertiliser. Hivos WEI, W. A. N. G. 2009. Application of Biogas Liquid and Biogas Residue in Soilless Culture of

Peppers [J]. Journal of Anhui Agricultural Sciences, 24, 67. WENKE, L., LIANFENG, D., & QICHANG, Y. 2009. Biogas slurry added amino acids decreased

nitrate concentrations of lettuce in sand culture. Acta Agriculturae Scandinavica Section B–Soil and Plant Science, 59, 260-264.

WRAP 2011a. Guidelines for Specification of Quality Compost for Use in Growing Media. Banbury.

WRAP 2011b. New Markets for Digestate from Anaerobic Digestion. Banbury: Written by: Rigby, H and Smith, S. R.

WRAP 2013a. Market expectations and requirements for digestate. Banbury: Written by: King, C., Bardos, P., Nortcliff, S.

WRAP 2013b. A survey of the UK organics recycling industry in 2012. WRAP Project RAK-005-002. Written by Horne J, Scholes P, Areikin E, Brown B.

WRAP 2015a. Bark admixtures: Formulation and testing of novel organic growing media using quality digestates for the production of containerised plants. Banbury: Written by: Dimambro, M. E., Steiner, H. J., Sharp, R., Brown, S.

WRAP 2015b. Options for the use of quality digestate in horticulture and other new markets. Banbury: Written by: West, H. M., Ramsden, S. J., Othman, M.

WRAP 2015c. The potential of anaerobic digestate fibre for horticulture. Banbury: Written by: Stainton, D., Cheffins, N.

WRAP 2015d. Use of quality digestates as a liquid fertiliser in the commercial production of strawberries Banbury: Written by: Dimambro, M. E., Steiner, J., Lillywhite, R., Keeling, C.

WRAP & ENVIRONMENT AGENCY 2010. Quality Protocol. Anaerobic Digestate. End of waste criteria for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste. Banbury.

WRAP & ENVIRONMENT AGENCY 2012. Quality protocol. Compost. Banbury: WRAP. WREDE, A. 2012. Gärrest aus Biogasanlagen als Substratzuschlagstoff bei der Rosenkultur im

Container. Landwirtschaftskammer Schleswig-Holstein. XIE, J. H., CHEN, G., YUAN, Q. X., LIN, G. Y., WANG, Z. S., GUO, C. Y., ZHONG, H. 2010.

Effects of combined application of biogas residues and chemical fertilizers on


greenhouse tomato's growth and its fruit yield and quality. Yong sheng tai xue bao= The Journal of Applied Ecology, 21, 2353-2357.

YI, L. U. 2011. Research on effects of Biogas Fertilizer on Growth of Celery in Greenhouse [J]. Journal of Anhui Agricultural Sciences, 4, 95.

ZANTA, G. 2001. Utilizzazione di digestati anaerobici di matrici organiche come fertilizzante nel vivaismo. Tesi di Laurea, Università degli Studi di Padova.

ZERO WASTE SCOTLAND 2010. Digestate market development in Scotland. Zero Waste Scotland, Stirling: Written by: A. Mouat, A. Barclay, P. Mistry and J. Webb.

ZHANG, J., ZHANG, M., SHAN, S., WANG, M. & CHEN, B. 2010. Nutritional Quality and Yield of Pai-tsai (Brassica chinensis L.) as Affected by Biogas Slurry Application under Soilless Culture. Bulletin of Science and Technology, 3, 17.

ZHAO, L., AO, Y. H., LIU, R. H. 2010. Effects of Different Ratio of Biogas Residue Media on the Growth and Chlorophyll Fluorescence Characteristics of Strawberry [J]. Journal of Shenyang Agricultural University, 2, 14.

ZHAO, S. G., REN, G. X., YANG, G. H., FENG 2007. Effect of spraying biogas slurry on Capsicum. Acta Agriculturae Boreali-occidentalis Sinica, 16, 128-131.

www.wrap.org.uk/organics

Documents

Final report Literature review: Digestate use in … Literature Review.pdf · Final report Literature review: Digestate use ... cost or expense incurred or arising from reliance on