Anaerobic digestion process and bio-energy in meat industry: A reviewand a potential

Ihsan HamawandNational Centre for Engineering in Agriculture (NCEA), Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba 4350 QLD,Australia

a r t i c l e i n f o

Article history:Received 27 November 2013Received in revised form23 November 2014Accepted 14 December 2014Available online 1 January 2015

Keywords:WastewaterAnaerobic digestionBiogasCo-digestionMeat industry

a b s t r a c t

Greenhouse gases especially methane has been proven to have a significant effect on global warmingand climate changes. Large share of methane is emitted to the environment from wastewater treatmentplants mostly from uncovered anaerobic digesters. The estimated methane emission is approximately618 Mt carbon dioxide-equivalents (CO2-e) globally. Methane emissions from uncovered anaerobicdigesters can be avoided by carrying out some modification to the treatment process and design. Thesepotential modifications were illustrated in details in this paper. The aims are to gain better under-standing of anaerobic digestion process and its performance. This paper is discussing and analysing thedifficulties associated with anaerobic digestion process specifically in meat industry and many methodsto overcome these problems. There are many ways for enhancing the performance of anaerobic digestionprocess such as through simulation, co-digestion, addition of surfactants, pre-treatment and optimaldigester design. It is obvious that solving the problems associated with anaerobic process may raiseinvestors' interest in covered anaerobic digesters and as a consequence will remarkably reduce emissionof greenhouse gases. Anaerobic digester would not only function as a water treatment process but as aresource of renewable energy as well.

& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382. Difficulties associated with digestion process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393. The main causes of these difficulties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404. Research approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.1. Co-digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.1.1. Co-substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.1.2. Waste sludge and surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.2. Digester design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.2.1. Innovative design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.2.2. Influent flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.2.3. Volumetric organic loading rate (OLR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3. Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.3.1. Potential usage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.3.2. Available software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3.3. BioWin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.4. Pre-treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475. Anaerobic digestion process benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.1. Biogas utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.2. Digestate utilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2014.12.0091364-0321/& 2014 Elsevier Ltd. All rights reserved.

E-mail address: Ihsan.hamawand@usq.edu.au

Renewable and Sustainable Energy Reviews 44 (2015) 37–51

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

1. Introduction

Climate change attributed to the greenhouse gases (GHGs)emissions has been the focus of research in the last decade. Theseresearches were initiated by the Kyoto Protocol with the objectiveof reducing GHGs emissions by 2008–2012 [1]. It was estimatedthat global anthropogenic methane emissions for 2010 wereapproximately 6875 Mt carbon dioxide-equivalents (CO2-e). Ascan be seen from Fig. 1, approximately 50% of these emissionscome from agriculture, coal mines, landfills, oil and natural gassystems, and wastewater. Wastewater is contributing in the emis-sions of 618 Mt carbon dioxide-equivalents (CO2-e) globally [2].The global methane emissions are projected to increase by 15% to7904Mt carbon dioxide-equivalents (CO2-e) by 2020 (see fig. 1) [2].Australia for example, based on the report submitted by theAustralian government to the United Nations framework conversionon climate change in April 2012, wastewater contributed in 2.8 Mtcarbon dioxide-equivalents (CO2-e) (20.1%) of total waste emission inAustralia alone (14.1 Mt CO2-e). The emission is mostly methane-generated from anaerobic digestion process. Fig. 2 shows the amountof methane released to the environment which is approximately 77%of the total methane generated in 2010 [3]. It has been reported thatmethane has greater global worming potency. It is 20 times moreeffective than carbon dioxide (CO2) in trapping heat in the atmo-sphere [4]. Recovery is an efficient method of reducing globalmethane emissions. Methane can be used to enhance economic

growth, promote energy security, improve the environment, andreduce greenhouse gas emissions [4]. The recovering of biogas fromwastewater has been increasing worldwide, particularly in Europeover the last 10 years [5]. It has become a mainstream commercialfuel produced from passive methane capture at landfills, from urbanwastewater and effluent treatment plants and from energy conver-sion methanisation plants fed with slurry, crop residues, foodprocessing waste, and household green waste. Despite the wide-spread application of biogas technology worldwide, Australia has avery limited portfolio of biogas projects [6].

The biogas industry is now moving beyond its historic focus onwaste treatment and management into energy generation, includ-ing the use of purpose-grown energy crops in some countries.German firms have led manufacturing and project development,driven by strong domestic demand and a feed-in tariff (FIT) forbiogas. By the end of 2010, there were approximately 6800 biogasplants in Germany [5]. Fig. 3 provides the number of operatinganaerobic digesters in selected European countries in 2011 asprovided by the International Energy Agency (IEA) Bioenergygroup. Biogas becomes an important resource of energy competi-tive to the other gas resources such as natural gas and coal seam gasand can significantly affect the energy economy in the world [7].Table 1 is a summary of worldwide biogas plants based at wastewatertreatment facilities. It shows that biogas can be produced from bothagricultural/food and sewage wastewater treatment industries. Theenergy gain can be as much as 2800Mwe as in Germany.

However there are many successful applications of coveredanaerobic digesters, there are many difficulties that may arise withdifferent feedstocks [9]. This paper is addressing problems thatmay associate with anaerobic digestion process especially in meatindustry and the methods of overcoming such difficulties. Increas-ing the biogas production quantity and quality of anaerobicdigestion process may encourage variety of industries to investin the covered anaerobic digesters. Due to the extensive literaturereview carried in this area, it has been found that biogas can beenhanced dramatically through co-digestion, better design, pre-treatment and adding innovative biodegradable surfactant.Also, simulation may help to mitigate risks before the design putunder construction. Some hypothesis and an innovative anaerobic

Fig. 1. Estimated and projected global methane emissions by source [2], 2010and 2020.

Fig. 2. Amount of methane released to the environment compared to the recovered[3]. Fig. 3. Number of operating anaerobic digesters in selected European countries [8].

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5138

pond design have been discussed as a contribution to solve someexistence difficulties associated with this process.

2. Difficulties associated with digestion process

By investigating some of the anaerobic digesters in the meatindustry, it has been found that there is currently a lack ofknowledge within anaerobic process regarding the design, opera-tion and upgrading these to covered anaerobic processes. Also, therecoverable quantity and quality of such gas remains unclear.Consequently there is a need for research into these areas toencourage investors to invest in covered anaerobic digesters andto mitigate the technical risks of the technology. For example, theanaerobic ponds at Churchill Abattoir (Ipswich, QLD) were putunder study to gather information and understanding the aboveissues. The purpose of the project was to obtain more deepunderstanding of the process and the difficulties associated. Also

gauge the performance of the anaerobic digester in terms of bothwaste treatment efficiency and subsequent biogas production.Despite lots of efforts were spent to measure the biogas flow rate,biogas quantity was unable to be accurately determined. This isdue to many substantial technical problems such as crust forma-tion over the top of the ponds and lack in the design parameters[10]. The crust layer over the pond led to remarkable damage tothe pond's cover. This also illustrated the poor performance of thepond in regard to the wastewater treatment quality and quantity.This problem is not unique to Churchill Abattoir and is a systemicproblem in the red meat processing industry which hinders thesuccessful uptake of technologies such as covered anaerobicponds. Similar results of CA WWTP have been reported withSouthern Meat (SM) covered anaerobic pond [11] and OakeyAbattoir (Oakey City). In case of Churchill abattoir, comparingthe design parameters of the ponds such as pond's volume, flowrate and organic loading rate, they are far away from any idealpond’s design to a large extent [12]. The design's parameters of the

Table 1Approximate number of biogas plants worldwide.

Country Approximate number of biogas plants Biogas yield/power generation Ref

Agriculture/food processing industry Sewage treatment industry

Germany 2012 47000, among this 2000 co-digestion 2800 MWe ADenmark 2012 460 farm scale biogas plants,

22 centralised biogas plantsB

Italy 2010 313 121 209 MWe CSweden 2012 14 farm plants 18 co-digestion plants 135 sewage treatment plants 1387 GWh /year DFrance 2012 40 on farm, 7 centralized, 80 industrial 60 sewage sludge, MSW 10 By 2020, 270 million cubic metres of

biogas will producedE

Switzerland 2011 76 co digestion, 22 industrial WW 460 (60 co digestion) 4140 GWh/year FAustria 2012 360 102.59 MW GUSA 2012 1800, 160 operational Ads Swine, Dairy,

Beef, poultry104 60 MW H

China 2011 40 million rural household biogas projectsand 4000 new large-sized biogas plantsnation-wide

19 billion cubic meters I

Canada 2012 10 5 MW JUK 2011 78 on-farm 220 75 MW KNetherlands 2010 90 LPoland 2011 10 57 7.245 MW MFinland 2012 73 biogas production sites (including

landfills)Total biogas production 139 millionm3 (630 GWh)

N

Turkey 2011 37, 23 industrial and 13 landfill 1 201 million m3 annually 91 MWinstalled capacity annually

O

Ireland 2012 4 agri 15 industrial, sewage sludge,municipal (biowaste), 7 landfill

100 Mm3 30–40 MWe P

No. Reference

A http://www.zernikeaustralia.com.au/pdf/18000_ExchangeNL_LR.pdfEnergy Fields, 2012, 2nd International CLAAS Symposium Biogas16–17 January 2012

B http://energy4farms.eu/biogas-plants-in-europe/biogas-plants-in-denmark/C http://www.greengasgrids.eu/?q=node/230

http://act-clean.eu/downloads/D5.1_ITALY_National_Report.pdfD http://energikontorsydost.se/userfiles/file/BiogasSydost/BioMethaneRegions/BasicDataonBiogas2012-komprimerad.pdf

http://www.iea-biogas.net/_download/publications/country/reports/2012/Country%20Report%20Sweden_Tobias%20Persson_Moss_04-2012.pdfE http://www.xergi.com/en/contact/newsletters/news4/november-2.html

http://www.iea-biogas.net/_download/publications/country-reports/2012/Country%20Report%20France_Olivier%20Theobald_Moss_04-2012.pdfF http://www.iea-biogas.net/_download/publications/country-reports/2012/Country%20Report%20Switzerland_Nathalie%20Bachmann_Moss_04-2012.pdfG http://www.fedarene.org/documents/projects/Biomethane/D212_CountrySpecificConditions/BMR_D…2.1.2_Conditions_Summary_EN_LEV.pdfH http://ase.tufts.edu/uep/degrees/field_project_reports/2011/Team_6_Final_Report.pdf

http://www.renewableenergyworld.com/rea/news/article/2012/04/biogas-technology-cow-power-catching-on-in-ushttp://www.americanbiogascouncil.org/biogas_foodWaste.asp

I http://ase.tufts.edu/uep/degrees/field_project_reports/2011/Team_6_Final_Report.pdfJ http://www.iea-biogas.net/_download/newsletter/2th%202012%20Newsletter%20Task%2037.pdfK http://www.ukbiogas.enagri.info/UK_Biogas_2011_Sample%20Pages.pdf

http://www.nnfcc.co.uk/news/number-of-uk-biogas-plants-grows-by-a-third-in-one-yearL http://www.biogasin.org/files/pdf/Biogas_financing_in_Holland.pdfM http://www.balticbiogasbus.eu/web/Upload/doc/Riga_20120201/6_Potential%20of%20biomethan%20production%20in%20BSR%20M%20Krupinski.pdfN http://www.iea-biogas.net/_download/publications/country-reports/2012/Country%20Report%20Finland_Outi%20Pakerinen_Moss_04-2012.pdfO http://www.iea-biogas.net/_download/publications/country/reports/april2011/Turkey_Country_Report.pdfP http://www.iea-biogas.net/_download/publications/country-reports/april2011/Ireland_Country_Report.pdf

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 39

pond are conflicting with that recommended in the literature [13].There is an accumulation of very thick crust on its surface whichreached in some places 1 m. Despite the efforts for many monthsto register the biogas production rate, it was unable to record it,this is likely due to shortage in the production. Once a simpletheoretical calculation is carried out using the measured reductionin COD, the results illustrated a potential of huge production ofbiogas. The theoretical biogas can be approximately as much as6000 m3/day when considering ideal biogas production of 0.35 L/gCOD reduction [13].

3. The main causes of these difficulties

While FOGs have the potential to produce large quantities ofmethane, their recalcitrant nature generally results in a number ofproblems. Some of the problems attributed to the build-up of FOGsinclude: clogging of pipes; foul odour generation; adhesion to thebacterial cell surface and reducing their ability to treat waste-water; and flotation of sludge and loss of active sludge [14]. FOGstend to accumulate on the surface of ponds to form a recalcitrantscum layer or crust [11,15]. However, primary treatment systemssuch as dissolved air flotation units (DAF) are capable of reducingFOGs [16], this will also reduce the potential of large biogasproduction and create another type of waste.

The formed crust on the surface of anaerobic pond is partici-pating in reduction the volume of the pond and the HRT [17]. Thismeans reduction in the pond’s efficiency. This crust as observed isa mixture of fats and floated sludge which count in the CODmeasurement [10]. Lipids have a tendency to form floatingaggregates and foam that may cause stratification problems dueto the adsorption of lipids into the biomass [18]. Slaughterhousesare known for their high lipid (FOG) content [19]. Process stabilitycould be negatively affected with the higher FOG content due topotential LCFA inhibition led to digestion failure due to acidifica-tion of the digester [15]. Inhibition of anaerobic digestion ofslaughterhouse wastes is attributed to the accumulation of highlevels of ammonia. Ammonia is resulting from the degradation ofthe high protein content of these wastes and to long chain fattyacids (LCFA) accumulation as consequence of lipids degradation[18]. Fat, grease and oil (FOG) count for the highest amount of CODamong the food waste industries [20]. FOG is poorly biodegradabledue to their low bioavailability, Lipids and long-chain fatty acidsresulting from lipid hydrolysis cause inhabitation of methanoginicactivity [10]. Also, FOG has a tendency to form floating aggregatesand foam that may cause stratification problems due to theadsorption of lipids into the biomass [18]. Despite that, FOG countsfor the highest amount of chemical oxygen demand (COD) [20]and offers significantly greater biogas yields [21] among the foodwaste industries. In order to increase the biogas production, FOGas an added ingredient is highly recommended.

An urgent research is required in order to overcome thedifficulties associated with anaerobic digestion process to encou-rage industries investment. Covered anaerobic digestion processcan contributes in remarkable advantages to meat industry. It is acheapest process for wastewater treatment, a source of renewableenergy (biogas), a source of agricultural fertilizer (digestate) andcan contribute in remarkable reduction in methane and carbondioxide release to the environment. Furthermore, it requires lowcapital and operation costs.

4. Research approach

Anaerobic digestion process has proven to be an excellentelement for wastewater treatment specifically wastewater from

food/meat industry. In addition, it has been shown by manyresearchers that some modification in the feeding components(co-digestion), adding surfactant to the feeding stream, ponddesign, pre-treatment and simulation may help to overcome manydifficulties may associate with the process. The following aremethods recommended to enhance the digestion process andhigher biogas production.

4.1. Co-digestion

4.1.1. Co-substrateBiogas can be produced from all kinds of feedstock as long as

they contain substrates such as carbohydrates, proteins, fats andcellulose. The theoretical gas production rate and yield aredependent on the feedstock type. In practical, the retention timeand the design of the digestion system are also affecting the biogasproduction [22]. Co-digestion is the least expensive and easiestmethod of optimization C:N ratio of the feedstock. Wastes withlow C:N are accompanied with high release of ammonia as muchas 4289 mg/L. The highest biogas yield is associated with wasteshad low concentration of ammonia and alkalinity of 1736 and8970 mg/L respectively [23]. In one study by Shanmugam andHoran [23], they reported optimum biogas yield of 0.145 and0.15 Nm L CH4/g VS reduction with wastes of C:N values of 15 and20 respectively. The same results for the optimum value of C:Nwere reported by many other researchers. The optimum condi-tions were achieved at controlled pH of 6.5 and feedstock C:N ratioof 15. The study has showed that blended wastewater from leatherindustry with municipal solid waste helped in reducing ammoniaconcentration and maximizes biogas production. The cumulativebiogas yield increased from 560 mL using leather wastewaterfraction alone, to 6518 mL with optimum blend [23]. Co-digestion has been proven to be able to overcome the long fattyacid (LFA) inhabitation and biomass floating issues by manyresearchers [9]. Co-digestion is crucial to enhance biogas produc-tion [9]. Co-digestion is highly recommended to be applied inwastes with high fat, oil and grease (FOG) content such as waste-water from meat industry. This is due to low degradability of FOGand potential of long-fatty-acid (LFA) inhabitation [21].

Co-digestion using different substrates can help in minimizingthe effect of the inhibitory compounds on the anaerobic process. Italso contributes in improving the stability, the performance of theprocess and digestion of the poorly digestible wastes such as fat orprotein [24]. In another study by Bayr et al. [25], a batch and aCSTR reactor with semicontinuous process were used to study theco-digestion of rendering and slaughterhouse wastes. Theyshowed that co-digestion of these materials are possible at lowOLRs (organic loading rates) and at mesophilic conditions. Thestudy has showed a methane production potential of 262–572 dm3

CH4/kg VS added. The OLRs was between 1 and 1.5 kg VS/m3 dayand HRT of 50 days at a temperature of 35 1C. The study has cometo a conclusion that in the long term the stability of the processcannot be granted. Further research was suggested to study thestability of the process and the inhabitation mechanism ofammonia. Table 2 is a summary of many research have beenconducted in the area of co-digestion mostly of abattoir waste. Tosummarize co-digestion can offer several benefits such as opera-tional advantages, improve nutrient balance, co-substrate hand-ling and fluid dynamics. Also, it may enhance the processeconomics through higher biogas yields and additional incomefrom the digestate [9].

4.1.2. Waste sludge and surfactantsThe following section is a hypothesis based on the current

literature review. The most and repeatedly co-digestion

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5140

component has been used with FOG is waste activated sludge(WAS) from industrial wastewater treatment plants (WWTPs). In astudy by Wan et al.[15], a thickened waste activated sludge(TWAS) obtained from the final clarifier in a WWTP was used asa co-substrate with FOG. In this study it has been shown for acertain ratio of FOG to TWAS the digestion was enhanced andregistered 137% increase in methane yield. Similar results to Wanet al. [15] have been obtained by Martin-Gonzalez et al. [37] andSilvestre et al. [38]. Martin-Gonzalez et al. [37] used the organicfraction of municipal solid waste from sewage treatment plant asco-substrate with FOG. They showed an increase in methane yieldby 145%. Silvestre et al. [38] reported an increase of 138% in themethane yield when a combination of sewage sludge (SS), amixture of 70% primary sludge and 30% activated sludge, andgrease waste (GW) were mixed. TWAS from primary sludge (PS)also used as a co-substrate with FOG by Kabouris et al. [39]. Theymonitored 295% increase in methane yield. Shanmugan and Horan[26] used wastewater from leather industry with biodegradablefraction of municipal solids waste (MSW). They found thatammonia concentration was reduced and biogas production wasmaximized from 560 mL using leather wastewater fraction alone,to 6518 mL with optimum blend. Many other studies have mon-itored a great enhance in methane yield when waste activatedsludge (WAS) was used. Sewage sludge, a combination of 50%waste activated sludge and 50% primary sludge from WWTP, wasused as a co-substrate with grease (GS) from grease trap in theWWTP [40]. The same combination of sewage sludge and greasewas also used by Luostarinen et al. [41]. Moreover, Li et al. [42]

carried out anaerobic digestion experiments with a combination ofconcentrated waste activated sludge (WAS), fats, oils and grease(FOG) and synthetic kitchen waste (KW). The waste activatedsludge was collected from a WWTP. Zhu et al. [43] used acombination of grease trap waste (GTW) with municipal wastesludge from the dissolved air flotation unit which was fed withprimary and secondary sludge. All these studies have monitoredenhance in both bio-degradability of the FOG and methane yieldwhen mixed with WAS.

The main relevance between these studies is the using ofprimary sludge (PS) and/or waste activated sludge (WAS) fromwastewater treatment plants (WWTPs) as the main component inthe co-digestion. The anaerobic digestion of FOG alone or in acombination with co-substrate with more than 50% has resulted infailure [15]. Addition of FOG to the WAS in an anaerobic co-digestion process has shown an excellent potential of enhancingthe biodegradability and methane production and yield. Based onabove, it is hypothysed that the good performance of WAS as a co-substrate in digestion of FOG may also be related to the adsorbedsurfactants on the sludge cells. It is well known that surfactanttends to adsorb onto sludge in the wastewater treatment plant[44]. It has been reported that anionic surfactant such as LinearAlkylbenzene sulfonate (LAS) concentration in sludge is between100 and 30,000 mg/kg [44]. Laboratory screening or digester testshave shown that LAS, and many other sulfonate surfactant, doesnot degrade under strictly anaerobic conditions [44]. Alcoholethoxylates (AE) is another surfactant that detected in sewagesludge at concentration of 23–141 mg/kg [45]. The linear (C9–C18)

Table 2Co-digestion studies using abattoir waste and other substrates from the food processing industry.

Industry waste used in co-digestion Reactor configuration Biogas yield and other comments References

Wastewater from leather industry withmunicipal solids waste (MSW)

BMP, anaerobic batch reactors of 500 mLcapacity

Biogas production was maximized from 560 mL using leatherwaste fraction alone, to 6518 mL with optimum blend.

[23,26]

Fruit and vegetable waste (FVW) withabattoir wastewater (AW)

Laboratory-scale anaerobic sequencing batchreactors ASBRs of 2 L effective volume

Better biogas yield than those obtained from digestions ofAW and FVW separately

[27]

Pig slaughterhouse by-products mixedwith pig manure

Semi-continuously fed CSTRs each with a totalcapacity of 5 dm3 and a working volume of3.2 dm3

Biogas production increased from 3.3 dm3/day to5.5 dm3/ day, corresponding to an overall specific yield of489 dm3/kg VS

[28]

Fruit waste and abattoir effluent 4 L batch plastic containers The cumulative volume of biogas and methane produced forthe 49 days retention period increased with increasing theproportion of abattoir wastes

[29]

Meat industry waste sludge (WS), cowmanure (CM), ruminal waste (RW) andpig and cow waste slurries (PCS)

1 L batch reactors with an active volume of400 mL operated under mesophilic conditionsof 35 1C

For example, the co-digestion experiment carried out with25% of WS and 75% of CM produced 11.7 L CH4/kg VSd, whilethe mixture of 75% of WS and 25% of CM produced 29.2 LCH4/kg VSd, respectively

[24]

Mixture of solid and liquid (blood, washingwater, manure) and wastes of meatindustry

2 L glass flask, the reactor was immersed in athermostatic water bath at 38 1C fitted with amagnetic stirrer

N/A [30]

Solid slaughterhouse waste, fruit-vegetablewastes and manure.

Four�2 L laboratory scale reactors workingsemi-continuously at 35 1C

Methane yields of 0.3 m3/kg VS added, with methane contentin the biogas of 54–56%

[31]

Slaughterhouse and other organic wastes(food industry and residue from ethanolproduction)

Two continuously stirred tank reactors (CSTR)with a total volume of 7400 m3 and a hydraulicretention time (HRT) of 45–55 days

Yearly total production of 9.6 million Nm3 [32]

Cattle/pig meat and fatty waste added toslaughterhouse wastewater treatmentplant

Anaerobic batch tests of 1000 mL glass flaks(500 mL working volume)

273–301 L CH4/kg COD in. [33]

Slaughterhouse waste together with blood(SB), manure (M), various crops residues(VC) and municipal solid waste (MSW)

Each reactor with 2 L capacity contained400 mL of inoculum under thermophilicconditions 55 C

Substrate combinations of SB:M:VC:MSW with the mixingratios of 1:1:1:1 and 1:3:4:0.5 were shown to have bestperformance with methane yields of 664 and 582 NmL CH4/gVS substrate

[34]

Poultry slaughterhouse waste mixed withfruit and vegetable waste

2 L sample vessel Semi-continuously-feddigesters

Biogas production was not successful [18]

Slaughterhouse waste (SHW) and theorganic fraction of municipal solid waste(OFMSW)

Semi-continuously fed digesters at 34 1C The biogas yield of the co-digestion systems doubled that ofthe SHW digestion system alone, 8.6 L/day

[35]

Co-digestion of slaughterhouse waste (SB)with various crops (VC)

BMP test [thermophilic conditions] 539 L CH4/kg VS loaded [36]

SB with MSW (municipal sewage waste) 613 L CH4/kg VS loadedSB with M (manure) 576 L CH4/kg VS loaded

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 41

are well biodegradable in anaerobic screening test with biogasproduction of 470%. The biodegradability of AE may be the reasonfor its low concentration in waste sludge. The biodegradability ofalcohol ethoxylates in a single anaerobic digester was determinedin the range of 54–74% [46]. The biodegradability of alcoholethoxylate decrease with increasing branching degree of thealcohol [47]. It can be interpreted that the adsorption of thesetwo surfactants, linear alkylbenzene sulfonate and alcohol ethox-ylates may have a significant effect on the ability of WAS indigestate the FOG.

Table 3 shows many studies that have been carried out toaddress the co-digestion of wastewater from meat industry andsewage sludge. It is clearly demonstrate the advantage of sewagesludge (more likely the absorbed surfactant) in enhancing theefficiency of the digestion process and the biogas production.

Fat, oil and grease FOG is highly resistant to biodegradation andcontributes to the high COD levels. Anaerobic treatment alone isnot very efficient at eliminating FOG. Wahaab and El-Awady [50],showed that the levels of fat, oil and grease in meat processingwastewater did not comply with regulatory discharge standardsfor the industrial wastewater into the sewage network afteranaerobic wastewater treatment using anaerobic sludge. The useof a surfactant, more favourite bio-surfactant, may enable theenhancement of anaerobic biodegradability of meat processingwastewater by solubilizing the fat, oil and grease [20]. It has beenreported, that surfactant such as sodium lauryl sulfate (SLS) at alow concentration contributes in increasing of biogas production.This was attributed to an increase of the bioavailability andsubsequent biodegradation of organic pollutants associated withthe sludge, promoted by the surfactant adsorption at the solid/liquid interface [51]. Surfactants, either chemical or biochemicalare a chemical option which aims to improve the biodegradabilityof fats, oils and greases by dissolving fats in the wastewater.Biodegradable surfactants are more favourable than chemicalsurfactants which may cause toxication toward the microbialcolonies in the digester [20]. Nakhla et al. [20] tested the impact

of a biosurfactant (BOD-balance) on the treatment of FOG-richrendering wastewater. The reduction of FOG concentration too800 mg/L increased total and soluble COD degradation rates by106%. Results from the full-scale mesophilic anaerobic digestionsystem indicated that the addition of the biosurfactant at doses of130–200 mg/L decreased FOG concentrations from 66,300 to10,200 mg/L over a 2-month-period. Linear alkylbenzene sulfo-nates (LAS) are the most widely used synthetic anionic surfactantsin cleaning. In a study by Gavala and Ahring [52] showed that theinhibitory effect of LAS is the main reason that anaerobic microbialenrichments on LAS have not been succeeded yet. It has aninhibitory action on the acetogenic and methanogenic step ofthe anaerobic digestion process. They reported that the upperallowable LAS to a municipal wastewater treatment plant thatemploys anaerobic technology should be 14 mg LAS (gVSS)�1. Inanother study by Garcia et al. [53], they showed that addition ofLAS to the anaerobic digesters increased the biogas production atconcentrations of 5–10 g/kg dry sludge but at higher surfactantloads it caused inhibition of the methanogenic activity. Othersurfactants have been studied by Pérez-Armendáriz et al. [51],they investigated the anaerobic biodegradability and inhibitoryeffects on the methane production of three different surfactants,two anionic: sodium lauryl sulfate (SLS) and sodium dodecylben-zene sulfonate (SDBS), and a cationic surfactant: trialkyl-methylammonium chloride (TMAC), in two different anaerobicsludges, granular and flocculent. The surfactants were tested atfive different concentrations, 5, 50, 100, 250 and 500 mg/L. SLSwas biodegraded at concentrations of 5, 50 and 100 mg/L withflocculent sludge and at 100 and 250 mg/L with granular sludge.However an inhibitory effect on methane productionwas observedin both sludges at 500 mg/L. The results indicate that TMAC wasslightly degradable at 50 and 100 mg/L with the flocculent sludge,and from 100 to 500 mg/L with the granular sludge. The resultsalso showed that SDBS was not biodegradable under anoxicconditions. In regards to AE, it has been tested for biodegradabilityand toxicity. All alcohol ethoxylates derived from straight chain

Table 3Summary of biogas yield in literature.

Industry waste used in Co-Digestion Reactor configuration Biogas yield References

Co-digestion of organic fraction of municipal solid waste (OFMSW) with different kinds of pureorganic such as commercial vegetable (coconut) oil; animal fat; cellulose; and protein

BMP test [mesophilic conditions] 450 L CH4/kgVS loaded

[48]

Co-digestion of animal by-products ABP (digestive tract content and drum sieve waste in theslaughterhouse was mixed with DAF sludge and grease trap sludge) mixture and sewage sludge

Constantly mixed reactor (CSTR)[mesophilic conditions]

430 L CH4/kgVS loaded

[49]

Co-digestion of fat, oil and grease (FOG) with thickened waste activated sludge (TWAS) Semi-continuously fed reactor (CSTRs)[mesophilic conditions]

598 L/kg VSloaded

[15]

Co-digestion of primary sludge (PS), thickened waste activated sludge (TWAS), and polymer-dewatered FOG

Semi-continuously fed reactor (CSTRs)[mesophilic conditions]

449 L/kg VSloaded

[39]

Co-digestion of sewage sludge (SS) was combined of 50% waste activated sludge and 50% primarysludge from WWTP and sludge from grease traps (GS)

BMP test 928 L/kg VSloaded

[40]

Continuous pilot-scale digestion[mesophilic conditions]

360 L/kg VSloaded

Co-digestion of mixture of sewage sludge from WWTP and grease trap sludge from a meatprocessing plant

BMP test 788 L/kg VSloaded

[41]

Semi-continuously fed rector (CSTRs) 463 L/kg VSloaded

Mesophilic conditionsCo-digestion of concentrated waste activated sludge (WAS), fats, oils and grease (FOG) BMP test [mesophilic conditions] 418 L/kg VS

loaded[42]

Co-digestion of seven different types of rendering plant wastes and three different slaughterhouseby-products including biosludge (sludge from wastewater treatment). Digested sludge from amunicipal wastewater treatment plant was used as inoculum.

Continuously stirred tank reactors(CSTRs) [mesophilic conditions]

720 L CH4/kgVS loaded

[25]

Co-digestion of source collected organic fraction of municipal solid wastes (SC-OFMSW) withsewage treatment plants fat, oil, grease waste (STP-FOGW)

Constantly mixed reactor (CSTR)[mesophilic conditions]

550 L CH4/kgVS loaded

[37]

Grease trapped waste (GTW) sample was taken from local restaurants and food processingfacilities mixed with septage. This then mixed with municipal sewage sludge (MWS) sample wastaken from the stream of thickened sludge from a dissolved air flotation unit fed with primaryand secondary sludge.

BMP test [mesophilic conditions] 1061 L CH4/kgVS loaded

[43]

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5142

primary or secondary alcohol undergo rapid and ultimate biode-gradation in anaerobic digestion process. The toxicity of the AEOincreased with the length of the alkyl chain and decreasing EO-chain length that means with increasing hydrophobicity [47]. Tothe best of my knowledge, no study has been carried out to test itseffect on FOG degradation.

However, some physical pre-treatment such as the removal offat and grease using screens or dissolved air flotation (DAF) may bea solution to reduce the difficulties associated with crust forma-tion, it will contribute in reducing the influent organic matter tothe digester. This will not only reduce the biogas production butalso will generate another type waste which requires treatment[39]. Further research into this field of FOG management should beconducted to determine the impact on biogas production andquality. To conclude, amending the wastewater with a biodegrad-able surfactant may be a proper choice. This will contribute inincluding the FOGs in the digestion process and prevent generat-ing a new waste by-product. Further studies are required to testthe variety of surfactant in order to identify cheap, environmen-tally friendly and efficient one.

4.2. Digester design

Digester design is typically based on organic loading rates andhydraulic retention times from pilot plants and observations ofexisting pond systems [54]. Hydraulic retention time refers to theamount of time that a liquid with soluble compounds remains in apond or digester, and is highly variable depending on temperatureand wastewater composition [55]. Generally, the desired goal is toachieve significant reductions in wastewater organic load with theleast hydraulic retention time (HRT) possible [11]. In a study byMarcos et al. [56], they suggested an HRT of 18 days to achieve thehighest degradation. Generally the hydraulic retention timerecommended for anaerobic digester is 10–12 days which issubjected to the BOD loading. For different consideration relatedto the pond design and difficulties of controlling the compositioninput to the pond, it is highly recommended to increase the HRT to20 days [12]. There are many important factors that recommendthis value of HRT among them are eliminate short circuiting,allowance for reduction of pond volume due to sludge and crustbuild up and more tolerating shock load. There are many designrecommendations for anaerobic ponds have been declared byresearch institutes and industry. For a low rate anaerobic reactorsuch as ponds where temperature, mixing, SRT and other environ-mental conditions are not regulated. The optimum loading rateshould be of 0.05–0.08 kg BOD/m3-day (see Table 4) [57]. In astudy by CSIRO [12] suggest that the anaerobic pond should bedesigned to handle 200–300 kg BOD for each 1000 m3 of pondvolume [12]. In another study by Environmental Protection Agencyat United State the BOD loading rate suggested to be 0.04–0.3 kg BOD/m3 [54] as shown in Table 5. The conclusion fromthese studies, BOD loading should not exceed 0.3 kg BOD/m3 andthe HRT should not be less than 20 days.

Anaerobic digester is usually continuous flow stirred tankreactor (CFSTR) for which hydraulic retention time (HRT) andsolid retention time (SRT) is nearly the same. Anaerobic treatmentof wastewaters requires long SRT to achieve better treatmentefficiency. The ratio of SRT/HRT�10–100 is required in order toraise the efficiency of the process [12]. The high ratio allows theslow growing methanogens to remain in the reactor for longertime. To achieve a high SRT irrespective to HRT, one choice is toadd a clarifier (settling tank), this will allow the settled biomass tobe recycled back to the reactor. Another choice is to add bafflesand locate the inlets and outlets of the wastewater to avoid short-circuiting and maximize solid retention time (SRT). This will bevery difficult in practice without tracer study [58]. In general, for

square ponds, inlet location and baffles may be necessary toprevent significant short-circuiting. Outlet location is also impor-tant, sheltered positions are preferred and it is usually found in thecorners of square ponds [59].

4.2.1. Innovative designDue to the difficulties associated with anaerobic digestion

especially in meat industry due to high concentration of fats, oilsand greases (FOGs) in the wastewater [60], an innovative design isrequired. The formation of thick crusts on the top of anaerobicdigesters is the main problem. It causes reduction in the digesterefficiency due to excluding large amount of the organic matterfrom the digestion process [60]. Due to the hot environment underthe cover and the large surface area of the pond these floatedmatter build up and dry on the top of the pond. Also, it preventsthese organics matter from participating in the digestion process.The build-up of the thick crust will lead eventually to breaking thecover. The crust is not easy neither cheap to remove. The design ofa pond should consider easy ways of collecting crust and biogas.Fig. 4 is a suggested pond's design (pilot-scale) that has a reducedsurface area using a partially submerged cover. The cover can bemanufactured from composite water-resistance materials. Thisdesign can be applied to a small scale pond (3 m�6 m) besidethe actual pond and can be fed by a by-pass from the main feedingpipe. The surface area can be reduced approximately to0.5 m�3 m which makes collecting the crust on a regular baseeasier. This design will be very useful in generating data in regardthe biogas production quantity, quality and crust mass balance.This design will exclude the weather effects on the pond abso-lutely and keep large proportion of the floated organic matter incontact with water.

The design considerations/parameters of this new digester'sdesign are based on an easy and efficient ways of collecting thecrust. As shown in Fig. 4, the cover is submerged in the digesterand has inclined walls, this will help to direct the floatingmaterials including the crust to the middle of the digester. Thecrust will accumulate at a relatively small surface area compare tothe surface area of the digester, this will not only assist in easiercollection of the crust but also keep the crust submerged in thewater until it collected. Keeping the floated materials submerged

Table 4Recommended design parameters for meat waste anaerobicdigesters [57].

Recommended design parameters are

Loading rate 0.05–0.08 kg BOD/m3 dayHydraulic detention time 20–40 daysDepth 3–5 mLength to breadth ratio 3:1Freeboard 0.5 m minInternal slope 2 –3:1 depending on soil

Table 5Anaerobic digestion design criteria [54].

Criteria Range

Optimum water temperature (oC) 30–35PH 6.6–7.6Organic loading 0.04–0.3 kg/m3 dayDetention time 1–50 days (temperature dependent)Surface area 0.2–0.8 haDepth 2.4–6 mOrganic loading represents BOD5

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 43

in the wastewater can enhance the degradability of these materi-als. The crust can be collected either manually by opening the‘collection point’ cover, and removing the materials periodically.Or can be removed using pumps at the end of the ‘crust removal’pipe. As the cover will be submerged in the digester and cleanwater will be placed on top of it, this will eliminate the digester'scover damage due to weather conditions. Also, this design willeliminate the rain from diluting and decreasing the HRT of thedigester. The rain water will mix with the clean water at the top ofthe digester and the excess water will overflow from the digesterthrough the ‘rain water’ weir. This new design separates the cleanwater above the top of the cover from the wastewater below thecover. The generated biogas will also be directed to the middle ofthe digester to the ‘collection point’, where it will then be directedby a pipe to the storing/purification facility. A floating or anattached peer will be required if the crust will be collectedmanually or else can be used to access the ‘collection point’ coverfor monitoring purposes. The inlet wastewater flow is recom-mended to be introduced to the digester at the bottom from oneside with high flow rate (using small influent pipe) to generatemixing action inside the digester in order to prevent sludge fromaccumulation at the bottom of the digester. The sludge will start tomigrate to the other side of the buffer's wall when it reached acertain concentration. This excess sludge can be collected from theother side of the buffer's wall where the wastewater is stagnantthrough a pipe at its bottom and recycle part of it to the digester.Finally, the clean water above the cover can be used as a fish farmand it will give the digester a natural and an environmentallyfriendly appearance.

4.2.2. Influent flow rateIn order to insure that the HRT is enclosed inside the recom-

mended range, the HRT should be set as a constant value between10 and 20 days. Based on this range of hydraulic retention timeand the reactor volume, the flow rate to the reactor can bepredicted from Eq. (1) as following:

Q ¼ VHRT

: ð1Þ

Table 6 presents a range of flow rates for the recommendedrange of HRT between 10 and 20 days. For example, for a pond of20 m long, 10 m wide and 5 m depth, the volume is around1000 m3.The table shows that the volumetric flow rate to thereactor should be enclosed between 100 and 50 m3/day based onthe recommended HRT.

4.2.3. Volumetric organic loading rate (OLR)Due to the complexity of controlling the ratio between the

hydraulic retention time (HRT) and the solid retention time (SRT),the design parameter of any digester can be based on volumetricorganic loading rate (VOLR). The most appropriate way to find thevolumetric organic loading rate (VOLR) in a pilot scale anaerobicdigester system is to measure it experimentally against theremoval efficiency of chemical oxygen demand (COD) inside thedigester [12]. The best VOLR will be the one which achieves thehighest removal of COD from the digester. The volumetric organicloading rate can be defined by the following equation. Eq. (2) wasrearranged to solve for the COD concentration of the influent (So).

So ¼ V � VOLRQ

: ð2Þ

In this study a range of VOLR was selected based on literature[54,57], then used to predict the COD concentration accompaniedwith a specific VOLR, reactor volume and flow rate. The BOD loadingto an anaerobic digester should not exceed 0.3 kg BOD/m3 day whichhas been recommended by many studies [54,57]. As shown inTable 7, the VOLR is recommended to be in the range of 0.05–0.08 kg BOD/m3 day [57]. In another study [54], the typical accep-table loading rates have been recommended in the range between0.04 and 0.3 kg BOD/m3 day, as can be seen in Table 7. Furthermore,the value of the VOLR is dependent on the temperature inside thedigester, however there is no studies that addressing the VOLR andHRT against the digester's temperature. As can be seen in Table 7, theHRT can vary between 1 and 50 days as results of different aspectsamong them the digester's temperature.

A range of values of VOLR was assumed between the minimumand the maximum values recommended by literature. The mini-mum value considered was 0.04 kg BOD/m3 day and the max-imum was 0.3 kg BOD/m3 day in order to investigate a vast rangeof applications. At each assumed value of the VOLR and a range ofinfluent flow rates, the COD concentration in the digester was

Fig. 4. Suggested Pond Design (reduced surface area anaerobic pond RSAP).

Table 6Examples of biogas plants in worldwide.

Exp. no. Reactor volume (L) HRT (day) Flow rate(L/day)

Flow rate(m3/day)

1 1,000,000 10 100,000 1002 1,000,000 12 83,333.3 83.33 1,000,000 14 71,428.5 71.44 1,000,000 16 62,500 62.55 1,000,000 18 55,555.5 55.56 1,000,000 20 50,000 50

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5144

predicted based on Eq. (2). As shown in Table 8, the COD value canvary from 800 to 12,000 mg/L depending on the flow rates andthe VOLRs.

Fig. 5 presents a correlation between the input flow rate andthe COD concentration of the inlet wastewater. The figure distin-guishes a favourable area to operate anaerobic digester based onboth the acceptable range of inlet flow rate and the volumetricorganic loading rate. It is obvious, at low COD concentrationbetween 500 and 2000 mg/L, the relationship between the twoparameters is linear. As the inlet flow rate increases, the CODconcentration of the inlet wastewater is decreasing in a linearpattern. In case of high COD concentration of the inlet fluidbetween 6000 and 12,000 mg/L, the relationship is taking a curveshape. This means that at low VOLR, high inlet flow rate can beachieved without a large decline in the influent COD concentra-tion. In contrast, at high VOLR, high flow rate can be achieved atlow influent COD concentration.

By fitting the curve of the high VOLR in Fig. 5 to a polynomialequation, other lines representing different values of VOLR weredrawn using the fitted equation, as shown in Fig. 2. Each curve inFig. 6 represents a specific VOLR which is also can be presented asa range of influent COD concentration. For example, for a waste-water with COD concentration of 3600 mg/L, the second curvefrom the top (red colour) with a range of COD between 3000 and6000 mg/L should be used to predict the suitable flow rate. Thiscan be done by drawing a horizontal line from the y-axis at CODvalue of 3600 mg/L, then at the point of intersection of this linewith the curve, a vertical line should be drawn. The intersection ofthe vertical line with the x-axis represents the value of flow rateshould this reactor operate at. For the case of wastewater withCOD content of 3600 mg/L, the VOLR is 0.15 kg BOD/m3 day, thereactor should operate at a flow rate of 0.57 mL/min whichachieves an HRT of 12 days. Fig. 6 present an interaction relation-ship between the HRT and the VOLR, for a specific value of VOLR,the HRT is dependent on the COD concentration of the inletwastewater to the digester (So/VOLR¼HRT). To run a digester ata constant VOLR, a controller should be added to the system thatchanges the inlet flow rate to the digester as a response to changesin the COD concentration of the inlet flow.

To summarize Section 4.2, it is highly recommended to controlthe flow rate of the wastewater to the digester in order to keep thevolumetric organic loading rate (VOLR) below the maximum valueof 0.3 kg BOD/m3 day and the hydraulic retention time HRT in therange between 10 and 20 days. This in order to avoid manyproblems may results by not following these recommendationssuch as; short circuit due to high flow rate and short HRT, build ofcrust due to high organic loading rate and reduce in biogasgeneration due to wash out of the activated sludge at low HRT.Table 9 in Section 4.3.1 shows the potential of generating biogas inthe range of 660–1300 m3/day when these variables (HRT andVOLR) with others such as temperature are applied correctly.

4.3. Simulation

4.3.1. Potential usageWhile an anaerobic digester is a very useful element in

treatment of wastewater, anaerobic digestion is a very complicated

chemical and biological process. It may require two to four monthsto start-up an anaerobic digester and an extra two to four monthsto analyse the efficiency of the process [13]. In addition, largenumber of measurements over a long period of time is required.Anaerobic digester requires a balance between the design para-meters of the digester, chemical and physical properties of theinlet wastewater, conditions inside the digester and biologicalaspects of the activated sludge. These variables have to be in acorrect balance in order to accomplish optimum nutrient removaland useful biogas generation rate. Due to complexity of anaerobicdigester, it is very difficult in practice to put all these variables inbalance and/or to identify problems that may affects the process.The recoverable quantity and quality of such gas remains unclear.Consequently there is a need to research these areas to mitigate

Fig. 5. The recommended range of COD concentration vs influent flow rate for acontinuous digester, the doted area.

Fig. 6. Recommended COD concentration vs influent flow rate for a continuousdigester, VOLR unit is in kg BOD/m3 day.

Table 8Recommended design parameters for meat waste anaerobic digesters [12,25].

Exp.no.

Reactorvolume(m3)

Flow rate(m3/day)

VOLR(kg BOD/m3 day)

Minimum,So, mg/L

VOLR(kg BOD/m3 day)

Maximum,So (mg/L)

BOD CODa BOD CODa

1 1000 100 0.04 400 800 0.3 3000 60002 1000 83.3 0.04 480 960 0.3 3600 72003 1000 71.4 0.04 560 1120 0.3 4200 84004 1000 62.5 0.04 640 1280 0.3 4800 96005 1000 55.5 0.04 720 1440 0.3 5400 108006 1000 50 0.04 800 1600 0.3 6000 12000

a COD value assumed as double as BOD concentration.

Table 7Range of inlet flow rate to anaerobic reactor.

Parameters Study 1 Study 2

VOLR (kg BOD/m3 day) 0.05–0.08 0.04–0.3HRT (day) 20–40a 1–50a

a Temperature dependent.

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 45

the technical risks associated with this technology. Modeling andsimulation may help to reveal and interpret these problems and atthe same time identify solutions [61,62,63].

In previous works by the author [64,65], BioWin software wasused to simulate chemical oxygen demand (COD) removal ratesand subsequent biogas generation rate from two abattoirs wherecrust (high FOG) accumulation was an issue, Churchil and South-ern meat abattoirs. In this study, it was shown by using simulationthat a large percentage of influent COD was present as a surfacecrust (floated at the surface), so it remained largely un-biodegradable. Field data effluent COD removal rates werematched to simulated rates predicted by BioWin when measuredinfluent COD was reduced to 30%, which inform that a significantportion of the inlet COD is not taking part in the anaerobicdigestion process. BioWin was able to predict approximate biogasproduction rates of the pond, however it was impossible to do sopractically due to the high accumulation of crust. The simulationprovided a preliminary assessment of pond performance and alsosubsequent biogas production rates. Table 9 shows more examplesof using BioWin in analysing and predicting the potential biogasgenerated. In these examples the biogas generated were mea-sured, and as shown in the table, BioWin was able to predict veryclose values of the generated biogas to that measured. Also,BioWin showed that this quantity of biogas can be generated onlywhen 50% of the inlet COD is consumed. The other 50% as expectedis stored in the crust over the top of the digester similar to thecases of Churchill and Southern meat abattoirs.

BioWin software (EnviroSim Associates Ltd., Canada) is easy touse, although it requires the user to have an extensive knowledgeand experience in regards to wastewater treatment processes [67].BioWin is a windows based computer simulation model developedby EnviroSim Associated Ltd. It has been reported, BioWin has theability to design a simple and a complicated wastewater treatmentplant whereas anaerobic digestion system is the main element ofthe plant [68,69,70]. Prediction the behaviour of wastewatertreatment systems despite its complexity or number of unitprocesses included becomes possible with BioWin simulationsoftware. The dynamic behaviour of the wastewater system canbe predicted under variable operation conditions and a wide rangeconfiguration of the process [71].

It will be valuable to be able to simulate the performance of ananaerobic lagoon during the design stage before any constructionor modification begins. Simulating the process will be very usefulto reduce time and effort required in analysing and optimizing theefficiency of anaerobic lagoons. Simulation can be used as anindirect tool to enhance biogas generation and COD removal bytesting many parameters related to the digester such as design

parameters, loading rate, and wastewater characteristic in ahypothetical digester and then select the design parameters thatprovide optimum results.

4.3.2. Available softwareAs wastewater treatment models become available, it was

natural to package the models in software, the early simulationswere reported by Andrews and Graef (1971). Nowadays, there areseveral simulator packages available in the market of wastewatertreatment, such as Aquasim, BioWin, Simba, STOAT and WEST.General purpose platforms like Matlab/Simulink are frequentlyused for simulation of wastewater treatment system control [72].

The opinion of many experts in the field of simulation ofwastewater treatment processes was considered in order tocompare the software available in the market such as BioWin,GPS-X and WEST. These experts are representing big wastewatertreatment companies and have applied many software in theirwastewater plant design. It has been reported that BioWincompare to GPS-X is a much more powerful simulation tool [13].BioWin can be used for complex and simple wastewater treatmentplants' analysis, in contrary, GPS-X can only be used for simplewastewater treatment plants [73]. BioWin software includes thekinetic model ASDM which is an excellent tool for modellingnitrogen and phosphorus conversions in anaerobic digester. Bio-Win, GPS-X and West are good simulation software, their usage ismore dependent on the application. BioWin is very easy to use andhas decent modelling features but it may be little slow and lackscustomization. WEST in the other hand is completely customizableand its speed is impressive [74] but it lacks steady state solver [75].BioWin may be slow when simulating dynamic process, this is dueto the large numbers of process rates and state variables thissoftware is dealing with. GPS-X has a fast dynamic simulator, buthas less number of state variables and process rates variables [76].Each simulator such as BioWin, GPS-X and West has theirstrengths and weaknesses depend on the application required tobe performed. These models' features such as speed, ability ofcustomizing elements, data processing, data display, controloptions, and built-in features are differs which makes each one apowerful tools in different applications [77].

4.3.3. BioWinSimulation of anaerobic digestion process can be carried out

using software such as BioWin ASDM model used by BioWin isrecognized by International Water Association (IWA) and it iscount for the most required parameters in a digestion process [78].Many literatures have addressed BioWin as excellent tool for

Table 9Real and simulated biogas production for real farm from literature [66].

Parameter Units Barhamfarm

BioWin simulation,50% CODcontribution

Corroll'sfarm

BioWin simulation,50% CODcontribution

Vestel farm BioWin simulation,50% CODcontribution

Flow rate m3/day 139 139 1182 1182 114 114Digester volume m3 24479 24479 26487 26487 2854 2854Depth m 6.1 6.1 7.3 7.3 4 4HRT days 176 176 22.4 22.4 25 (design) 25Loading rate kg BOD/m3/

day0.088 – 0.186 – 1.29 –

CODn mg/L 31500 31500 8500 8500 66000 66000BODn mg/L 15474 15474 4175 4175 32421 32421Type of digester Tem. range Ambient

tem.5-32 Ambient tem. 5-32 Mesophilic (35

oC72)35

Optimum Biogasproduction

m3/day 940 925 793–850 993 663–1330 1454

Methane content in biogas % 63.774.7 63 68–80 80 Not reported 62

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5146

design and analysis of wastewater treatment plants (WWTP). Therecent setting of the default parameters in BioWin was studied byDe Hass and Wentzel [79], they showed that the recent defaultparameters are more realistic compared to the old versions. In onestudy by Elbeshbishy et al. [80], they have achieved a very goodfitting of the experimental data with that predicted from BioWinwhile using the default software kinetic coefficients and stoichio-metric parameters. Also, the calibrated BioWin software was ableto predict properly most of the influent and effluent fractions suchas COD, BOD, TSS, and TKA. BioWin has been used to simulationlarge systems of wastewater which are combined of many ele-ments including anaerobic digester [81,65]. Furthermore, BioWinwas able to predict the biodegradability of organic compounds inthe same order of the experimental finding [82]. In another studyby Dhar et al. [83], however, all the kinetic and stoichiometricparameters were kept at default values accept one the hydrolysisrate. The methane production rate and VSS removal simulated byBioWin were in good agreement with the measured data. More-over, it has been reported that BioWin is able to simulate otherkinds of bioreactors successfully. In a one study by Eldyasti at el.[84], they studied treating of landfill leachate in a pilot scalecirculating fluidized bed reactor. They illustrate that BioWinprediction of many major wastewater effluent parameters suchas TKN, NH4-N, NO3-N, TP, PO4-P, TSS, and VSS with an averagepercentage error (APE) of 0–20%. The study show the betteraccuracy of BioWin compare to other software. BioWin wascalibrated by adjusting the wastewater fractions using measureddata experimentally [84]. In another study by Hafez et al. [85],they showed that BioWin has skills in predicting biomass con-centration in CSTR bioreactor with average percentage erroraround 5%. Also, it successfully showed ability in predicting manyother parameters among them hydrogen production rates andhydrogen yield compared to measured data with a low absoluteerror of 4%. This has been done by calibrating the wastewaterfractions included in BioWin and decoupling the SRT from theHRT. The model's process stoichiometry was first calibrated usingexperimental data. Trial and error method was used to achieve thebest fit of the experimental data with that predicted by BioWin[85]. A study by Blair et al. [86] used BioWin because it providesthe best estimate of biodegradation when compared with mea-sured first-order rate constants.

It is obvious from the literature that BioWin can reliably beused as a design and an analysis tool for wastewater treatmentplants especially with a suitable calibration [78]. However, to myknowledge, BioWin has not been used to simulate industrialponds/lagoons (only by previous study of the author) and/or at alab-scale level.

4.4. Pre-treatments

Due to the high concentration of fat and grease in abattoirwastewater, pre-treatment is required to reduce these insolublematters and/or increase its solubility. Physical mass transferfrom the solid (fat) to the liquid phase is limited due to itshydrophobic characteristic [87]. And when degrade it presentssome long chain fatty acids to the solution which may inhibitmethanogenic organisms [88]. Pre-treatment such as theremoval of fat and grease using screens, settling tanks ordissolved air flotation are important to eliminate future pro-blems with crust formation and reduce maintenance costs. Thecharacteristics of abattoir wastewater before settling and after24 h settling time was studied by Amuda and Alada [89], theresults show highest removal was achieved for the total sus-pended solid (TSS) of around 65%. In a study by Massé andMasse [90], they showed the efficiency of DAF units for manyslaughterhouses. The reduction of TCOD and SCOD were

approximately 22–35 and 0–16, respectively. In a combinationprocess of screening, settling and dissolved air flotation (DAF),these processes can result in 75–80% BOD5 (SS and FOG)removal from slaughterhouse wastewater. And had the addi-tional advantage of removing large quantities of nitrogen andphosphorus [91]. Another physical method is saponification orexposure to low frequency ultrasound which may assist insolubilising these recalcitrant organics [92]. Anaerobic bafflereactors improve the efficiency of typical lagoon systems byimproving sludge distribution, mixing and the increases thesolid retention time [93]. Adding fibrous physical carriers havealso been studied, microbial biofilms attached to the carrierswere found to be more tolerant and less prone to being washedout during shock loading [94].

Chemical pre-treatment of abattoir wastewater is anothermethod with better capability. Al-Mutairi et al. [95] investigatedthe use of the coagulation/flocculation process to remove organicmatter from slaughterhouse wastewater by adding aluminiumsalts and polymer compounds. The maximum chemical oxygendemand (COD) removal efficiency was reported to be in the rangeof 45–75%. In a study by Massé and Masse [90], they showed that achemical-DAF unit can reduce TCOD and SCOD by 58% and 26%,respectively. The chemical used in this process was ferric chloridecoagulants. In another study by Masse et al.[96], sodium hydroxideand three commercial lipases of plant, microbial and animalorigins were tested. In regards to NaOH addition, the study doesnot recommend NaOH hydrolysis pretreatment for fat particlesdue to the high doses of NaOH required and the resulting increasein pH, alkaline. Recently, enzymatic products are becoming moreavailable commercially. The first enzyme was a pork pancreaticlipase called pancreatic lipase 250 (PL-250, Genencor Interna-tional, Rochester, NY). Pancreatic lipase 250 is claimed to beefficient for hydrolysing triglycerides containing LCFAs with morethan 12 carbons, such as those in animal fat. The second enzymewas a bacterial lipase extracted from Rhizomucor miehei calledlipase G-1000 (LG-1000, Genencor International, Rochester, NY).Lipase G-1000 is reported to hydrolyse natural fats, such as oils,beef tallow, butter fats and lard oil, with a preference for shorterchain fatty acids (o12 carbons). The third enzyme was a plantlipase called EcoSystem Plus (ESP, Neozyme International, New-port Beach, CA). Neozyme claims that ESP effectively breaks downfat particles in aerobic or anaerobic environments. It was con-cluded that PL-250 was the best pretreatment to hydrolyse fatparticles. Also, the tests have shown that pancreatic lipaseappeared more efficient with beef fat than pork fat, possiblybecause beef fat contains less polyunsaturated fatty acids thanpork fat. In regards to the efficiency of these enzymes, for example,in samples receiving 500 and 3500 mg/L of LG-1000, the SCODincreased by 6% and 27%, respectively.

In a study by Jensen, et al. [97], they suggest that conventionaltreatment processes such as anaerobic lagoons are not an opti-mized treatment strategy. This is because of different anaerobicbiodegradability and degradation rates between streams within aslaughterhouse. Therefore separate and specialized treatment ofred waste (rendering and slaughter floor) and green waste(paunch and offal waste) is recommended.

Luste and Luostarinen [98] studied hygienization (70 1C,60 min) of anaerobic co-digestion of a mixture of wastes frommeat-processing industry and of sewage sludge. They showed thathygienization has improved the efficiency of the digestion process,as an indication methane production raised to a level above thehighest OLR applied.

Reverse osmosis (RO) is another way of pre-treatment forconcentration of meat industry wastewater prior to treatment byanaerobic digestion (AD). In a study by Beszédes et al. [99], ADexperiments were conducted on the RO concentrate and combined

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 47

with appropriate pre-treatment methods. To find the best pre-treatment method for highest biogas production, the effect ofgrease mixing, alkaline and acidic condition combining thermalpre-treatment were evaluated. The AD tests showed good decom-position ability for the RO concentrate, and the highest biogasproduction was achieved by the combination of alkaline conditionwith heating at 70 1C.

It is obvious that pre-treatment of meat industry wastewater isrequired in order to reduce/eliminate issues such as crust forma-tion and inhabitation due to its high content of fat, grease and oil(FOG). Removing the FOG from the wastewater and recycle it tothe rendering room may be a good solution to this issue. But,enhancing the availability (solubility) of these materials for diges-tion by any cost-effective method is preferable due to their highbiogas content.

5. Anaerobic digestion process benefits

Biogas produced from waste materials can play significant rolein the future of energy demands globally. It comes with manyadvantages compared to other sources of bioenergy. Anaerobicdigestion has been considered as the most energy efficienttechnology of bioenergy production, the added benefit is thereduction in GHG emission. Furthermore, the digestate resultedfrom the process can contribute in reducing or substitutingmineral fertilizer uses in agriculture industry [100]. Anaerobicdigester can generate revenue from sale of the process's productsor offer savings by using the products on-site. Biogas can be soldeither as Methane or as electricity while the liquor and the fibrecan be sold as rich organic fertilizers [101].

5.1. Biogas utilization

Anaerobic process produces a mixture of gases such asmethane, carbon dioxide and contains smaller amounts of hydro-gen sulfide and ammonia. Biogas produced saturated with watervapour and trace amount of other gases [100]. The mixture ofbiogas can transformed into other kinds of energy such as thermal,electrical or mechanical. The calorific value of the biogas mixtureis about 6 kWh/m3, this corresponds to about half a litter of dieseloil [102]. As an example of utilization of biogas is the WWTP atFrankenförde, Germany. There are two anaerobic reactors in theplant with capacity of 450 m3 for each reactor. The reactors

receiving industrial waste from slaughterhouses and from proces-sing edible oil, these two wastes were added to pig manure. Thebiogas produced from these two reactors is purified by passing thegas through two columns of bog iron ore (gasreinigung) whichthen stored in a 75 m3, 20 mbar membrane storage tanks. Thepurified biogas is 70% methane content which contributes inproduction of 6.96 kWh/m3 in a gas engine/generator set com-bined heat and power plant. Around 30% (2.1 kWh) of this energyis transformed to electricity and the rest (4.2 kWh) in to heat [103].Other examples from Europe, the biogas produced from anaerobicponds/lagoons/reactors from different WWTPs facilities in Europe areillustrated in Table 9. The main uses of the biogas produced are asource of energy for CHP-plant, Gas fired boiler, and/or CHP-plant/gasboiler [104].

5.2. Digestate utilization

The digestate that resulted from anaerobic process has thepotential to be used as bio-fertiliser. The quality of the digestateis essential in order to be accepted to replace mineral fertilisers incrop production. The features of high quality digestate arenutrient content, PH, free of inorganic impurities, sanitized andsafe in regard to pathological and chemical content. The digestionprocess cannot degrade all the contaminant compounds in thefeedstock to the digester. This requires excluding any feedstockthat may have potential to contaminate the digestate [105]. Thedigestate can also be sold as a dried fertilizer which reduces thepotential of any pathogenic problems. Also dying will participatein reducing the weight of the fertilizer and increase its shelf time[106,107]. Table 10 shows many examples of utilization ofdigestate around the world. The most common use of thedigestate is as bio-fertilizer in farming industry. In many casesthe digestate dried and then sold as fertilizer and/or can be solidas a liquid fertilizer directly which depend on the quality of thedigestate.

6. Conclusion

Anaerobic digestion is a very useful and cheap wastewatertreatment process. The added benefit is biogas and digestate as by-products. Covered anaerobic digesters can offer many advantagesamong them are offset the cost of wastewater treatment plants,

Table 10Examples of biogas plants in the worldwide.

Location Type Size andcapacity

Substrate utilization Biogasyield

Biogas utilization Digestateutilization

Electricitygenerated

Germany Jühnde,Co-digestion,centralizedplants

3000 m3 Whole plant silage and grass, liquidmanure and wide diversity of crops andeven weeds

CHP plant with 700 kWelectrical and 750 kW thermalpower

5000 MWhelectricity per year

Italy CRPA co-digestionplant, onsite

Two 1200 m3

completelystirred tanks

Cattle manure together withagricultural residues and energy crops(forage, maize silage, onions andpotatoes residue, beet pulps and otherseasonal biomasses).

86,131m3/month,0.730 m3/kg VS

Biogas is burned in two co-generators (CHP) that cansupply 115 and 240 kW ofelectrical power

154,885 KWh/month

Denmark Ribe BiogasPlant,co-digestion,centralizedplants

3�1745 m3,53 C

cattle, pig, poultry and mink slurry fromlivestock farms with waste fromabattoirs, digestible fatty organic wastesfrom food and fish processing industriesand from medicinal industry and withflotation sludge from a poultry abattoir

5.5 millionNm3/year

CHP-plant/gas boiler Liquidfertilizer

130,000 GJ ofenergy annually

Sweden Linköpingbiogas plant

2�3700 m3

stirred tankdigesters, HRT30 days

Manure for pigs and cattle, abattoirwaste, industrial organic waste,household waste, Others. (solid abattoirwaste is minced before it enter thedigester)

7.7 millionm3/year

Upgrade the biogas throughPSA-plant

Bio-fertilizerto farming

Total biogasproduction48,000 MWh/yBiogas delivered tovehicles45,000 MWh/y

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–5148

reduce GHGs emissions, achieve cleaner wastewater and save incarbon tax, and generate revenue from both the biogas anddigestate. At the same time, anaerobic digestion is a complexbiological process where many variables have significant effects onits performance. These variables can be related to the design of thedigester, process conditions, influent wastewater characteristic,and pre-treatment processes. Anaerobic digestion processes in thered meat industry facing challenges that hinder investments incovered anaerobic digesters. This paper has addressed the diffi-culties that associated with the digestion process in this industry.It has been widely reported that FOGs is the main problem whichcauses reduction in the process efficiency and formation of crustwhich leads to breaking the digester cover and reduce biogasproduction. This paper also addresses many methods to overcomethese problems such as co-digestion, use of surfactant, pre-

treatment, simulation and innovative design of the digester. Themost reasonable solution can be recommended as a result ofinterpretation of literature is adding biodegradable surfactants andinnovative pond design. These two will contribute in including theFOGs in the digestion process, increase biogas production and noextra waste will be generated. Furthermore, simulation of suchprocess could reveal potential risks and associated costs notcaught in capacity planning.

Acknowledgement

The authors would like to thank the National Centre ofEngineering in Agriculture (NCEA) at the University of SouthernQueensland (USQ) for providing facilities and resources in order to

Table 10 (continued )

Location Type Size andcapacity

Substrate utilization Biogasyield

Biogas utilization Digestateutilization

Electricitygenerated

Switzerland Bern WWTP Three digesters,each with avolume of6000 m³.

Sewage sludge. Fatty sludge from thefood industry is used as co-substrate.

14400 Nm³/d

CHP-unit, energy use of thesludge drying plant and toheat water boiler

Drying thedigesterresiduals

Ukraine Blagodatnoe Slaughterhouse waste 20 t/day 2400 m3/day

Russia MokriySemenek

pig liquid manure 150 t/day þsilage15 t / day

600 N el kW

Greece Xanthi cattle liquid manure 300 tþdung5 tþsilage 5 t/day

1063

USA Chino,California,centralized

Cattle manure, liquid waste from foodindustry

18.813 m³per day

CHP: 4�30 kWe Micro-gas-turbines

1500 kWe

Canada Kensington,PrinceEdwardIsland

4�5500 m³,steel tank

Potato residues, oil, potato starch Biogas is used for heatingpurposes – hot waterproduction

12 MWth

Austria St. Martin/Innkreis, onfarm

Pig slaughtering process such as pigblood, minced hind gut includingcontent and fat from dissolved airflotation

4.7 MWh/d ofelectricityand7 MWh/dof heat

80% of the heat demand in theslaughterhouseis covered bythe biogas driven CHP

Finland Kalmari farm 1000 m3

mesophilic,continuousstirred reactor

cow manure and confectionery by-products with smaller amounts ofenergy crops and mainly grass silage

CHP and Gas boiler, electricityself-sufficient, electricity issold to the grid, and vehiclefuel sales exceeded1000 MWh in 2011

Used asbio-fertilizer

Electricity 75MWh/year Heat150 MWh/yearBiomethane fortraffic Fuel1000 MWh/year

Netherlands Collaborationbetween themunicipality,a local energycompany anda farm

2�2500 m3

CSTRContinuouslyStirred TankReactor, HRT 50days

50% organic manure and additionalsubstrates such as corn, grass and wasteproducts from the food industry

CHP unit Fertilizer 7 million kWh ofelectricity annually

Location Reference

Germany http://ec.europa.eu/energy/renewables/bioenergy/doc/anaerobic/016bm_015_1993.pdfhttp://www.stowa-selectedtechnologies.nl/Sheets/Sheets/Co.Digestion.html

Italy http://www.ramiran.net/doc08/RAMIRAN_2008/Piccinini.pdfDenmark http://www.ub.edu/bioamb/PROBIOGAS/centralcodig_descrip2000.pdf

There are 18 example of centralized co-digestion plants in Denmarkhttp://www.iea-biogas.net/_download/Success%20Story%20Ribe2012.pdf

Sweden http://www.biogasmax.eu/media/d2_11_biogasmax_iwes_vfinal_nov2010__095398400_1109_10022011.pdfSwitzerland http://www.biogasmax.eu/media/d2_11_biogasmax_iwes_vfinal_nov2010__095398400_1109_10022011.pdfUkraine http://zorg-biogas.com/upload/pdf/References_en.pdfRussia http://zorg-biogas.com/upload/pdf/References_en.pdfGreece http://zorg-biogas.com/upload/pdf/References_en.pdfUSA http://www.kriegfischer.de/texte/Industrial_Big_Biogas_Plant_North_America.pdf

Krieg & Fischer Ingenieure GmbH, 140 biogas plants in: Germany, Japan, Netherlands, Austria, SwitzerlandLithuania, Italy, Slovakia, Canada, USA, Spain, FranceMore examples at: http://www.americanbiogascouncil.org/biogas_foodWaste.asp

Canada http://www.kriegfischer.de/texte/Industrial_Big_Biogas_Plant_North_America.pdfAustria http://www.iea-biogas.net/_download/st_martin.pdfFinland http://www.iea-biogas.net/_download/success-story-kalmari2012.pdfNetherlands http://www.iea-biogas.net/_download/success_story_zeewolde2011.pdf

I. Hamawand / Renewable and Sustainable Energy Reviews 44 (2015) 37–51 49

commence this review paper. The NCEA is a centre aims to developsolutions for a sustainable and profitable rural sector throughapplied engineering, research, training and commercialization.

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