Composting and storage of organic household waste with different litter amendments. I: carbon turnover

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  • Composting and storage of organic household waste with dierentlitter amendments. I: carbon turnover

    Y. Eklind *, H. Kirchmann

    Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden

    Received 31 May 1999; received in revised form 8 December 1999; accepted 14 December 1999

    Abstract

    Composting of source-separated organic household wastes is becoming a more common practice in several countries. Carbon

    decomposition dynamics during composting are important for an overall understanding of the process. We investigated over 590

    days losses of organic C and decomposition of C constituents in artificial organic household waste mixed with six dierent litter

    amendments; straw, leaves, hardwood, softwood, paper and sphagnum peat. Litter addition was necessary to achieve an aerobic

    process. Samples were analysed for dry matter, ash, organic C, volatile fatty acids, and lignin, cellulose and hemicellulose fractions.

    Calculated by first-order kinetics, residual amounts of dry matter were 2263% and of organic C 1161%, and both amounts were

    highest in the peat mixture and lowest in the control without litter addition. Rate constants for dry matter and organic C de-

    composition were highest in the leaf mixture and lowest in the control. The initial lignin content in the mixtures was highly cor-

    related (R2 0:91) with the residual amount of organic C. A lag phase, of varying length, in lignin decomposition was present insome but not all cases. Cellulose decomposition was slower in leaf, hardwood and softwood mixtures than in paper and straw

    mixtures. The results showed that the characteristics of litter amendments greatly influence the composting process. 2000Elsevier Science Ltd. All rights reserved.

    Keywords: Carbon loss; Litter amendments; Cellulose; Hemicellulose; Lignin; Volatile fatty acids; Decomposition rate

    1. Introduction

    Separation of organic household wastes at source isbecoming a more common practice in several Europeancountries and composting is one biological option fortreating this waste before it is used in agriculture orhorticulture. However, composting of source-separatedorganic household wastes often presupposes that littermaterial is added to act as a bulking agent to improvestructure and enhance aeration, to adsorb excess liquids,and to provide microorganisms with an extra energysource to balance the normally high N content. Littermaterials added can vary considerably in both physicaland chemical properties, such as bulk density, waterholding capacity, composition of C constituents (as re-viewed by Jenkinson, 1981 and Lynch, 1987), andthereby may have dierent eects on composting. Therehave been earlier studies of the eects of dierent litteradditives on the regulation of bulk density in composts

    (Liao et al., 1995), on N losses (Brink, 1995) as well ascontent and form of N in the compost produced (Martinet al., 1993; Meyer and Sticher, 1983) and propertiesrelated to plant growth (Kobayashi et al., 1994).

    Carbon decomposition dynamics during compostingare important for an overall understanding of compo-sting, as C compounds provide the energy for the de-gradation process. Volatile fatty acids are a type of Ccompound that may be formed during the compostingprocess (Kirchmann and Widen, 1994), and since theyare phytotoxic can cause problems in cultivation(DeVleeschauwer et al., 1981). Changes in C compo-nents in relation to organic C have been used to definethe degree of decomposition or bio-maturity of com-posted city refuses (Inoko et al., 1979; Harada et al.,1981), grass compost (Mahmood et al., 1987) andfarmyard manure (Levi-Minzi et al., 1986).

    The aim of the present study was to investigate themass loss and C turnover during composting, matura-tion and storage of household waste mixtures with dif-ferent litter amendments. Results of the N turnover ofthe same experiment were described by Eklind andKirchmann (2000).

    Bioresource Technology 74 (2000) 115124

    * Corresponding author.

    0960-8524/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 0 4 - 3

  • 2. Methods

    2.1. Materials used for composting

    Based on a survey of the dierent types of materialssorted into the compostable fraction in households(Nilsson et al., 1993), a standardized, artificial, organichousehold waste was prepared consisting of potatoesand carrots, chopped to about 1 cm3, meat meal andbone meal (commercial products), mixed in proportionsof 65%, 15%, 13% and 7% of dry matter, respectively.The potatoes and carrots were ecologically grown to besure to exclude residues of pesticides, and used withoutshoots. The carrots were unwashed and therefore had ahigh ash content (Table 1).

    The litter additives used were straw from winter wheatof up to 10 cm length; autumn leaves, mostly consisting ofAcer platanoides and Aesculus hippocastanum, dried andcut with a lawnmover into 23 cm2 pieces; shavings fromBetula spp. (hardwood); shavings from Pinus silvestrisand Picea abies (softwood), up to 23 cm2 in size; driedand milled pulp of waste paper; and sphagnum peat; boththe latter in lumps of up to 2 3 6 cm (Table 1). Litterwas added so that the same ratio of litter C to household

    waste C in all litter-amended mixtures was achieved.Moreover, the intention was that the mixtures shouldhave initial C to N ratios within normally recommendedlimits, resulting in initial C to N ratios of 2234 and alitter C proportion of about 50% of the organic C. Arti-ficial, organic household waste without additives wasused for control. The initial water content was 5865% ofthe fresh weight in the litter-amended mixtures and 74%in the control. Dry bulk densities of the mixtures were0.090.19 kg dm3, with the lowest values in straw andthe highest in leaf mixtures, and 0.43 kg dm3 in thecontrols without litter additives. The dry bulk densitieswere determined by weighing dried material after uni-form compaction in a 1 l beaker.

    2.2. Experimental performance

    Composting was carried out in 125 l, insulated bins,rotatble around their horizontal axes, octagonal incross-section and with ventilation holes in the side walls.The bins were filled to the same volume (about 90% oftotal volume) and as the litter amendments had dierentbulk densities, dierent total initial weights were used(Table 2). They were then placed in a climatic chamber

    Table 1

    Properties of materials used in the experiment

    Material Dry matter (% of fw) Ash (% of dm) Carbon (% of dm) Nitrogen (% of dm) C to N ratio

    Raw materials

    Potatoes 19.6 5.1 41.3 1.37 30

    Carrots 10.8 43.2 24.8 0.82 30

    Meat meal 93.4 42.9 28.0 7.64 4

    Bone meal 95.2 46.0 26.3 7.37 4

    Artificial household

    wastea27.8 19.2 36.2 2.84 13

    Litter amendments

    Straw 95.6 8.5 43.2 0.47 92

    Leaves 91.7 30.8 36.5 1.13 32

    Hardwood 94.6 0.4 47.2 0.07 638

    Softwood 93.5 0.7 47.3 0.06 736

    Paper 95.8 6.9 43.8 0.10 438

    Peat 54.1 2.0 48.2 0.92 52

    a Consisted of the wastes mentioned (potatoes, carrots, meat meal and bone meal; 65%, 15%, 13% and 7% of dry matter, respectively).

    Table 2

    Proportions of artificial household waste and litter amendments used to achieve the same litter C addition, expressed in total amounts of dry matter

    per bin. Control was artificial organic household waste without litter addition

    Waste mixture Artificial household

    waste (kg DM)

    Amendment (kg DM)

    Straw Leaves Softwood Hardwood Paper Peat

    Household waste + Straw 2.86 2.60

    Household waste + Leaf 5.30 6.16

    Household waste + Softwood 6.22 4.98

    Household waste + Hardwood 6.18 4.82

    Household waste + Paper 4.18 4.06

    Household waste + Peat 4.00 3.66

    Control 18.58

    116 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115124

  • with an ambient temperature of 1723C. Daily openingof lids and manual rotation of the bins (4.5 turns) wasdone to favour aeration and homogenous conditions.Temperature was measured daily in the centre of themixture masses until it reached the ambient tempera-ture.

    Samples were taken from the decomposing materialsnine times during the experimental period (0, 7, 14, 21,35, 70, 106, 177 and 590 days). Samples were pooledfrom 10 subsamples of about 250 ml and kept at )24Cfor later analysis. Sampling and all following analyseswere done in duplicate. The excess of pooled materialnot needed for analysis was immediately returned to thebins. On each sampling occasion, the mixture masseswere weighed, and the water content was determinedand adjusted by water addition to about 50% of thewater holding capacity. Initial water holding capacitiesof the straw, leaf and control mixtures were 3.17 g H2Og1 dry matter; of hardwood mixture 3.78 g H2O g1; ofsoftwood mixture 3.31 g H2O g

    1; of paper mixture 4.56g H2O g

    1 and of peat mixture 7.13 g H2O g1 drymatter. After reaching ambient temperature, the com-post masses were kept in the bins to maintain controlledconditions also during maturation. On day 177, thecompost masses were removed from the bins, placedinto open plastic bags and stored indoors at about 17Cuntil day 590. The relatively long experimental periodwas choosen to check chemical changes in compostmaterial also during maturation and long-term storage.The materials were turned and water was added severaltimes during storage to compensate for evaporation.

    2.3. Chemical analyses

    Ashing was done at 550C. Organic C in mixtureswas measured by dry combustion and IR determinationof CO2 evolved (LECO analyser, USA). Cellulose andlignin fractions were determined using the acid deter-gent fibre (ADF) and permanganate method as de-scribed by Goering and Van Soest (1970). Neutraldetergent fibre (NDF) was also determined by Goeringand Van Soests method, but modified by using trieth-ylene glycol instead of ethylene glycol monoethylether.In addition, amylase was added to the ND solutionbefore boiling (Jeraci and Van Soest, 1990). The hemi-cellulose fraction was calculated by subtraction of NDFfrom ADF values. All these analyses were carried outon dried (60C) samples.

    Total N in mixtures was measured by the Kjeldahlmethod (Kjeltec, Tecator, Sweden). In samples con-taining significant amounts of nitrate and/or nitrite, to-tal N was determined by a modified Kjeldahl methodwhere the sample is pretreated with salicylic acid andthiosulphate (Bremner and Mulvaney, 1982). Presenceof volatile fatty acids (VFA) was used as an indicator ofanaerobic conditions in the waste mixtures. VFA con-

    centrations were determined using high pressure liquidchromatography (HPLC): 10 g frozen material wasthawed and extracted with 40 ml deionized water inbottles. Samples were shaken for 30 min at 6C andcentrifuged for 20 min at 3000 revolutions min1. Oneml of the solution was put in Eppendorf tubes, 50 ll 1MH2SO4 was added and the samples were centrifuged for20 min at 12 000 revolutions min1. Two hundred ll ofthe solution was used for the HPLC analysis. All theseanalyses were performed on wet, thawed material.

    2.4. Calculations and statistical analysis

    Data on C constituents were expressed on a C-basisassuming the following C content: lignin 63% C, cellu-lose 45.5% C and hemicellulose 58% C (Swift et al.,1979). Losses of dry matter and organic C, and the de-composition dynamics of lignin, cellulose and hemicel-lulose over time were fitted to the following exponentialdegradation function by the least-squares technique us-ing the SAS procedure NLIN (SAS Institute, 1985)

    M Mo 100M0 ekt;where M is the remaining mass (%), M0 the potentialresidual mass (%), k the decomposition rate (day1) andt is time (days).

    When a lag phase was present the function givenabove was fitted from the last measured point beforedecomposition started, although the period could onlybe approximated and could lie between two measuredpoints.

    The potential residual amount, M0, represents a re-calcitrant part of the organic matter or C constituentstudied, that does not degrade during the particular timeperiod. As a result, in composting and incubationstudies the degradation function will not reach zero as itdoes in long-term soil studies.

    The values of M0 and k, respectively, were comparedby t-test, using the standard errors of the parameterestimates received from the NLIN procedure. The timeneeded for 50% degradation of the organic C and Cconstituents, the so-called half-life, was calculated usingthe decomposition rate constants derived from thementioned function. Correlations between dierentquality and decomposition variables were tested with thesoftware JMP (SAS Institute, 1989).

    3. Results

    3.1. C to N ratios, and concentrations of C constituentsand volatile fatty acids

    The initial C to N ratio was 13 in the control and 2234 in the mixtures with litter additives. Only a smallchange in the C to N ratio, from 28 to about 20, was

    Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115124 117

  • found in the peat mixture during composting, matura-tion and storage whereas a considerable change wasapparent in the straw mixture, from 28 to 7 (Table 3).

    The control had low initial concentrations of ligninand cellulose (0.9% and 2.9% of ash-free dry matter,respectively), whereas hemicellulose concentration was14.5% (Table 4). In the mixtures with litter amend-ments initial lignin concentrations accounted from7.3% to 20.8% of ash-free dry matter, cellulose from11.3% to 33.8%, and hemicellulose from 12.9% to26.4%. Concentrations of lignin and hemicellulose in-creased during the experimental period but not theconcentration of cellulose. After 590 days of decom-position, concentrations of lignin varied between 8.3%and 36.0%, of cellulose between 3.8% and 35.5% and ofhemicellulose between 18.0% and 51.2% of ash-free drymatter.

    Concentrations of volatile fatty acids (VFA) were lowduring the whole experimental period in mixtures withlitter amendments. Only traces of acetate and lactatewere recorded during the first weeks. In the leaf mixturefor example, the highest total concentrations of VFAwas on a dry matter basis 1.5 mg VFA g1 compost onday 14. In the same mixture, the C present in VFAconstituted only 0.2% of the organic C present. In thecontrol, however, in addition to acetate and lactate,butyrate, propionate and also formate were found. Thehighest total concentration of VFA in the control was15 mg VFA g1 on a dry matter basis, and 2.2% on a Cbasis. However, no VFAs were recorded on day 70 orlater in the control.

    3.2. Dry mass loss and decomposition rates

    Dry mass losses during composting, maturation andstorage are shown in Fig. 1. The dry mass losses after590 days were largest in the control (about 80%), fol-lowed by the hardwood-, paper-, straw- and softwood-amended mixtures (about 70%), the leaf-amended mix-ture (about 50%) and the peat-amended mixture (about40%) (Table 3). The correlations between losses of drymass and organic C were very high (R2 0.981.0).

    Potential residual amounts (M0) of dry matter variedbetween 22% and 63%, of organic C between 11% and61%, and were highest in the peat-amended mixture andlowest i...

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