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Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 231–245

Contents lists available at SciVerse ScienceDirect

Process Safety and Environmental Protection

journa l h o me p age: www.elsev ier .com/ locate /psep

naerobic co-digestion of fat, oil, and grease (FOG): A reviewf gas production and process limitations

. Hunter Long, Tarek N. Aziz, Francis L. de los Reyes III, Joel J. Ducoste ∗

epartment of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, NC7695-7908, United States

a b s t r a c t

The addition of readily available high strength organic wastes such as fats, oils, and grease (FOG) from restaurant

grease abatement devices may substantially increase biogas production from anaerobic digesters at wastewater

treatment facilities. This FOG addition may provide greater economic incentives for the use of excess biogas to

generate electricity, thermal, or mechanical energy. Co-digestion of FOG with municipal biosolids at a rate of 10–30%

FOG by volume of total digester feed caused a 30–80% increase in digester gas production in two full scale wastewater

biosolids anaerobic digesters (Bailey, 2007; Muller et al., 2010). Laboratory and pilot scale anaerobic digesters have

shown even larger increases in gas production. However, anaerobic digestion of high lipid wastes has been reported

to cause inhibition of acetoclastic and methanogenic bacteria, substrate, and product transport limitation, sludge

flotation, digester foaming, blockages of pipes and pumps, and clogging of gas collection and handling systems. This

paper reviews the scientific literature on biogas production, inhibition, and optimal reactor configurations, and will

highlight future research needed to improve the gas production and overall efficiency of anaerobic co-digestion of

FOG with biosolids from municipal wastewater treatment.

© 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: FOG; Anaerobic digestion; Methane; Wastewater; Grease; Biogas

. Introduction

iogas from the anaerobic degradation of wastewater biosolidsay be beneficially used in boilers or dryers for process

eat, or can be used in micro-turbines or internal combus-ion engines for generating electricity (Crawford and Sandino,010). The US EPA estimates that over 80% of municipal-ties that utilize anaerobic digestion flare excess biogasEPA, 2007). Typical digester gas energy recovery systemsan recover 20–40% of the energy requirement for wastew-ter treatment plants that use the activated sludge processCrawford and Sandino, 2010). Fat, oil, and grease (FOG) col-ected from the food service industry has been cited toncrease biogas production by 30% or more when addedirectly to the anaerobic digester and may allow wastew-ter treatment plants to meet over 50% of their electricity
emand through on-site generation (Suto et al., 2006; Bailey,007; Kabouris et al., 2008, 2009a,b; Davidsson et al., 2008;
∗ Corresponding author. Tel.: +1 919 515 8150; fax: +1 919 515 7908.E-mail address: [email protected] (J.J. Ducoste).Received 28 February 2011; Received in revised form 30 September 20

957-5820/$ – see front matter © 2011 The Institution of Chemical Engioi:10.1016/j.psep.2011.10.001

York et al., 2008; Parry et al., 2008; Luostarinen et al., 2009;Muller et al., 2010). Despite the reported benefits of co-digestion, studies investigating the anaerobic digestion ofhigh-strength lipid wastes, have also reported a wide assort-ment of operational challenges. These operational challengesinclude the inhibition of acetoclastic and methanogenic bac-teria, substrate, and product transport limitation, sludgeflotation, digester foaming, blockages of pipes and pumps,and clogging of gas collection and handling systems (Hanakiet al., 1981; Koster and Cramer, 1987; Hwu et al., 1998a;Shin et al., 2003; Pereira et al., 2004; Jeganathan et al.,2006; Shea et al., 2010). The aim of this review is to aggre-gate and compare the numerous laboratory, pilot, and fullscale anaerobic digestion research studies on high-strengthlipid wastes and discuss how results from these studiescan be used to address current and future challenges inthe adoption of the co-digestion of FOG with wastewaterbiosolids.

11; Accepted 5 October 2011neers. Published by Elsevier B.V. All rights reserved.

http://www.sciencedirect.com/science/journal/09575820

www.elsevier.com/locate/psep

mailto:[email protected]

dx.doi.org/10.1016/j.psep.2011.10.001

232 Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 231–245

2. The FOG waste stream

2.1. FOG generation and management

FOG is a term commonly used to define the layer of lipid-rich material from wastewater generated during cooking andfood processing. The direct release of FOG into the col-lection system is considered by most municipalities to beillegal. Once in the collection system, FOG can accumulateon pipe walls, potentially forming hardened deposits througha chemical reaction or a physical aggregation process (Heet al., 2011, in press). These deposits lead to a reduction inconveyance capacity and ultimately to sanitary sewer over-flows that cost municipalities millions of dollars each yearin cleaning, repairing, and maintenance fees (US EPA, 2011).For this reason, municipalities implement pre-treatment stepsto aid in the removal of grease from kitchen waste streams.Grease removal is most commonly accomplished by the useof grease abatement devices referred to as “grease traps” or“grease interceptors”. Grease abatement devices are gener-ally non-mechanized flow-through gravity separation devicesthat retain suspended grease and food solids by providingsufficient time for flotation/sedimentation of the influentwaste. Grease traps are typically around 50 gallons (190 L) insize and are installed inside the food preparation facility,directly below the sink. Grease interceptors, however, are typ-ically 1000–2000 gallons (3785–7570 L) in size and are generallyinstalled below ground and outside of the building. The termgrease trap waste (GTW) and FOG have been used interchange-ably throughout the literature, but for the sake of clarity theterm GTW will be used throughout this paper to refer to theentire contents of the grease abatement device including FOG,water, and food particles. Collecting or pumping out GTW atregular intervals can help prevent FOG from entering the col-lection system.

According to a survey conducted by the National Renew-able Energy Laboratory (NREL) in thirty US metropolitanareas, FOG is generated at a rate of approximately 1.9 gallonsFOG/person/year (7.1 L FOG/person/year) (Wiltsee, 1998). Thisis an estimate of only the FOG portion of GTW and doesnot include the excess water and food solids, which makeup the remainder of the GTW. However, this estimate alsoincludes the quantity of FOG that arrives at the WWTP mixedwith raw sewage in the sanitary sewer line. FOG is difficultto recover once it has been mixed with raw sewage and theNREL study likely overestimates the quantity of recoverableFOG. A separate survey of the Wake County North Carolinametropolitan area (population 866,410) determined that thereis an average of 18.7 gallons of GTW/person/year when con-sidering the entire contents of the grease abatement device(70.6 L GTW/person/year) (Austic, 2010). Based on these twoestimates, approximately 5.9 billion gallons (22 billion liters)of GTW and 10% of this volume, 0.6 billion gallons (2.2 billionliters) as recoverable FOG are generated annually in the UnitedStates, a population 312 million (Census Bureau, 2011).

Once collected, GTW disposal options may include landapplication, landfilling, composting, rendering for man-ufacturing lubricants or industrial soaps, incineration,anaerobic co-digestion, or biodiesel production (Wiltsee,1998; Rohm, 2005; Chung et al., 2010). Land application,while one of the cheapest disposal options, is regulatedat the US national level by 40 CFR Part 257 (Rohm, 2005).
Maximum quantities of wastes that may be land appliedare dependent on how the land is used, soil characteristics,
location, nutrient, and organic load of the waste. For example,in the state of North Carolina, the GTW land application limitis generally 30,000 gallons/acre/year (280.6 m3/hectare/year)before dewatering (Dayton, 2010). Other estimates haveclaimed that GTW may be land applied at loads ranging from4 to 8 dry tons/acre/year (9–18 dry metric tons/hectare/year),which would equate to 16,000–32,000 gallons/acre/year(150–300 m3/hectare/year) if the total solids are 6% (Rohm,2000, 2005). Land application has been cited as improvingthe soil organic carbon content and may prevent nitrogenleaching (Rashid and Voroney, 2004). However, depending onGTW application and soil characteristics, land applicationmay also require additional nutrient loading to sustain cropgrowth (Rashid and Voroney, 2004). Composting, another GTWdisposal option, may result in an end product that can be soldas a soil amendment. In addition, the composting processwill mitigate the potential for methane that would have beenproduced if the GTW had been landfilled. However, energymust be expended during the composting process (Brownet al., 2008). Although land application and composting mayoffset the use of existing soil amendments, no research hasbeen performed to assess the relative utility of the variousGTW direct-to-land applications.

GTW has a high biochemical oxygen demand, a large frac-tion of lipids, and contains roughly 7000–10,000 BTUs/pound(4.5–6.5 kW h/kg) when dewatered (Dayton, 2010). Accord-ingly, GTW has energy value-added potential for beneficialuse in incineration, biodiesel production, and anaerobic co-digestion. The FOG portion of GTW may be recovered for useas a biodiesel feedstock. However FOG was found to accountfor approximately only 0–15% by volume of GTW, with anaverage of about 2–3% (Suto et al., 2006; Gabel et al., 2009;Chung et al., 2010), and the remaining waste (approximately97% by volume) still needs some form of disposal. Further-more, FOG from GTW has high free fatty acid content, whichmay require an additional acid catalyzed pretreatment stepin addition to the typical alkaline catalyzed transesterifica-tion of triglycerides for biodiesel production (Canakci and VanGerpen, 2001). Pilot studies have shown that the conversion ofGTW to biodiesel is feasible (Jolis et al., 2010). However, a com-prehensive comparison of GTW disposal options is needed todetermine the best alternative. For example, the minimal pre-treatment requirements of anaerobic co-digestion may makeit a better disposal option than incineration or biodiesel pro-duction.

There are still numerous operational concerns in imple-menting anaerobic co-digestion of GTW with primary andsecondary sludge. Of these operational concerns, the inhibi-tion of methane generation as a result of FOG or its derivativesis one of the most common (Hanaki et al., 1981; Koster andCramer, 1987; Angelidake et al., 1992; Rinzema et al., 1994;Hwu and Lettinga, 1997; Alves et al., 2001b; Pereira et al.,2003; Shin et al., 2003; Kim et al., 2004). Though enhancedbiogas production during anaerobic co-digestion of FOG hasbeen frequently reported in recent years, there has been nodiscussion of inhibition concerns or the potential for inhi-bition during anaerobic co-digestion. Many of the studies todate have focused on the application of high rate anaerobictechnologies for treating high lipid industrial wastes (Kosterand Cramer, 1987; Angelidaki and Ahring, 1992; Hwu et al.,1998a,b; Pereira et al., 2002a,b, 2003, 2004, 2005; Jeganathanet al., 2006; Alves et al., 2001a,b, 2009; Cavaleiro et al., 2009;
Kim and Shin, 2010), and a recent summary of high rate anaer-obic technology for the treatment of lipid wastes has been

Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 231–245 233

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ublished (Alves et al., 2009). High rate anaerobic technologyypically utilizes anaerobic treatment processes with a morelug flow characteristic, such as up-flow anaerobic sludgelanket reactors (UASBs) whereas US municipal wastewaterreatment plants typically employ continuously stirred tankeactor (CSTR) anaerobic treatment processes. Consequently,here is a need to analyze the similarities and differencesetween anaerobic co-digestion and high rate anaerobic tech-ologies for treatment of lipid wastes.

.2. GTW characterization

he chemical characteristics of grease trap waste can varyreatly depending on the type of restaurant or food ser-ice establishment, the grease abatement device configurationi.e., size, inlet/outlet piping, number of baffles), and the greasebatement device pump out frequency. Numerous municipal-ties utilize a 25% rule, requiring that the grease abatementevice is pumped clean before the top floatable layer and bot-om sludge layer account for a combined 25% of the deviceolume (City of Santa Ana, 2004). Other municipalities mayequire that the grease abatement device is pumped clean at

predetermined frequency, such as every 90 days (Westernirginia Water Authority, 2006). GTW may have a higher or

ower BOD, FOG, and total solids content depending on therequency of pump outs. BOD, FOG, and total solids may alsoary depending on the physical characteristics of the greasebatement device, i.e., flow through velocity, hydraulic reten-ion time, or average temperature. A report by De los Reyesnd He (2009) evaluated physical and chemical parametersf seven different locations within two separate grease inter-eptors (GIs), including the influent and effluent. Parametershat were monitored include pH, oxidation–reduction poten-ial (ORP), dissolved oxygen (DO), chemical oxygen demandCOD), biochemical oxygen demand (BOD), total solids (TS),olatile solids (VS), and oil and grease concentration. Theirssessment indicated that substantial differences in chemicalnd physical characteristics can be observed between differ-nt grease interceptors, and within a single interceptor atifferent times (De los Reyes and He, 2009).

GTW typically forms three layers within the grease abate-ent device; top floatable layer (primarily FOG), middle

queous layer (organic rich wastewater) and bottom sludgeayer (food particles and other settleable solids) (Suto et al.,006). The volume of each layer will vary greatly dependingn the type of grease abatement device and how frequently

t is pumped clean. Table 1 provides a summary of the char-cterization of GTW from restaurants. As expected, the GTWollected from restaurant grease abatement devices is highn fat content. In these six particular studies, there is alightly higher concentration of unsaturated fat than satu-ated fat. Oleic acid (C18:1) was the most common fatty acidound in GTW. This result was expected as oleic acid is alsohe most common fatty acid found in wastewater treatmentlants (Viswanathan et al., 1962). Total solids were found toange from 2% to 22%, before dewatering (Bailey, 2007; Sutot al., 2006; Fonda et al., 2003; York et al., 2008; Cockrell,007; Chakrabarti et al., 2008; Schutz, 2008). Long chain fattycids (LCFA), which are thought to inhibit methane generationHanaki et al., 1981; Koster and Cramer, 1987; Angelidake et al.,992; Rinzema et al., 1994; Hwu and Lettinga, 1997; Alves et al.,001b; Pereira et al., 2003; Shin et al., 2003; Kim et al., 2004)
ccur in high concentrations in GTW and may cause opera-ional problems during co-digestion. The low pH measured in
GTW may require specific materials to prevent corrosion inpumps, pipes, and storage tanks. Overall, a broader character-ization of GTW is needed to establish better predictions of thedigestibility of this material.

3. FOG anaerobic digestion

3.1. Degradation process

LCFAs, the primary component of FOG, are degraded anaero-bically via the �-oxidation pathway to acetate and H2, whichare subsequently converted to methane. �-oxidation beginswhen the fatty acid is activated with coenzyme A and theresulting oxidation leads to the release of acetyl-CoA and theformation of a fatty acid chain, which is shortened by two car-bons. Acetyl-CoA is oxidized by way of the citric acid cycle andthe process of �-oxidation is repeated (Madigan et al., 2006).The following reaction expresses the degradation of long chainfatty acids via the �-oxidation pathway.

CH3(CH2)nCOOH + 2H2O → CH3(CH2)n−2COOH

+ CH3COOH + 2H2(Kimet al., 2004)

Sousa et al. (2009) and Alves et al. (2009) have provideda detailed summary of the �-oxidation degradation pathwayof fatty acids and a description of the microbial communitycapable of degrading LCFAS. Saturated fatty acids follow thetraditional �-oxidation pathway, however, the exact pathwayfor degradation of unsaturated fatty acids is undeterminedand two possible pathways have been suggested. Earlier stud-ies have suggested that the degradation of unsaturated LCFAfirst required complete saturation followed by the typical �-oxidation pathway (Novak and Carlson, 1970). Another studysuggested that �-oxidation of unsaturated fatty acids mightoccur before fatty acid saturation (Roy et al., 1986). Multiplestudies have shown that palmitic acid C16:0 is a key inter-mediate in the degradation of oleic acid C18:1 (Lalman andBagley, 2000; Jeganathan et al., 2006; Cavaleiro et al. 2009).However, the degradation of stearic acid C18:0 did not form theintermediate palmitic acid C16:0 (Lalman and Bagley, 2000).Additionally stearic acid C18:0 was not observed as an inter-mediate in the anaerobic degradation of oleic acid C18:1 orlinoleic acid C18:2 (Lalman and Bagley, 2001). The observationthat oleic (C18:1) and linoleic (C18:2) acid degrades to palmiticacid (C16:0) with no intermediate stearic acid (C18:0) sug-gests that the degradation of unsaturated fatty acids may notrequire complete fatty acid saturation before �-oxidation canoccur. Additionally, Sousa et al. (2006) reported that an anaer-obic bacterial culture that was enriched on palmitate (C16:0)could not degrade oleate (C18:1). However, they also reportedthat a culture enriched on oleate could degrade palmitate sug-gesting that a different consortium of bacteria may be involvedin the degradation of saturated and unsaturated fatty acids.

Approximately 14 syntrophic bacteria have been identi-fied as being able to degrade fatty acids in pure cultureor coculture with hydrogen-consuming microorganisms. Allbelong to the families Syntrophodonadaceae and Syntrophaceae(McInerney et al., 2006; Zhao et al., 1993; Wu et al., 2006; Sousaet al., 2007c; Jackson et al., 1999). Only four of these microor-ganisms, Syntrophomonas sapovorans, Syntrophomonas curvata,Syntrophomonas zehnderi, and Thermosyntropha lipolytica are
able to degrade unsaturated LCFA with more than 12 carbonatoms. The unsaturated LCFA degradation pathway and the

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Table 1 – Characterization of GTW samples.

Reference Kabouris et al.(2009a,b)

Parry et al.(2008)

Suto et al. (2006) Canakci (2007) Bailey (2007) Wimmer (2010) De los Reyes and He(2009)

Sample Polymerdewatered FOG

Restaurantgrease(dewatered)

27 Different restaurant greasesamples

Brown grease 7 Samples takenfrom 5000 gallongrease truckloads

Samples from 5grease haulers inthe DC area

Samples from two foodservice establishmentsin NC

Total solids 42.4% 97.2% nd nd 1.8–21.9% 20,000 mg/L Floatable layer:2500–303,400 mg/LBottom layer:7300–51,900 mg/L

% VS/TS 96.5 100 96.2 nd 88.9–98.6 90 Floatable layer: 66–99Bottom layer: 84–99

pH 4.03 5.79 nd nd 4.3–4.8 nd 3.9–6.2COD Total COD:

1211 g/kg wetsample

Total COD:439,000 mg/LSoluble COD:152 mg/L

Floatable layer: 478,000 mg/LAqueous layer: 66,200 mg/LSludge layer: 107,061 mg/L

nd 20,000–68,000 mg/L 15,000 mg/L nd

BOD nd nd Floatable layer: 33,767 mg/LAqueous layer: 21,721 mg/LSludge layer: 53,367 mg/L

nd nd nd nd

Carbohydrates 15% of VS 0.5% of VS nd nd nd nd Floatable layer:99–1464 mg/LBottom layer:890–7416 mg/L

Protein 7% of VS 0.3% of VS nd nd nd nd Floatable layer:227–4371 mg/LBottom layer:323–9209 mg/L

Fat (% of VS) 78% 99.5% nd nd nd nd ndSaturated fat (% of fat) 37.9% nd 48.6% 37.03% nd nd ndCaprylic acid (C8:0) nd nd 0.9% nd nd nd ndDecanoic acid (C10:0) nd nd 1.3% nd nd nd ndLauric acid (C12:0) nd nd 3.0% nd nd nd ndMyristic acid (C14:0) nd nd 8.4% 1.66% nd nd ndPalmitic acid (C16:0) nd nd 23.1% 22.83% nd nd ndStearic acid (C18:0) nd nd 9.8% 12.54% nd nd ndArachidic acid (C20:0) nd nd 2.1% nd nd nd ndPolyunsaturated fat (% of fat) 7.4% nd 15.3% 12.91% nd nd ndLinoleic acid (C18:2) nd nd 15.3% 12.09% nd nd ndLinolenic acid (C18:3) nd nd nd 0.82% nd nd ndMonounsaturated fat (% of fat) 39.5% nd 36.10% 45.49% nd nd ndPalmitoleic acid (C16:1) nd nd nd 3.13% nd nd ndOleic acid (C18:1) nd nd 36.1% 42.36% nd nd ndTrans fat (% of fat) 15.2% nd nd nd nd nd nd

nd, not determined.


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icroorganisms involved in the degradation are still not com-letely known. Additional research is needed and may lead torocess improvements for anaerobic degradation of unsatu-ated LCFAs.

.2. Operational concerns

ne concern with anaerobic co-digestion is that longhain fatty acids (LCFA) may have a detrimental effect onethanogenic bacteria when introduced at sufficiently high

oncentrations or loading rates (Hanaki et al., 1981; Kosternd Cramer, 1987; Rinzema et al., 1994; Hwu and Lettinga,997; Lalman and Bagley, 2002; Shin et al., 2003; Kim et al.,004). Researchers have suggested that the detrimental effectn methanogenic bacteria may be due to: sludge flotation andashout (Hwu et al., 1998a; Jeganathan et al., 2006); trans-ort limitation from bacteria being coated in a layer of LCFAshereby hindering the cells access to substrates and its abil-ty to release biogas (Pereira et al., 2004); or a LCFA toxicityffect on methanogenic bacteria (Koster and Cramer, 1987;wu et al., 1996; Hwu and Lettinga, 1997; Alves et al., 2001a,b).igester foaming is another operational concern associatedith anaerobic digestion of lipids (Ganidi et al., 2009; Cockrell,

007; Muller et al., 2009). Alves et al. (2009) provided a reviewf the three proposed inhibition mechanisms; sludge flotation

eading to washout, substrate, and product transport limi-ation, and inhibition of acetoclastic and hydrogenotrophic

ethanogens. Many of the experiments investigating poten-ial inhibition mechanisms were conducted in batch reactors,igh rate anaerobic reactors such as UASB, or expandedranular sludge bed (EGSB). Multiple studies have suggestedaximum concentrations of LCFAs, above which anaerobic

igesters are likely to experience excessive methanogen inhi-ition. Table 2 displays inhibition concentrations for various

ong chain fatty acids.

.2.1. Inhibition of methanogenic bacteriaany of the experiments, described in Table 2, were con-

ucted on synthetic wastes composed of single fatty acids.TW is a mixture of lipids, fatty acids, food particles, soaps,etergents, and other substances that may be discharged fromood service establishments. It is still unclear whether similarnhibitory levels apply to the more diverse GTW. Research toate suggests the possibility of either exacerbated inhibitionr reduced challenges with an anaerobic digester feed con-aining a mixture of distinct LCFAs. For example, Koster andramer (1987) reported that a mixture of LCFAs will collectivelyxert greater inhibition than a single LCFA. Yet, Hwu et al.1998b) observed that the presence of co-substrates enhancedverall lipid COD removal. These inconsistencies indicate thatxperimental anaerobic co-digestion tests involving GTW andunicipal sewage sludge need to be performed to quan-

ify GTW inhibition concentration limits. Co-digestion studiesresented later in Section 4.3 demonstrated improved digesteras production, but did not test the upper concentration oroading rate limit of GTW co-digestion

The exact nature and mechanism of the proposed toxicffect of LCFAs on methanogenic bacteria is not wellnderstood. However, researchers first suggested that theechanism for LCFA inhibition of methanogenic and aceto-

enic bacteria was due to a surfactant effect causing the LCFAo damage the cell membrane (Galbraith and Miller, 1973b).
he surface tension reducing properties of LCFAs may cause
rreversible changes to the bacterial cell wall or cell membrane.

This surfactant effect may alter the nature of the cell mem-brane allowing protons to cross the membrane in bothdirections rather than only exiting the cell (Galbraith andMiller, 1973c). The damage to the cell membrane may alsoresult in the microorganism’s inability to regulate the energyflow (i.e., the synthesis of ATP would be inhibited) (Galbraithand Miller, 1973c). Coles and Lichstein (1963) also concludedthat oleic acid (C18:1) may alter the permeability of the cellmembrane causing the organism to lose its ability to regulateintracellular pH.

Hanaki et al. (1981) identified that long chain fatty acidscaused an increasing lag phase in methanogen activity asthe concentration of LCFA increased. Hanaki et al. (1981) alsonoted that shock loading of LCFAs may severely retard theanaerobic digestion process. LCFA methanogenic inhibitionwas initially reported to be irreversible and that bacteria couldnot build up a tolerance to LCFA inhibition (Angelidaki et al.,1992; Rinzema et al., 1994). Furthermore, Angelidaki et al.(1992) reported that the toxic effect of the LCFAs oleate andstearate on anaerobic thermophilic digestion of cow manurewas permanent and growth did not occur when the inhib-ited cultures were diluted to non-inhibitory concentrations.Rinzema et al. (1994) also concluded that, no ‘adaptation’or ‘recovery’ of bacteria takes place, and that recovery ofmethanogenesis after a toxic load is due to the growth ofthe surviving (approximately 0.2%) acetotrophic methanogenpopulation. These findings indicate that adding high lipidmaterials such as GTW to an anaerobic digestion process hasthe potential to completely shut down the process, a riskthat most municipalities would not be willing to take. How-ever, recent studies by Pereira et al. (2003, 2004) have shownthat even when methanogenic activity was severely inhib-ited due to LCFA loading, anaerobic bacteria were still able toco-digest the adsorbed LCFA. This result contradicts the previ-ously accepted idea that LCFA adsorption to anaerobic sludgecauses changes to the cell membrane, which induce lysis andbactericidal effects (Coles and Lichstein, 1963; Galbraith et al.,1971; Galbraith and Miller, 1973a,c; Kabara et al., 1977). It ismore likely that the exposure of anaerobic sludge to high LCFAconcentrations will result in a lag phase in which methaneproduction may initially be decreased, but will not result ina bactericidal affect in which the bacterial cells are destroyed(Hwu et al., 1998a; Pereira et al., 2004, 2005; Cirne et al., 2006).

The severity of LCFA toxicity on methanogenic bacteriais thought to increase with an increasing number of dou-ble bonds in the LCFA (Kodicek and Worden, 1945; Demeyerand Henderickx, 1967; Galbraith et al., 1971; Kim et al., 2004).The increase in toxicity could be due to an increase in LCFAsurface area as a result of increasing the number of LCFAdouble bonds. The increase in LCFA surface area will resultin a greater area of the methanogenic bacteria being occu-pied by each LCFA molecule and consequently fewer LCFAmolecules may be required to occupy a sensitive site on thebacterium (Galbraith et al., 1971). The severity of LCFA toxi-city is also thought to increase in the presence of a mixtureof LCFAs rather than a single LCFA (Koster and Cramer, 1987).However, the suggested bactericidal effect of LCFA may againpoint to differences between continuously stirred tank reac-tors (CSTRs) and anaerobic treatment processes with a moreplug flow characteristic, such as upflow anaerobic sludge blan-ket reactors (UASBs). Due to higher dilution effect, bacteriain CSTRs may be contacted with lower LCFA concentrations
even when operated at similar LCFA loading rates to a UASBreactor. Pereira et al. (2002a) compared oleic acid anaerobic


Table 2 – LCFA methanogenic activity inhibition concentration.

References Experimental setup Loading Effect

Koster and Cramer (1987) Batch tests conducted onsludge from UASB reactor

10 mM Caprylic acid (C8:0) 50% Acetoclastic methanogenicactivity loss

Koster and Cramer (1987) – 5.9 mM Capric acid (C10:0) 50% Acetoclastic methanogenicactivity loss

Koster and Cramer (1987) – 4.3 mM Lauric acid (C12:0) 50% Acetoclastic methanogenicactivity loss

Koster and Cramer (1987) – 4.8 mM Myristic acid (C14:0) 50% Acetoclastic methanogenicactivity loss

Koster and Cramer (1987) – 4.35 mM Oleic acid (C18:1) 50% Acetoclastic methanogenicactivity loss

Angelidaki et al. (1992) Batch tests on sludge fromcow manure digester

0.2 g/L oleate (C18:1) Increase lag phase of methaneproduction

Angelidaki et al. (1992) – 0.5 g/L oleate (C18:1) No growthAngelidaki et al. (1992) – 0.5 g/L stearate (C18:0) Increase lag phase of methane

productionAngelidaki et al. (1992) – 1.0 g/L stearate (C18:0) No growthRinzema et al. (1994) Stirred batch reactors

inoculated with biomass6.7 mM capric acid Acetogenic and methanogenic

population is killedHwu and Lettinga (1997) Batch test on 4 different

sludges from UASB andUSSB reactors

0.35–1.75 mM oleate (55 ◦C) 50% Acetoclastic methanogenicactivity loss

Hwu and Lettinga (1997) – 0.53–2.27 mM oleate (40 ◦C) 50% Acetoclastic methanogenicactivity loss

Hwu and Lettinga (1997) – 2.35–4.30 mM oleate (30 ◦C) 50% Acetoclastic methanogenicactivity loss

Alves et al. (2001b) Fixed bed reactor 80 (not acclimated to oleicacid) – 137 (acclimated tooleic acid) mg/L oleic Acid

50% Acetoclastic methanogenicactivity loss

Pereira et al. (2003) Fixed bed reactor 2000 mg COD/g VSS oleicacid based synthetic waste

Upper limit of anaerobic sludgecapacity (methanogenic activitystopped above this concentration)

Shin et al. (2003) Batch tests with acclimatedgranular sludges

2700–2850 mg COD/L(oleate)

50% Acetoclastic methanogenicactivity loss

Shin et al. (2003) – 3530–3610 mg COD/L(oleate)

50% Methanogenic activity loss(propionate degradation)a

Shin et al. (2003) – 550–620 mg COD/L(linoleate)

50% Acetoclastic methanogenicactivity loss (acetate degradation)a

Shin et al. (2003) – 760–1050 mg COD/L(linoleate)

50% Methanogenic activity loss(propionate degradation)

Shin et al. (2003) – 3890–4400 mg COD/L(palmitate)


Shin et al. (2003) – 4310–4410 mg COD/L(palmitate)


Shin et al. (2003) – 3800–4480 mg COD/L(stearate)


Shin et al. (2003) – 4400–4410 mg COD/L(stearate)


a Shin et al. (2003) performed methanogenic inhibition tests with acetate or propionate as the main substrate.

degradation under static and stirring conditions and deter-mined that the methanization rate of adsorbed substrate(oleic acid) was enhanced under stirring conditions. Further,research is needed to confirm the reported LCFA toxicity and todetermine if CSTR anaerobic treatment processes have lowerLCFA toxicity levels.

3.2.2. Sludge flotation and washoutRecently, researchers have suggested that a bactericidal effectwas incorrectly diagnosed and that LCFA inhibition may sim-ply be a result of sludge flotation and washout, transportlimitations, or a combination of both (Pereira et al., 2004). Hwuet al. (1998a) observed that the onset of sludge flotation in acontinuous UASB treatment process occurred at LCFA loadingrates exceeding 0.09 g COD/g VSS-d with complete flotation
occurring at LCFA loading rates above 0.2 g COD/g VSS-d. Theseresults suggest that flotation and washout rather than toxicity
may be the cause of inhibition since the LCFA loading rate atwhich flotation occurs, is below the reported toxicity levels formethanogenesis (Hwu et al., 1998a).

UASB treatment processes may be operated at lowhydraulic retention times provided that the sludge blanket,containing the active bacteria, has a sufficient solids reten-tion time for active bacterial growth (Rittman and McCarty,2001). UASB reactors depend on the establishment of a densesludge bed in which the biological process occurs. The sludgebed is designed to remain in the reactor and must havegood settling properties to avoid washout (Seghezzo et al.,1998). Sludge flotation may lead to sludge washout and reac-tor failure as the reactors are designed with sufficient solidsretention time for bacterial growth, only when the sludgeblanket is maintained at the bottom of the reactor (Rittman
and McCarty, 2001). Several researchers have observed sludge-washout effects during the digestion of lipid-rich waste in


UHnkwt

catsmUbmata2

3DbeacnSfSttum(adaGs2cttdlec

3Tiaht(uwiapssm

ASB reactors (Rinzema et al., 1993; Hawkes et al., 1995;wu et al., 1998a). Alves et al. (2009) have even developed aovel anaerobic reactor for removal of LCFA from wastewaternown as an Inverted Anaerobic Sludge Blanket (IASB) reactor,hich advantageously utilizes sludge floatation to maintain

he sludge bed.Municipal anaerobic digesters are designed to behave as

ontinuously stirred tank reactors. In the operation of CSTRnaerobic treatment processes, the hydraulic and solids reten-ion times are the same. Consequently, the occurrence ofludge flotation is less of an issue in CSTR anaerobic treat-ent processes compared to upflow processes such as the

ASB (Rittman and McCarty, 2001). Sludge washout has noteen cited as a problem in anaerobic co-digestion of GTW withunicipal sludge in municipal wastewater treatment CSTR

naerobic treatment processes. However digester foaming andhe formation of a scum layer has been observed in GTWnaerobic co-digestion (Jeganathan et al., 2006; Kabouris et al.,008; Shea et al., 2010).

.2.3. Digester foamingigester foaming may lead to blockages of gas mixing devices,inding of sludge pumps, fouling gas collection pipes, andven tipping of floating digester covers from foam expansionnd collapse (Ganidi et al., 2009). One concern with anaerobico-digestion of GTW is that a high FOG loading may cause sig-ificant digester foaming (Cockrell, 2007; Muller et al., 2009;ober et al., 2010). Surface active agents (surfactants) such asatty acids, oil, or grease may contribute to digester foaming.urfactants have both hydrophilic and hydrophobic proper-ies. The hydrophobic end of the surfactant moves towardhe air phase and the hydrophilic end moves toward the liq-id phase leading to a decrease in surface tension, whichay result in foaming if air bubbles are present in solution

Ganidi et al., 2009). Jeganathan et al. (2006) observed FOGccumulation that led to digester foaming and reduced degra-ation in an upflow anaerobic sludge blanket (UASB) reactort a loading rate of 5 kg COD/m3 d. Anaerobic co-digestion ofTW at the Riverside Water Quality Control Plant in River-ide California did not experience any digester foaming (Bailey,007). Additionally, Muller et al. (2010) reported that foamingould be minimized during anaerobic co-digestion of GTW athe Annacis Island WWTP in Vancouver, Canada by loweringhe stand pipe level and modifying the operating procedureuring offloading. Additional data is needed to determine

oading rates, mixing requirements, or other operating param-ters that will reduce the risk of digester foaming duringo-digestion of GTW.

.2.4. Substrate and product transport limitationransport limitation is often cited as a mechanism for LCFAnhibition of anaerobic bacteria (Pereira et al., 2004). LCFAsdsorb to anaerobic sludge leading to encapsulation andindrance of substrate diffusion into the sludge, as well asransport of methane and other products out of the sludgePereira et al., 2003). Adsorption of LCFAs to anaerobic gran-lar sludge was dependent on the concentration of LCFAs,ith increasing LCFA concentrations resulting in increasing

nitial adsorption rates (Hwu et al., 1998a,b). Interestingly, thedsorption of an LCFA mixture on anaerobic sludge granulesroceeded much faster than the adsorption of oleate as aole LCFA source (Hwu et al., 1998a). Pereira et al. (2004) also
urmises that transport limitations, from LCFA adsorption,ay be responsible for the observed lag phases which have
previously been attributed to a bactericidal effect addressed inSection 3.2.1 (Coles and Lichstein, 1963; Galbraith and Miller,1973c; Hanaki et al., 1981; Koster and Cramer, 1987; Rinzemaet al., 1994; Hwu and Lettinga, 1997; Lalman and Bagley, 2002;Shin et al., 2003; Kim et al., 2004).

It should be noted that Elefsiniotis and Oldham (1994)reported that CSTR anaerobic digesters had higher lipid degra-dation (63–83%) than UASB reactors (48–67%). Elefsiniotis andOldham concluded that the increased performance of lipiddegradation in CSTRs is probably due to the stronger mixingconditions that lead to better lipid dispersion and increasedcontact between substrate and enzyme. Other researchershave found similar results, citing enhanced LCFA degrada-tion with enhanced mixing (Pereira et al., 2002a). Pereira andcolleagues go on to suggest that suspended sludge is advan-tageous over granular sludge due to a higher capacity of LCFAadsorption and degradation but notes that retention in thereactor may be problematic. Further research is needed todetermine if the increased mixing intensity of CSTR anaerobicdigestion process decreases the LCFA substrate and producttransport limitation effect. It is possible that multiple inhi-bition mechanisms may be occurring simultaneously in theanaerobic digester, making it difficult to ascertain the exactcause of digester performance problems (Pereira et al., 2003).

4. Effective co-digestion implementations

As discussed previously, there are substantial differences inthe composition of GTW and reactor designs for the co-digestion of GTW. Numerous municipalities across the USA,Canada, and Europe have implemented full scale anaerobicco-digestion of GTW and municipal sewage sludge at wastew-ater treatment plants (Chung et al., 2010). GTW or other highlipid wastes have also been successfully co-digested with cowmanure, swine manure, and food waste (Zitomer et al., 2008;Neves et al., 2009; Parry et al., 2009; Creamer et al., 2010).The following discussion will focus on GTW co-digestion withmunicipal sewage biosolids as it is municipalities that mustalso oversee the appropriate disposal of GTW. Anaerobic co-digestion studies have pointed to multiple factors affecting gasproduction including % FOG as volatile solids, reactor tem-perature, pH, hydraulic residence time, reactor size, feedingapproach (continuous or batch), and whether the reactor isoperated in one phase or is separated into multiple phases.

4.1. Variations in reactor operational conditions

4.1.1. Process temperatureFOG co-digestion can be performed under mesophilic orthermophilic conditions. Suto et al. (2006) recommended ther-mophilic temperature conditions (55 ◦C) for FOG co-digestiondue to the reactor’s increased ability to degrade LCFAs andthe formation of a smaller scum layer compared to a reac-tor operating in the mesophilic temperature range (35 ◦C).Under thermophilic conditions, lipids become more acces-sible to microorganisms and their lipolytic enzymes due toincreased diffusion coefficients and lipid solubility in aqueousmedia with increasing temperature (Chipasa and Medrzycka,2006). However, thermophilic operation does not always offersufficiently higher volatile solids reduction to justify the costassociated with the increased energy for heating (Lynch and
Fitgerald, 2009). Additionally, thermophilic bacteria may bemore sensitive to LCFA inhibition than mesophilic bacteria


(Hwu and Lettinga, 1997). Kabouris et al. (2009a) also deter-mined that thermophilic co-digestion may be favorable whenthere is a need for near-complete destruction of degradablevolatile solids and increased methane production. However,they also noted that thermophilic treatment may increasethe nutrient release. In addition, financial and process designconsiderations should be taken into account on a case-by-case basis in determining if thermophilic or mesophilicco-digestion is preferred.

4.1.2. Single phase vs. two-phase co-digestionA two-phase reactor can accommodate larger FOG loadingrates without encountering stuck digester problems (Hanakiet al., 1987; Kabouris et al., 2009a,b). The acidogenic phase,the first phase in a two-phase process, allows for a degreeof LCFA saturation and degradation. Kim (2004) reported 19%of LCFAs were degraded and 12% of unsaturated LCFA weresaturated in the acidogenic phase operated with an HRT of0.76 days. Converting unsaturated LCFA to saturated LCFA issignificant since inhibitory effects of unsaturated LCFAs arethought to be more severe than saturated LCFAs (Hanaki etal., 1981; Lalman and Bagley, 2002) and LCFA saturation maybe required for �-oxidation (Sousa et al., 2009). Beccari et al.,1998 also reported that the acidogenic phase transformed theunsaturated LCFAs to palmitic acid, thus reducing the lipidinhibition of methanogenesis in the second phase. A two-phase reactor may lead to a higher percentage of digestionand consequently larger gas production. Kim and Shin (2010)reported that a two-phase system composed of an acidogeniccontinuously stirred tank reactor followed by a methanogenicupflow bed reactor, treating a high lipid wastewater from amilk and ice cream factory, resulted in 1.2 times the CODremoval, 1.9 times lipids removal, and 1.4 times the methaneproduction compared with a single-phase system. Kim (2004)reported that a single-phase system deteriorated at loadingrates above 1.38 kg LCFA–COD m−3 day−1 while a two-phasesystem was still performing satisfactorily.

4.1.3. Feeding sequenceReactor performance was initially thought to be better whenFOG is fed continuously rather than in batches (Angelidakiand Ahring, 1992). Cavaleiro et al. (2009) showed that an ini-tial batch feeding process may be required for acclimationin a continuously fed FOG operation. However, Coelho et al.(2007) studied the effect of intermittent feeding compared tocontinuous feeding of a UASB reactor using a dairy wastew-ater and demonstrated that intermittent feeding may allowanaerobic reactors to maintain stable operation at a higherorganic loading rate compared to a system with continuousfeeding. Further research is needed to determine if a contin-uous or intermittent GTW feeding cycle will achieve betterdegradation and methane production in a typical municipalwastewater treatment CSTR mesophilic anaerobic digester.

4.2. Digester gas production

GTW typically has a higher methane potential than munic-ipal wastewater sludge. Anaerobic digestion of fat has atheoretical yield of 22.8 ft3 biogas/lb (1430 mL biogas/g) com-pared to 14.8 ft3 biogas/lb (930 mL biogas/g) for protein and13.3 ft3 biogas/lb (840 mL biogas/g) for carbohydrates (Alveset al., 2009). Batch experiments determined the methane
potential of GTW to be in the range of 14.5–22.5 ft3/lb VSreduced or 909–1404 mL/g VS reduced (Davidsson et al., 2008;
Kabouris et al., 2008, 2009a; Luostarinen et al., 2009). These val-ues are in agreement with the theoretical yield for lipids (Alveset al., 2009). Proteins and carbohydrates can be convertedinto biogas with 50–58% methane while fats can be convertedinto biogas with 66–73% (Gujer and Zehnder, 1983) methane.Digesters with FOG and sludge produce more methane thandigesters with sludge alone due in part to the lower (morenegative) mean oxidation state of carbon in fats as comparedto carbohydrates and proteins (Gujer and Zehnder, 1983). Theequation below by Gujer and Zehnder describes an empiricalrelationship between the mean oxidation state and COD/TOClevels.

Mean oxidation state = 1.5 ×[

CODTOC

]− 4,

where COD, chemical oxygen demand and TOC, total organiccarbon degraded (Gujer and Zehnder, 1983). Primary municipalsludge is generally composed of lipids (10–21%), cellu-lose (18–32%), protein (17–29%), volatile acids (4–6%), andash (20–27%) (O’Rourke, 1968; Eastman and Ferguson, 1981;Higgins et al., 1982), while activated sludge is composedof lipids (5–12%), cellulose (7%), protein (32–41%), and ash(25–41%), percent dry matter (US EPA, 1979). Recent researchperformed by Xia and de los Reyes (submitted) shows thatGTW contains 900–1400 mg/L and 3700–4100 mg/L of proteinin the FOG (top) layer and bottom (settled solids) layer, respec-tively. The FOG layer contained about 750 mg/L carbohydrateand the bottom layer contained 2100–3400 mg/L carbohydrate.Multiple studies have shown that the co-digestion of GTWwith primary sludge and thickened waste activated sludge cansignificantly increase biogas production.

4.3. Results from co-digestion studies

Full scale, pilot scale, and lab scale studies of co-digestionof FOG with municipal sewage sludge have been conductedacross Europe, the USA, and Canada (Suto et al., 2006; Bailey,2007; Cockrell, 2007; Davidsson et al., 2008; Kabouris et al.,2008; Parry et al., 2008; Kabouris et al., 2009a,b; Luostarinenet al., 2009; Muller et al., 2010). These studies have reportedincreased biogas production and in one scenario a net reduc-tion in total dewatered biosolids (York et al., 2008). A thoroughinvestigation is necessary to determine the factors thatattributed to the variance in anaerobic co-digestion perfor-mance.

Table 3 provides a summary of full scale GTW anaerobicco-digestion experiments and Fig. 1 and Table 4 provides asummary of lab and pilot scale experiments. Fig. 1 displays acomparison of the total gas production without GTW to gasproduction with GTW at various loading rates defined as % VSby mass FOG. In these tests, sewage sludge that consist of pri-mary and secondary sludges, was first anaerobically digestedwithout GTW addition and then anaerobically digested withGTW under the same process conditions (see Table 4).

The results from full, lab, and pilot scale studies show thatthe addition of GTW causes an increase in the gas produc-tion potential of the digester feedstock. This gas productionincrease, however, may vary drastically depending on the %FOG loading, reactor configuration, mixing intensity, and pos-sibly other variables. Studies 1 and 2 reported an almost 200%increase in digester gas production (Kabouris et al., 2009a,b)while studies 3 and 7 reported a maximum increase in digester
gas production of 13% and 27%, respectively (Parry et al., 2009;Davidsson et al., 2008). It should be noted that studies 1 and 2


Table 3 – Full Scale GTW Co-digestion implementations.

References Location Reactor Loading Rate Response

Bailey (2007) Riverside, CA 28.1–30.4% GTW by volume 81.9% Digester gas increase, 9.5%BTU increase, 5–6% increase inmethane content

Cockrell (2007) Watsonville, CA Average 143,000 gallonsGTW/month

>50% Digester gas increase

Muller et al. (2010) Vancouver,British Columbia

9.5% GTW by volume 32.4% Digester gas increase

uecrot1erebwrsdda

tcvoettOv

D

sed a two-phase reactor configuration with a small volumenabling the reactors to approach theoretical ideal mixingonditions, which are unlikely to be achieved in a full scaleeactor (Kabouris et al., 2008). Studies 3 and 7 were conductedn larger single-phase reactors that may have contributed tohe decreased digester gas production compared to studies

and 2 (Davidsson et al., 2008; Parry et al., 2008). Full scalexperiments were unable to attain the digester gas increaseeported in studies 1 and 2 (Bailey, 2007; Cockrell, 2007; Mullert al., 2010). Full scale loading rates were recorded in % GTWy volume whereas the laboratory and pilot scale experimentsere able to directly measure the VS content of the GTW and

ecord the loading rate based on % VS by mass. Thus, the fullcale data may not be directly comparable to the experimentalata and further research and monitoring would be needed toetermine the GTW volatile solids loading rates to full scalenaerobic digesters.

Kabouris et al. (2009a) lab scale results suggest that whenhe reactor is operated near ideal mixing conditions, theo-digestion of FOG with primary sludge (PS) and waste acti-ated sludge (WAS) demonstrated an increased gas productionf 197%. This increased gas production is higher than thexpected theoretical increase in gas yield when consideringhe individual gas production from FOG, PS, and WAS, usinghe method described in WEF MOP No. 8, 2010 and Parkin and
wen (1986), and assuming a FOG gas yield of 22.8 ft3 biogas/lbolatile solids reduced (Alves et al., 2009). This higher gas
Fig. 1 – Digester gas productata from Parry et al. (2008) is recorded in mL methane/g COD ad

yield may indicate the possible existence of synergistic activ-ity. However, full scale experiments displayed an increase indigester gas production that are closer to estimates basedsimply on the theoretical gas yield. This possible synergis-tic condition has not been previously confirmed as a reasonfor enhanced gas yield and additional research is needed todetermine the conditions for the onset of synergistic activity.

In addition to an increase in biogas production, biogas BTUvalue, and biogas methane content, the co-digestion of FOGwith municipal sludge has also been reported to decreasethe volume of biosolids by 33%, decrease the weight by 28%,and improve the biosolids dewaterability when FOG was co-digested with primary and secondary sludge at a municipalwastewater treatment plant in Millbrae, CA (York and Magner,2009). The anaerobic digesters in this study were operated witha very long solids retention time (38–58 days), which may notbe typical of most municipal wastewater treatment plants.Further research is needed to determine the specific reasonsfor the decreased biosolids volume and the improvement indewaterability to determine if these results are primarily dueto GTW co-digestion.

5. Conclusions and future researchdirection

GTW anaerobic co-digestion with biosolids from municipalwastewater treatment plants has already been recognized as

ion with FOG addition.ded.

240

Process

Safety

an

d En

viro

nm

enta

l Pro

tection

9

0

( 2

0 1

2 )

231–245

Table 4 – Description of FOG co-digestion experiments.

References Source of sludge Source of FOG Initial biogasproduction

Maximumbiogasproduction

% Increasea FOG feedconcentrationat max gasproduction

Reactorconfiguration

Temp.b SRT

Kabouris et al. (2009a) Primary andthickened wasteactivated sludgefrom municipalwastewatertreatment plant.Pinellas county, FL

Polymerdewatered FOGfrom greasehaulers. Sat. fat:(37.9%)C16:0 > C18:0 > C20:0Unsat. fat: (62.1%)C18:1 > C20:1 > C18:2 > C16:1

159 (mL CH4/g VSadded)


197 48% VS Two phase CSTR1 L – acid phase4 L – Methanephase

M Acid phase –1 dayMethane phase –12 day

Kabouris et al. (2009a) Primary andthickened wasteactivated sludgefrom municipalwastewatertreatment plant.Pinellas county, FL

Polymerdewatered FOGfrom greasehaulers. Sat. fat:(37.9%)C16:0 > C18:0 > C20:0Unsat. fat: (62.1%)C18:1 > C20:1 > C18:2 > C16:1



179 48% VS Two phase CSTR1 L – acid phase4 L – methanephase

T Acid phase – 1dayMethane phase –12 days

Parry et al. (2009) Co-thickenedsludge from amunicipalwastewatertreatment plant inRenton, WA

Restaurantgrease

267 (mL Biogas/gCOD added)

302 (mL Biogas/gCOD added)

13 33% VS CSTR – 5 gallons M 25 Days

Suto et al. (2006) 60% thickenedwaste activatedsludge, 40%primary sludgefrom municipalWWTP in Oakland,CA

FOG from 27differentsamples (seeTable 2 for fattyacid profile)

900 mL/h 1740 mL/h 93 35% VS CSTR30 L – semicontinuousfeeding

M 20 Days

Suto et al. (2006) 60% thickenedwaste activatedsludge, 40%primary sludgefrom municipalWWTP in Oakland,CA

FOG from 27differentsamples (SeeTable 2 for fattyacid profile)

870 mL/h 1800 mL/h 106.9 50% VS CSTR30 L – semicontinuousfeeding

T 20 Days

Davidsson et al. (2008) 50% wasteactivated sludge,50% primary sludgefrom WWTP inMalmö, Sweden

Grease trapsludge

325 mL CH4/g VSadded


109 60% VS Batch methanepotential reactor

M nr

Process

Safety

an

d En

viro

nm

enta

l Pro

tection

9

0

( 2

0 1

2 )

231–245

241

– Table 4 (Continued)

References Source of sludge Source of FOG Initial biogasproduction

Maximumbiogasproduction

% Increasea FOG feedconcentrationat max gasproduction

Reactorconfiguration

Temp.b SRT

Davidsson et al. (2008) 50% wasteactivated sludge,50% primary sludgefrom WWTP inMalmö, Sweden

Grease trapsludge



27 30% VS CSTR – batch fedonce per day

M 10–13 Days

Kabouris et al. (2008) Primary andthickenedsecondary sludgefrom WWTP inPinellas county, FL

Dewatered FOGsample



175 41% VS Batchbiodegradabilitytests

M nr

Kabouris et al. (2008) Primary andthickenedsecondary sludgefrom WWTP inPinellas county, FL.Used sludge fromrun 1 as inoculum

Dewatered FOGsample



137 33% VS Batchbiodegradabilitytest (used sludgefrom run 1 asinoculum

M nr

Luostarinen et al. (2009) Sewage sludgefrom a WWTP inMikkeli, Finland

Grease trapsludge from ameat processingplant

278 m3 CH4/VSadded

463 m3 CH4/VSadded

66 46% VS CSTR – 5 LFed once per day

M 16 Days

nr, not reported.a % increase = (biogas production in control reactor/biogas production with FOG) − 100%.b M, mesophilic (∼35 ◦C); T, thermophilic (∼55 ◦C).


an economical and environmentally sustainable method ofFOG disposal and an approach for increasing digester gas pro-duction (Bailey, 2007; Suto et al., 2006; Cockrell, 2007; Schutz,2008; York and Magner, 2009; Kabouris et al., 2009a,b). How-ever, research is needed to provide a better understanding ofthe process, improve process efficiency, and realize the maxi-mum benefits of FOG co-digestion. The following provides anoutline of future research needs.

As a result of the wide range of GTW characteristicsdescribed in Table 1 and the absence of a wide range charac-terization of GTW, a thorough study of GTW characterizationis needed. This characterization should be coupled to theGTW management strategy of municipalities to determine anypotential links between waste management and digestibil-ity of GTW. Municipalities often enforce a grease abatementdevice pump out policy based on frequency (i.e. pump outevery ninety days) or by device capacity (i.e. pump outmust occur before oil and solids layer exceeds 25% of thegrease abatement device contents). Further research is neededto determine if there is an optimal time or grease abate-ment device capacity that will result in the most beneficialco-digestion material. This knowledge would enable munic-ipalities to implement grease abatement device policies thatwould not only improve the operation of the device, but wouldalso result in a higher quality co-digestion material.

Saturated fatty acids are known to degrade via the �-oxidation pathway. However, it is still unknown whetherunsaturated fatty acids must first be completely saturatedbefore degradation can proceed via �-oxidation (Sousa et al.,2009). Additional research is needed to identify the initial stepsin the degradation pathway of unsaturated fatty acids thatmay lead to specific reactor configurations or pretreatmentbased on the saturated and unsaturated fatty acid content ina waste. In addition, a more comprehensive sampling effort isneeded to determine if fatty acids of specific chain length andsaturation are more common to certain restaurants or types ofgrease abatement devices. To date, most of the inhibition testshave been performed on samples of pure free fatty acids, i.e.,oleic, stearic, and palmitic acid or a combination of pure fattyacids in UASB, EGSB, or fixed bed reactors (Koster and Cramer,1987; Angelidaki et al., 1992; Rinzema et al., 1994; Hwu andLettinga, 1997; Alves et al., 2001a,b; Pereira et al., 2003; Shinet al., 2003). Some studies have shown a synergistic effect ofinhibition in which the methanogenic inhibition of multiplefatty acids is more severe than with single fatty acids (Kosterand Cramer, 1987). These studies, however, may not be repre-sentative of the type of GTW nor reactor configuration that amunicipal WWTP is likely to receive. Additional lab and pilotscale experimental tests are needed to determine what effecta mixture of oil, grease, fat, food solids, and detergents willhave on anaerobic co-digestion inhibition and how this inhi-bition may vary with reactor configuration. It has also beenhypothesized that the specific LCFA content of the sludge maydetermine the dominant inhibition mechanisms (i.e. flotation,transport limitation, or toxicity) (Pereira et al., 2004).

Anaerobic co-digestion of FOG with municipal sewagesludge has been reported to increase the volatiles solids reduc-tion (York et al., 2008). The city of Millbrae POTW is theonly GTW co-digestion facility that has reported an over-all decrease in dewatered biosolids production. Additionalresearch is needed to determine if this result is specific to a cer-tain GTW stream or reactor operation, and further, if this result
may be expected at other co-digestion sites (York et al., 2008).Hwu et al. (1998b) also reported that the addition of easily
biodegradable co-substrates such as glucose were a prereq-uisite to increasing the overall efficiency of LCFA degradation.Additional research is needed to determine if the co-digestionof FOG with municipal sewage sludge can be expected toincrease reactor efficiency, and if the increase in efficiencymay be predicted based on the type of reactor and type of co-digested material. Furthermore, it is important to determineif co-digestion of FOG alters the characteristics of the residualsludge biosolids and to identify any necessary modificationsto the post treatment of digested sludge. This knowledge isessential to identify the true cost or benefit of co-digestion.

There is a broad need for more published data from lab,pilot, and particularly full scale anaerobic co-digestion. Thisdata could lead to the establishment of models that mayaccurately predict the impact of adding co-digestion mate-rial on anaerobic digestion processes (i.e. methane production,volatile solids destruction, flotation, inhibition, etc.). As seenin Table 4, there has been a wide range in reported bio-gas production increase due to co-digestion of GTW. Moredata is needed to understand the differences in reactor per-formance and also to identify optimal reactor conditionssuch as % FOG loading, solids retention time, temperature,and reactor configuration. Once this research need has beenaccomplished, anaerobic co-digestion of GTW with municipalsewage biosolids will provide additional options for GTW dis-posal as well as strengthen the wastewater treatment plant’srenewable energy portfolio.

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Alves, M.M., Vieria, J.A.M., Pereira, R.M.A., Pereira, M.A., Mota, M.,2001b. Effects of lipids and oleic acid on biomass developmentin anaerobic fixed-bed reactors. Part II: oleic acid toxicity andbiodegradability. Water Res. 35 (1), 264–270.

Alves, M.M., Pereira, M.A., Sousa, D.Z., Cavaleiro, A.J., Picavet, M.,Smidt, H., Stams, A.J.M., 2009. Waste lipids to energy: how tooptimize methane production from long-chain fatty acids(LCFA). Microb. Biotechnol. 5, 538–550.

Angelidaki, I., Ahring, B., 1992. Effects of free long-chain fattyacids on thermophilic anaerobic-digestion. Appl. Microbiol.Biotechnol. 37 (6), 808–812.

Austic, G., 2010. Feasibility Study: Evaluating the Profitability of aTrap Effluent Dewatering Facility in the Raleigh Area. For ECOCollections through the Biofuels Center of North Carolina, pp.1–21.

Bailey, R.S., 2007. Anaerobic digestion of restaurant greasewastewater to improve methane gas production and electricalpower generation potential. In: Proceedings of the 80thAnnual Technical Exhibition and Conference of the WaterEnvironment Federation, 13–17 October 2007, San Diego, CA,pp. 6793–6805.

Beccari, M., Majone, M., Torrisi, L., 1998. Two-reactor system withpartial phase separation for anaerobic treatment of olive oilmill effluents. Water Sci. Technol. 38 (4–5), 53–60.

Brown, S., Kruger, C., Subler, S., 2008. Greenhouse gas balance forcomposting operations. J. Environ. Qual. 37 (4), 1396–1410.

Canakci, M., Van Gerpen, J., 2001. Biodiesel production from oilsand fats with high free fatty acids. Trans. ASAE 42 (5),1203–1210.

Canakci, M., 2007. The potential of restaurant waste lipids asbiodiesel feedstocks. Bioresour. Technol. 98 (1), 183–190.

Cavaleiro, A.J., Salvador, A.F., Alves, J.I., Alves, M., 2009.Continuous high rate anaerobic treatment of oleic acid based


CC

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