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Renewable Energy 68 (2014) 304e313

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Renewable Energy

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Biogas production from brown grease using a pilot-scale high-rateanaerobic digester

Pengchong Zhang a,b,1, Che-Jen Lin a,c,*, James Liu c, Pruek Pongprueksa d, Simon A. Evers e,Peter Hart f,**aCollege of Environment and Energy, South China University of Technology, Guangzhou, 510006, ChinabDepartment of Chemical Engineering, Lamar University, Beaumont, TX 77710-10053, USAcDepartment of Civil Engineering, Lamar University, Beaumont, TX 77710-10024, USAdDepartment of Mechanical Engineering, Lamar University, Beaumont, TX 77710-10028, USAeMeridian Bioenergy, Inc., The Woodlands, TX 77380, USAfMeadWestvaco Corporation, Richmond, VA 23219-0501, USA

a r t i c l e i n f o

Article history:Received 29 May 2013Accepted 30 January 2014Available online 3 March 2014

Keywords:Brown greaseBiogasAnaerobic digestionRenewable energy

* Corresponding author. Department of Civil EnBeaumont, TX 77710-10024, USA. Tel.: þ1 409 880 87** Corresponding author.

E-mail addresses: Jerry.Lin@lamar.edu (C.-J. Lin), p1 Current address: Oil & Gas Division, Concept

Siemens Energy Inc., Houston, TX, USA.

http://dx.doi.org/10.1016/j.renene.2014.01.0460960-1481/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Food wastes are typically disposed of in landfills for convenience and economic reasons. However,landfilling food wastes increases the organic content of leachate and the risk of soil contamination. Asound alternative for managing food wastes is anaerobic digestion, which reduces organic pollutionand produces biogas for energy recovery. In this study, anaerobic digestion of a common food waste,brown grease, was investigated using a pilot-scale, high-rate, completely-mixed digester (5.8 m3). Thedigestibility, biogas production and the impact of blending of liquid waste streams from a nearby pulpand paper mill were assessed. The 343-day evaluation was divided into 5 intensive evaluation stages.The organic removal efficiency was found to be 58 � 9% in terms of COD and 55 � 8% in terms of VS ata hydraulic retention time (HRT) of 11.6 � 3.8 days. The removal was comparable to those found inorganic solid digesters (45e60%), but at a much shorter HRT. Methane yield was estimated to be 0.40e0.77 m3-CH4@STP kg-VSremoved

�1 , higher than the typical range of other food wastes (0.11e0.42 m3-CH4@STP kg-VSremoved

�1 ), with a mean methane content of 75% and <200 ppm of hydrogen sulfide in thebiogas. The blending of selected liquid wastes from a paper mill at 10 vol% of brown grease slurry didnot cause significant reduction in digester performance. Using a pseudo-first-order rate law, theobserved degradation constant was estimated to be 0.10e0.19 d�1 compared to 0.03e0.40 d�1 forother organic solids. These results demonstrate that brown grease is a readily digestible substrate thathas excellent potential for energy recovery through anaerobic digestion.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Using biogas as an alternative source of energy is gaining moreattention globally in recent decades [1,2]. There has been anincreasing number of studies performed to evaluate the conversionof waste streams such as animal manure, municipal solid wastes,energy crops, municipal bio-solids and food wastes to biogas [2e5].According to International Energy Association (IEA), there are over9000 anaerobic digesters in operation using these materials to

gineering, Lamar University,61.

eter.hart@mwv.com (P. Hart).ual & Engineering Services,

produce biogas [6]. For instance, w15% of organic wastes are beingconverted annually in Germany [7,8]. The practice of convertingwastes to energy provides a two-fold benefit of environmentalprotection and energy recovery.

Brown grease (BG) is a mixture consisting of trapped grease,sewage grease, and black grease collected in grease interceptors(traps) of restaurants and food industries [9]. In the United States,there are 1.84 million tons of BG produced every year [10]. Mostcollected BG eventually ends up in landfills. In the US, the landfillcost for BG varies by region and was up to US$110 per metric tonnein 2002 [10]. This results in a very high direct disposal cost. Inaddition, the moisture content in BG can lead to soil and waterpollution, making the soil sterile and unable to support plant life[11]. Because of these drawbacks, the European Union enacted ageneral ban on landfilling organic waste in 2005 [11]. An earlier

Nomenclature

AD anaerobic digesterALK total alkalinityBAL balance tankBG brown greaseBMP biochemical methane potentialCOD chemical oxygen demandCSTR continuous stirred-tank reactorEPA Environmental Protection AgencyFAC facultative tankFC foul condensate, a liquid waste stream from kraft mill

processGPD gallons per dayHRT hydraulic retention time

LCFA long chain fatty acidOLR organic loading rateORP oxidation reduction potentialSPL screw press liquor, a liquid waste stream from paper

millST sedimentation tankSTP standard temperature and pressure (0 �C, 1

atmosphere)TN total nitrogenTP total phosphorousTS total solidsTSS total suspended solidsVFA volatile fatty acidsVS volatile solidsVSS volatile suspended solids

P. Zhang et al. / Renewable Energy 68 (2014) 304e313 305

study suggested that 14 � 106 m3 of methane could be produced inthe US annually by converting the generated BG into biogas [12].This is a substantial amount of renewable bio-energy. Recoveringthe energywhile eliminating thewaste input to landfills yields botheconomic and environmental benefits [13].

Anaerobic digestion is a treatment process capable of pro-ducing biogas from organic wastes. The benefits of anaerobicdigestion include smaller reactor size in terms of organic loading,lower air emissions and a smaller amount of generated sludgecompared to aerobic biological treatment [14]. Greasy wastessuch as BG have been added as a lipid-rich co-substrate in earlieranaerobic digestion studies for sewage sludge [15,16], municipalwastewater [17e19], and the digestible fraction of municipal solidwastes [20,21]. Typically it is blended at 2e50% of the primarysubstrate’s organic loading to improve the biogas yield andmethane content [15e21]. However, higher lipid loading (>50% ofthe substrate) can cause long chain fatty acid (LCFA) inhibitions[15,16,22], scum and foam formation [23] and fat cloggingproblems [17,23]. To our knowledge, there are few studiesdevoted to investigating the degradability and biogas productionusing BG alone.

Biochemical methane potential (BMP) data provide an esti-mate of biogas production from digesting an organic substrateanaerobically. Combining the BMP data and the associated ki-netics of substrate degradation, the waste treatment efficiencyand the cost benefits of an anaerobic digestion process can beoptimized [24,25]. BMP measurements are typically carried out inlab-scale experiments. Although effective and easy-to-control,the estimated BMP can be up to 50% higher than the biogasproduction in full-scale digesters where operational constrainsexists [26]. This leads to a large uncertainty in estimating thebiogas production from commercial-scale operation. Investi-gating methane production in a pilot-scale operation can providea more practical estimate of the energy values of digestedsubstrates.

The objective of this study is to employ a pilot-scale anaerobicdigestion system to assess the methane yield and treatment effi-cacy of BG. Kinetic evaluation was performed to estimate the sub-strate utilization rate constant. Various liquid waste streams froman adjacent paper mill including foul condensate (FC) and screwpress liquor (SPL) were blended as an effort to minimize the wateruse in the feed. The effects of process parameters including sub-strate composition, hydraulic retention time (HRT), organic loadingrate (OLR) and operational changes on the system performance arereported.

2. Materials and method

2.1. Pilot-scale system setup

The high-rate anaerobic digestion system employed in this workcomprises of three CSTRs and a clarifier: a balance tank (BAL), afacultative tank (FAC), an anaerobic digester (AD), and a finalsedimentation tank (ST). The system schematic and the corre-sponding mixing and transferring pumps are presented in Fig. 1.The BAL and FAC are rectangular shaped tanks having an adjustablevolume of 0.2e1.0 m3. AD is a cylindrical tankwith a total volume of7.6 m3 (1.6 m in diameter and 3.8 m in height) with adjustablereaction volumes of 4.3, 5.8 and 7.6m3. The nominal reactor volumein this study was 5.8 m3. It has a Plexiglas window at the top forobserving themixed liquor in the digester. BAL and FACweremixedwith submergedmixing pumps. Mixing in ADwas accomplished byre-circulating the digester liquor. The mixing was checked regularlyby taking suspended solid measurements at different reactorheights to ensure a completely mixing condition. The sedimenta-tion tank (1.5 m3) has a cylindrical shape with a conical bottom at1:1 slope.

The BG feedstock is a solid phase substrate with a high chemicaloxygen demand (COD). Before feeding to BAL, the BG feedstock wasre-suspended in tap water to form BG slurry (water:BG ¼ 1:1 involume) using a hand-held mixing pump. The re-suspended BGslurry was then mixed with tap water and/or co-substrates in theBAL yielding a substrate concentration of 25,000e50,000 mg L�1

COD. After this homogenization process, the substrate was pumpedto FAC for pre-digestion. A commercial facultative culture powder(Meridian Bioenergy) was introduced (at 1:5000 mass ratio ofbioculture to COD) to FAC for bio-augmentation at 12-h HRT. Thepurpose of this process is to initially introduce a series of microbialstrains, degrade possible LCFA and to eliminate potential processinhibitors as reported earlier [15]. Afterwards, the pre-digestedsubstrates were pumped continuously into AD for digestion. TheAD effluent was transferred to ST gravitationally for sedimentation.

2.2. Evaluation schedule

The evaluation period lasted for 343 days. Excluding the systemstart-up, maintenance and feeding transition periods, process datawere collected for 238 days. The evaluation was divided into fiveintensive evaluation periods (IeV). During each operating period, asteady stage (S1 to S5, defined as a state with relatively consistentbiogas production and organic removal) was selected for intensive

Fig. 1. Schematic diagram of the pilot-scale high-rate anaerobic digester.

Table 1Feeding characteristics and reactor configuration during the evaluation. Data were collected in five different periods for analysis.

System start-up I II III IV V

Date 4/13/11e7/26/11 7/27/11e8/7/11 8/8/11e10/24/11 10/25/11e12/7/11 12/8/11e2/29/12 3/1/12e3/21/12Days of operation / 1e12 13e90 91e135 136e217 218e238Days of intensive

evaluationa/ 1e12 (S1) 34e45 (S2) 107e133 (S3) 184e217 (S4) 218e238 (S5)

Sedimentation tank No No Yes Yes Yes YesFeeding BG BG BG BG þ FC BG BG þ SPLInfluent COD (mg L�1) / 34,510 � 2557 56,570 � 3894 26,570 � 6264 33,881 � 9176 30,200 � 1503Influent VS (mg L�1) / 13,965 � 1262 23,937 � 1625 10,139 � 754 13,224 � 3236 13,225 � 1891OLRb (kg-VS m�3d�1) / 2.0 � 0.2 2.7 � 0.3 (2.0 � 0.3) 1.0 � 0.2 (0.8 � 0.2) 0.8 � 0.2 (0.6 � 0.2) 1.2 � 0.3 (0.9 � 0.3)HRT (days) / 7.3 � 0.6 8.9 � 0.9 15.2 � 1.1 15.8 � 1.9 11.0 � 0.1Activity Seeding and

initiatingEstablish BGsteady state

Add ST Establish BG þ FC steady state Back to BGsteady state

Establish BG þ SPLsteady state

a S1eS5 stands for five selected stages with intensive evaluation and stable data consistency.b OLR and HRT were calculated based on AD only. The OLR in the parentheses for S2eS5 were based on AD þ ST.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313306

measurement and data analysis. Table 1 summarizes the evaluationschedule and the corresponding operating parameters in eachstage.

During S1 (day 1e12, Table 1), the systemwas operated withoutST and the AD effluent was considered as the final effluent. Tounderstand the effect of solid recycle, ST was added to the systemduring S2 (day 13e90). During S3eS5 (day 91e238), the systemflow follows the schematics in Fig. 1. The supernatant (overflow) ofST was the final effluent and the underflow was returned to AD. InS3 and S5, FC and SPL was fed respectively as co-substrates toreduce tap water use and to investigate their impact on the systemperformance. The S4 period was used to re-establish baselineconditions between the FC and SPL co-substrate additions.

2.3. Chemical analysis and biogas measurement

Daily liquid samples were taken from five sampling points: BALeffluent, FAC effluent, AD effluent, ST underflow (return sludge) andST overflow (final effluent). Gas samples were drawn from a gassampling outlet at top of AD at least twice a day. Table 2 shows theanalytical measurements and frequencies for each sample.

Table 2Analytical parameters and frequency for each sampling point.

Sampling point After BAL After FAC After AD

Analytical frequencies Daily Daily DailyMeasurements pH, T, OLR,

COD, VFApH, T, DO, ORP, TN, TP,VFA, ALK, TS, VS, TSS, VSS, COD

pH, T, DO, ORVFA, ALK, TS

The pH, dissolved oxygen (DO), oxygen reduction potential(ORP) and temperature (T) weremeasured on site using a calibratedportable meter with appropriate probes (Hach HQ40D with pH101,LBOD101, and IntelliCAL ORP probes) after samples were drawnfrom the reactors. All laboratory measurements were performedwithin 2 h after sampling, therefore no sample preservation wasperformed. The total solids (TS), volatile solids (VS), total sus-pended solids (TSS) and volatile suspended solids (VSS) weremeasured according to EPA Standard Methods 2540 [27]. Dissolvedparameters including dissolved COD (dCOD), total nitrogen (TN),total phosphorous (TP), volatile fatty acids (VFA), total alkalinity(ALK), sulfide and sulfate were measured after the samples werecentrifuged (AccSpin� 400) for 30min at 6000 rpm. These analyseswere performed using EPA approvedmethods (Hach Company DOC316.5, EPA 310.2, 350.1, 353.2, and 365.3, CFR 136.3, 141.5) with atime-lapse heating reactor (Hach DRB200) and a spectrophotom-eter (Hach DR3800). The cumulative volume of produced biogaswas measured continuously using a digital gas flow meter (FILL-RITE Model 820, TTS Corp., IN, USA). The composition of biogas wasmeasured using a portable gas analyzer with an infrared detector(Model GEM 2000, Landtec Inc., Colton, CA, USA). Standard gases

ST recyclingsludge

ST up-flow(final effluent)

Biogas (gas point on AD)

Daily Daily At least twice a dayP, TN, TP,, VS, TSS, VSS, COD

pH, T, TS, VS,TSS, VSS, COD

pH, T, DO, COD Flowrate, content ofCH4, CO2, H2S, and O2

Table 3Substrate characteristics. Brown grease was used as the primary substrate and the other two liquid wastes were used as co-substrates in part of the evaluation.

Parameter Brown Grease (BG)a (m � s, n ¼ 17) Foul condensate (FC) (m � s, n ¼ 11) Screw press liquor (SPL) (m � s, n ¼ 13)

COD (mg L�1) 910,634 � 229,993 2973 � 142 4498 � 2020dCOD (mg L�1) / 2740 � 125 609 � 189TS (mg L�1) 437,778 � 91,348 406 � 104 8768 � 7957VS (mg L�1) 372,111 � 77,646 210 � 14 3742 � 1666VS/TS ratio 0.85 � 0.06 0.53 � 0.1 0.5 � 0.1TSS (mg L�1) / 357 � 58 4048 � 1750VSS (mg L�1) / 339 � 46 1997 � 875VSS/TSS ratio / 0.83 � 0.25 0.49 � 0.06Alkalinity (mg L�1 as CaCO3) / 205 � 50 /pHb 6.51 � 0.77 9.28 � 0.18 8.44 � 0.83TN (mg L�1)c / 52.2 � 4 2.3 � 0.1TP (mg L�1)c / 0.24 � 0.09 0.41 � 0.04Sulfide (mg L�1) / 52.2 � 20.5 /Sulfate (mg L�1) / <40 /Moisture content (wt%) 56 � 9 / /

a Here BG stands for pre-treated brown grease in solid phase, thus the unit of COD, TS, and VS is mg kg�1.b pH of brown grease was measured by suspending 100 g brown grease in 1 L tap water that has a pH of 8.05 and alkalinity of 55 mg L�1 as CaCO3.c In aqueous phase.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313 307

(50% CO2, 50% CH4, dry air and 1000 ppm H2S) were used for thecalibration of the gas analyzer daily. The methane yield of thesubstrates was estimated as the ratio of the volume of CH4 (at thestandard temperature and pressure, STP) produced in the digesterto the VS of the substrate digested (m3-CH4@STP kg-VSremoved

�1 ).During the five intensive evaluation periods (S1eS5, Table 1),

analyses of four replicates of randomly selected samples for all theabove-mentioned parameters typically had a relative standarddeviation less than 5 % except for solids, which had larger analyticalerrors (<10%). The completeness of data used for process perfor-mance evaluation was over 90%.

2.4. Substrate characteristics

The BG feedstock of this study was obtained daily from a wastemanagement plant in Houston, TX, where the raw BG was screenedto remove large particles, stabilized by lime addition, and thenflocculated and dewatered. The processed BG is a light-brown,sticky, greasy solid with a perished food smell. In different stagesof evaluation, it was emulsified into water at different concentra-tions (Table 1). After re-suspension in tap water, the slurry has aneutral pH range (6.5e7.5), suitable for biological treatment such asanaerobic digestion. No nutrients were added because of the

Table 4Anaerobic digestion operating parameters and system performance in five selected stage

Stages I II III

pH 7.34 � 0.05 / 7.12 �T (�C) 36.0 � 0.7 36.3 � 0.7 34.3 �DO (mg L�1) 0.01 � 0.00 / 0.06 �ORP (mV) �209 � 14 �228 � 24 �243TN (mg L�1) 591 � 83 409 � 37 237 �TP (mg L 1) 3.4 � 2.4 1.5 � 0.4 0.9 � 0Alkalinity (mg L�1 as CaCO3) 3087 � 282 / 1455 �VFA (mg L�1 as HAc) 274 � 97 / 199 �COD removal efficiency (%) 42.1 � 6.7 50.6 � 5.8 73.8 �VS removal efficiency (%) 26.8 � 7.9b 37.1 � 4.3b 72.7 �CH4 content (%) 74.3 � 2.0 74.6 � 1.0 75.9 �CO2 content (%) 22.3 � 1.3 / 23.9 �H2S content (ppm) 38.2 � 4.1 / 147.2CH4 yield (m3-CH4 kg-VSremoved

�1 ) 0.40e0.49 0.58e0.77 0.49

a Typical value of operating parameters including pH, T, ORP, TN, TP, VFA and alkalinityof methane yield were based on earlier literature.

b For comparison purpose, VS removal efficiency in S1 and S2 has not been corrected

balanced nutrient composition of BG was deemed sufficient forbiological treatment [28].

The characteristics of BG feedstock, FC, and SPL are shown inTable 3. Since the BG has an extremely high organic content(w1 kg-COD kg-BG�1, Table 3), the feeding stream was diluted tothe range of 25,000e50,000 mg L�1 COD. FC and SPL have a rela-tively low COD level and solids content comparedwith BG (Table 3).In addition, their mild alkalinity (Table 3) effectively offset the mildacidity in BG. FC is a liquid substrate with relatively low solidcontent (TS ¼ 400 mg L�1), its major organic content is in thedissolved phase (dCOD is >90% of total COD, Table 3). SPL has a TScontent less than 1.0 wt%. Its dCOD concentration is <20% of totalCOD concentration (Table 3), indicated the major organic content isin the solid phase.

3. Results and discussion

3.1. Performance of anaerobic digester

The operating parameters of AD and system performance duringthe five intensive study periods (S1eS5, Table 1) are summarized inTable 4. For comparison, the typical values for anaerobic digestionsystems are also included. During the evaluation, the pH in AD was

s.

IV V Typical rangea

0.08 7.10 � 0.07 7.01 � 0.17 6.5e8.5 [52]1.8 34.3 � 2.1 37.9 � 1.0 35e40 [52]0.04 0.15 � 0.05 0.10 � 0.03 /

� 40 �247 � 37 �263 � 23 m400w�150 [53]74 314 � 50 306 � 46 60e1000 [54].4 2.3 � 1.1 2.2 � 0.4 6e50 [54]457 2478 � 291 2204 � 222 1500e5000 [55]

76 394 � 84 469 � 378 <1800 [53]11.0 61.7 � 12.3 53.5 � 8.7 /7.4 57.9 � 13.2 56.4 � 9.9 /1.9 74.6 � 1.8 75.4 � 1.0 /1.9 25.2 � 1.8 24.2 � 1.0 /� 34.8 185.2 � 28.1 371.7 � 127.6 /

0.48 0.45 0.11e0.42 [3,24,44e50]

were based on the description of typical anaerobic digestion systems. Typical values

by biomass calculation.

Fig. 2. VFA concentrations before and after AD. The alkalinity in AD is also shown.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313308

controlled at neutral range (7.01e7.34, optimal pH 6.9e7.6 foranaerobic digestion) and the temperature was maintained in themesophilic range (34.3e37.9 �C). The low DO concentration (0.05e0.20 mg L�1) and ORP value (<�200 mV) indicated strictly

Fig. 3. Concentrations of (a) COD, and (b) VS before and after AD. Their removal efficien

anaerobic conditions were maintained during the evaluation. TNand TP concentrations in the system were 230e600 mg L�1 as ni-trogen and 1e4 mg L�1 as phosphorous, indicating adequate ni-trogen but possible phosphorus deficiency. VFA concentration inADwas generally lower than 500mg L�1 as acetic acid (HAc) exceptduring the maintenance period (Fig. 2). The average total alkalinitywas 2122 mg L�1 as CaCO3, suggesting absence of in vitro VFAtoxicity [29,30]. These operational parameters were controlled inthe typical range of healthy digesters (Table 4).

The pre-digestion process in FAC effectively eliminated thenegative effects of the lipid-rich substrate. Earlier studies have re-ported that fat scum foaming by greasy substrates may be a concern[31,32]. However, no significant scum formation was observed inthe AD during the evaluation. The surface fat scum was rapidlydiminished within an hour in FAC [33], possibly because of theeffectiveness of the pre-digestion process breaking down LCFAsthat form scum. Perle et al. (1995) conducted a lab-scale experi-ment of dairy wastewater and noted a similar result [34].

The pre-digestion process also produced VFAs that acceleratedthe anaerobic digestion process. To verify the VFA productionthrough the bio-augmentation process, a facultative experimentusing diluted BG slurry (10,000 mg L�1 COD) with the commercialfacultative culture was conducted in a lab-scale reactor (0.5 L).Experimental results show that after 12-h of pre-digestion at roomtemperature, the pH was decreased from 7.03 to 5.55, and VFAconcentration was increased from 148 to 293 mg L�1 as HAc. In thepilot-scale system, the facultative pre-digestion efficiently

cies are shown on the secondary axis. The five selected stages (S1eS5) are marked.

Fig. 4. Volatile fractions of (a) VS/TS and (b) VSS/TSS in FAC and AD.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313 309

augmented VFA generation for methanogenesis in AD. Fig. 2 showsthe VFA level in FAC and AD as well as the total alkalinity in AD. TheFAC effluent has about 2 times higher VFA concentrations (600e1800 mg L�1 as HAc, Fig. 2) compared to the FAC influent. Theaverage VFA concentration decreased from w800 mg L�1 in FAC tow413 mg L�1 as HAc in AD (Fig. 2).

Fig. 3 shows the COD and VS variation and removal efficiencyduring the five intensive evaluation periods (S1 to S5, Table 1). In S1,only BG was fed to the AD at a mean daily flow rate of 0.568 m3 d�1

(150 gallon per day, GPD) without recycling of anaerobic sludge.The COD in the influent and effluent was w33,000 mg L�1 andw20,000 mg L�1 respectively. Since the organic content of effluentis too high for the subsequent aerobic treatment, a ST was added onday 13 to remove the solid content of the digester effluent and toincrease the anaerobic biomass in AD with sludge recycling. In S2,0.151 m3 d�1 (40 GPD) of the anaerobic sludge was recycled to AD.The total daily flowwas slightly reduced to 0.503m3 d�1 (133 GPD),i.e., the feeding streamwas reduced to 0.341-0.378m3 d�1 (90e100GPD). After the modification, the influent and effluent organiccontents in S1 and S2 were compared.

In S2, the overall feed concentration to AD was higher (45,000e60,000 mg L�1 COD) than S1 (30,000e40,000 mg L�1 COD) becauseof the recycled sludge (Fig. 3). The mean influent and effluent VSconcentrations were 13,965 and 9448mg L�1 in S1. In S2, due to theadded organic load of recycled sludge, the mean influent VS

increased to 23,937 mg L�1 and the effluent VS increased to12,078 mg L�1. During S1 and S2, the OLR and HRT were calculatedbased on the flow into and out of AD. Stage I used a lower con-centration (w14 g-VS/L) and a higher flowrate (150 GPD), whileStage II used a higher concentration (w24 g-VS/L) and a lowerflowrate (90e100 GPD). This resulted in w30% increased of OLR(from 2.0 to 2.7 kg VS m�3 d�1) in Stage II (Table 1). Since thefeeding flow rate in S2 was reduced from 0.568 m3 d�1 to0.503m3 d�1, The HRTof S2 increased to 8.9 days comparedwith S1(7.3 days, Table 1). The COD and VS removal of S2 was slightlyhigher than S1 (Table 4), possibly because of the increased amountof biomass in AD which improved the digester performance.

Starting from day 91 (S3), the data analysis included both ADand ST as a digestion system. During S3eS5 (day 91e238), theinfluent COD concentration was kept at w26,500 mg L�1 (Table 1)and the influent VS was in the range of w11,000 mg L�1. The flowrate in S3 and S4 was reduced to 0.246e0.322 m3 d�1 (65e85 GPD,HRT 15e16 days). The flow rate in S5 was maintained at0.397 m3 d�1 (105 GPD, HRT 11 days, Table 1). During these periods,the ST overflow had a relatively consistent COD and VS concen-tration (COD w 10,000 mg L�1 and VS w 3500 mg L�1, Fig. 3). Thisimplies that, the effluent reached a stable level to be treatedaerobically after ST. Similar results for palm oil mill effluent werereported by Basri et al. (2010) [35].

FC and SPL were introduced as co-substrates at 10 vol% duringS3 and S5, respectively. The addition of the co-substrates did notimpose significant impact upon the methane yield (Table 4).Compared to the periods feeding BG only (S4), the COD and VSremoval efficiency of S3 increased from w60% to w74% (COD,Table 4 and Fig. 3) and w55% to w73% (VS, Table 4 and Fig. 3)respectively. This is probably because the organic content of FC isrelatively easy to degrade (>80%) [36e38]. SPL did not appear tosignificantly affect the COD and VS removal efficiency (S5, Fig. 3).

To understand the organic removal in the system at differentstages, the volatile ratios of solids (VS/TS and VSS/TSS) before andafter AD were monitored. The volatile ratio is an indicator oforganic degradation and of inorganic accumulation since theinorganic mass fraction of solids is conserved during digestion[39]. Fig. 4 shows the VS/TS ratio (Fig. 4a) and VSS/TSS ratio(Fig. 4b) in FAC and AD, respectively. Before AD, the volatile ratiofor BG was in the range of 0.70e0.90. After AD, the volatile ratiowas reduced to the range of 0.60e0.70, suggesting the organiccontent of the feed has been effectively removed in the digester.During S3eS5, the addition of co-substrate also affected the vol-atile ratio, and the varying tendency was in accordance with CODremoval efficiency (Figs. 3 and 4). During the entire operatingperiod, the average COD and VS removal efficiency were 58% and55% (Table 4), lower than those previous reported for lipid-richwastewaters (w90%) [40] but comparable to those of typicalsolid digesters (45e60%) [41].

Fig. 5 shows the scatter plot between the organic removal andorganic loading rate in terms of VS (Fig. 5a) and COD (Fig. 5b).During S1 and S2, the COD and VS removal efficiency were notsignificantly affected by OLR variation, resulting in a linear in-crease of VS and COD mass digestion with respect to the appliedOLR (Fig. 5). However, the organic removal efficiency in S2 wasgreater than that in S1, as reflected by the change of COD and VSin the feed and digester effluent (Fig. 3). S3 to S5 had the similartrend of linear increase of digested COD and VS mass with respectto the OLR, indicating that the system should have a highertreatment capacity than the OLR range applied during theevaluation.

The daily biogas production during the evaluation is summa-rized in Fig. 6. In S1, the biogas production was 5e6 m3 d�1. Thebiogas production was higher in S2 (w7 m3 d�1) because of the

Fig. 5. Scatter plots between organic loading rate (OLR) and organic removal in terms of (a) VS and (b) COD. The points in the frames show the data during S1 and S2 when theoperational parameters were different from those in S3-S4.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313310

higher organic load and recycled biomass (Fig. 5a). During S3eS5,the average daily biogas productionwas lower than S1 and S2 sincethe system OLR was reduced (Table 1). In S3, the COD removal ef-ficiency was higher than S4 and S5 (Table 4), leading to higherbiogas production (w5.6 m3 d�1) compared to that in S4 and S5(w3.5 m3 d�1). The easily digested dCOD in FC accounted for thehigher biogas production in S3. The lower biogas production in S4and S5 (as compared to that of S3) was caused by the lower OLRapplied (S4, Table 1) and the slightly lower organic removal (S5,Table 4), respectively. Generally, the biogas production trend in S3eS5 was consistent with the organic removal shown in Fig. 3, sug-gesting that the biogas production was not significantly affected bythe addition of co-substrate.

3.2. Methane yield and kinetic analysis

The produced biogas had a consistently high CH4 content(w75%, Table 4). The other major gas (CO2) consisted of the otherw25% by volume (Table 4) and trace gases (e.g., H2S). From day160e175, the system was recovered from system maintenance andthe methane content built up from 40% to 75% quickly (Fig. 6).During the entire evaluation, the average H2S concentration was189 ppm, significantly lower than the level that may cause H2Stoxicity (w1500 ppm) [42].

The cumulative CH4 production and digested VS in S3 to S5 areshown in Fig. 7. The methane yields of S3 to S5 were calculated asthe ratio of the two slopes. The value was reported based on VS

removal because the organic content of the BG feed was mainly inthe suspended solid phase. The methane yield of BG in S3 to S5 wasconsistently in the range of 0.45e0.49 m3-CH4 kg-VSremoved

�1 (All gasvolumes mentioned hereafter have been normalized to STP).

For the first two stages (S1 and S2), the apparent VS removalefficiency (27e37%, Table 4) was significantly lower than in S3eS5(55e75%, Table 4) because ST had not been introduced to thesystem. Under this condition, the effluent VS during S1 and S2contains substantial amount of biomass produced from theanaerobic digestion of brown grease. During S3 to S5, ST was usedto collect and recycle most of the generated biomass back to theAD, resulting in the higher apparent BG removal efficiency. Toestimate the BG conversion into biogas during S1 and S2, a massbalance on solids before and after the AD was performed asfollowed:

ð1� f ÞF ¼ ð1� aÞX þ ð1� bÞY (1)

aX þ bY ¼ M (2)

Eq. (1) represents the mass balance of the fixed (inorganic)solids. Where f is the volatile fraction of influent BG substrate ob-tained from measurement (0.808 in S1 and 0.816 in S2), F is themass flow of influent total solid (kg d�1). a and b are the volatilefraction (VS/TS) of biomass and undigested BG substrate, respec-tively (a ¼ w0.80 [43]). X and Y are the mass flow of biomass andundigested BG (kg d�1). Eq. (2) represents the VS composition in

Fig. 6. Measured daily biogas production and CH4/CO2 content.

Fig. 7. Cumulative CH4 production at STP and cumulative VS digested during fiveselected stages (S1eS5). The slopes of each linear stage were used to calculate corre-sponding CH4 yield. In S1 and S2, the mass of digested VS was corrected by biomasscalculation.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313 311

the effluent, whereM is the mass flow of VS in the effluent (kg d�1).Using the solid measurements, we estimated that the generatedbiomass constitutes 25e40 wt% in the effluent.

Based on the mass balance results, the cumulative CH4 pro-duction and the digested VS during S1eS5 were plotted in Fig. 7.The methane yield was then calculated as the ratio of the slopes of

Table 5Comparison of reported and calculated first order degradation rate constants and metha

Substrate Reactor type Pseudo-first-o

Brown grease CSTR 0.10e0.19Municipal sludge CSTR 0.175Municipal sludge Batch 0.2e0.4Corn stover Batch /Rice straw /Canary grass CSTR 0.03e0.04Sunflower oil cake Batch /Winter wheat Batch /Waste activated sludge Batch /Waste activated sludge þ fatty wastewater Batch /Synthetic kitchen waste Batch /Synthetic kitchen waste þ municipal grease waste Batch /

the two lines in the respective period. The estimated methane yieldin S1 (0.40e0.49m3-CH4 kg-VSremoved

�1 ) was comparable to the yieldin S3 to S5 (0.45e0.49 m3-CH4 kg-VSremoved

�1 ). S2 has a highermethane yield (0.58e0.77 m3-CH4 kg-VSremoved

�1 ) because of theshorter hydraulic retention time and higher organic loading(Table 1) in the presence of recycled biomass. Under such condition,only the readily degradable fraction of the substrate in the feed wasdigested, leading to a higher methane yield and lower organicremoval efficiency (Table 4). In S3eS5, the lowered organic loadingand higher HRT improved the organic removal at the cost ofreduced methane production. In practice, the mode of processoperation will depend on the treatment objective (better organicremoval or higher methane yield). Also, the added co-substrate (FCand SPL) did not adversely affect the methane yield during S3eS5(Table 4).

A pseudo-first-order kinetic model was applied to analyze thesubstrate utilization. Similar approaches have been used earlier[3,24,44e50]. The substrate concentration was calculated based onVS. For a CSTR at steady state, the effluent concentration (C) can beestimated as:

C ¼ C01þ kq

(3)

where C0 is the influent substrate concentration (mg L�1 VS), kis the first-order substrate utilization rate constant (day�1)and q is the HRT (d). The estimated k value was in a relativelyconsistent range of 0.10e0.19 d�1 throughout the evaluationprocess.

For comparison, previously reported methane yields of foodwastes and their first-order kinetic parameters are shown in Table 5[3,24,44e50]. Generally, the degradation rate constants were in therange of 0.03e0.4 d�1. The rate constant obtained in this study(0.10e0.19 d�1) had probably been adversely affected by the greaterdifficulty of controlling the digestion conditions (temperature andmixing) in a pilot-scale system due to the ambient temperaturevariation (>15 �C diurnal change). It was slightly lower than that ofmunicipal solid sludge in batch reactors (0.2e0.4 d�1, [51]), com-parable to that of municipal solid sludge in CSTR (0.175 d�1, [44]),and higher than that of canary grass in CSTR (0.03e0.04 d�1, [3]).The measured methane yield of BG in this pilot study ranged from0.40 to 0.77 m3-CH4 kg-VSremoved

�1 . Because of the higher lipid con-tent of BG, the value is substantially higher than themethane yieldsof typical sugary and sludge substrates (0.11e0.42 m3-CH4 kg-VSremoved

�1 , Table 5). The biogas quality produced by BG is excellent(w75%, Table 4), possibly also due to the high lipid content of BG.These pilot-plant data suggest that BG can be effectively digestedanaerobically for high quality biogas production.

ne yields.

rder rate constant (d�1) Methane yield (m3-CH4 kg-VSremoved�1 ) Reference

0.40e0.77 This study0.309 [45]/ [44]0.239 [49]0.225 [50]0.19e0.33 [3]0.107e0.227 [46]0.311e0.360 [47]0.116 [48]0.362 [48]0.117 [24]0.324e0.418 [24]

P. Zhang et al. / Renewable Energy 68 (2014) 304e313312

4. Conclusions

In this study, the anaerobic digestion of waste brown grease(BG) in a pilot-scale systemwas investigated. A mean COD removalof 58% and a mean VS removal of 55% were achieved. The organicremoval efficiency was comparable with those found in typicalsolid digesters. Kinetic analysis showed that the pseudo-first-orderdegradation rate constant of BG was in the rage of 0.10e0.19 d�1.After anaerobic treatment process, the effluent had a consistenteffluent organic strength (COD w 10,000 mg L�1) that can betreated aerobically. It was concluded that BG was a readily digest-ible substrate as a sole substrate.

The pilot-scale system produced biogas of excellent quality (75%CH4 content), with a methane yield in the range of 0.40e0.77 m3-CH4 kg-VSremoved

�1 . The addition of paper mill waste streams (foulscondensate and screw press liquor) as the co-substrates did notadversely affect the methane yield. BG has the industrial potentialto be anaerobically treated as an energy feedstock and there hasbeen ongoing commercial effort to build large-scale digesters usingBG as the primary substrate. Using BG for biogas production couldserve as a profitable model for converting waste to renewableenergy.

Acknowledgment

This study was supported by MeadWestvaco Evadale TX facility(Project No: MWV0001). The authors would like to thank DipendraWagle, Sophia Yang, Yolanda Wang, Brandon Corace and Erik Cor-ace for their assistance in the field work and laboratory analysis.The assistance of Gary Colson, Michael Clapper and Robert Sasser insupporting this work and in obtaining and supplying paper millsamples is greatly appreciated. The administrative support ofStewart Cairns, Reid Sweet and Thomas Sitton are alsoacknowledged.

References

[1] Hilkiah Igoni A, Ayotamuno MJ, Eze CL, Ogaji SOT, Probert SD. Designs ofanaerobic digesters for producing biogas from municipal solid-waste. ApplEnergy 2008;85:430e8.

[2] Nges IA, Escobar F, Fu X, Bjornsson L. Benefits of supplementing an industrialwaste anaerobic digester with energy crops for increased biogas production.Waste Manag 2011;32:53e9.

[3] Kacprzak A, Krzystek L, Pa�zdzior K, Ledakowicz S. Investigation of kinetics ofanaerobic digestion of Canary grass. Chem Papers 2012;66:550e5.

[4] Sivakumar P, Bhagiyalakshmi M, Anbarasu K. Anaerobic treatment of spoiledmilk from milk processing industry for energy recovery e a laboratory to pilotscale study. Fuel 2012;96:482e6.

[5] Lay J-J, Lee Y-J, Noike T. Feasibility of biological hydrogen production fromorganic fraction of municipal solid waste. Water Res 1999;33:2579e86.

[6] Internation Energy Association. Country Reports of Task 37: Energy fromBiogas. http://www.iea-biogas.net/country-reports.html; November 2013[accessed December 2013].

[7] Gallert C, Henning A, Winter J. Scale-up of anaerobic digestion of the biowastefraction from domestic wastes. Water Res 2003;37:1433e41.

[8] Weiland P. Anaerobic waste digestion in Germany e status and recent de-velopments. Biodegradation 2000;11:415e21.

[9] Burgess MN. What to do with brown grease, plumbing systems & design;2010.

[10] Tyson KS. Brown grease feedstocks for biodiesel. National Renewable EnergyLaboratory; 2002.

[11] Murto M, Bjornsson L, Mattiasson B. Impact of food industrial waste onanaerobic co-digestion of sewage sludge and pig manure. J Environ Manage2004;70:101e7.

[12] Dichtl N. Thermophilic and mesophilic (two stage)anaerobic digestion.J Charter Inst Water Environ Manage 1997;11:98e104.

[13] Dearman B, Bentham RH. Anaerobic digestion of food waste: comparingleachate exchange rates in sequential batch systems digesting food waste andbiosolids. Waste Manage 2007;27:1792e9.

[14] Gujer W, Zehnder AJB. Conversion processes in anaerobic digestion. Water SciTechnol 1983;15:127e67.

[15] Noutsopoulos C, Mamais D, Antoniou K, Avramides C. Increase of biogasproduction through co-digestion of lipids and sewage sludge. Global Nest J2012;14:133e40.

[16] Neczaj E, Bien J, Grosser A, Worwag M, Kacprzak M. Anaerobic treatment ofsewage sludge and grease trap sludge in continuous co-digestion. Global NestJ 2012;14:141e8.

[17] Long JH, Aziz TN, Reyes DL. Anaerobic co-digestion of fat, oil and grease (FOG):a review of gas production and process limitations. Process Safety EnvironProtect 2012;90:231e45.

[18] Bouchy L, Perez A, Camacho P, Rubio P, Silvestre G, Fernandez B, et al. Opti-mization of municipal sludge and grease co-digestion using disintegrationtechnologies. Water Sci Technol 2012;65:214e20.

[19] Wan C, Zhou Q, Fu G, Li Y. Semi-continuous anaerobic co-digestion of thick-ened waste activated sludge and fat, oil and grease. Waste Manag 2011;31:1752e8.

[20] Martin-Gonzalez L, Castro R, Pereira MA, Alves MM, Font X, Vicent T. Ther-mophilic co-digestion of organic fraction of municipal solid wastes with FOGwastes from a sewage treatment plant: reactor performance and microbialcommunity monitoring. Bioresour Technol 2011;102:4734e41.

[21] Martin-Gonzalez L, Colturato LF, Font X, Vicent T. Anaerobic co-digestion ofthe organic fraction of municipal solid waste with FOG waste from a sewagetreatment plant: recovering a wasted methane potential and enhancing thebiogas yield. Waste Manag 2010;30:1854e9.

[22] Zhu Z, He Q. Enhancing biomethanation of municipal waste sludge withgrease trap waste as a co-substrate. Renew Energy 2011;36:1802e7.

[23] Valladao ABG, Cammarota C, Freire DMG. Performance of an anaerobic reactortreating poultry abattoir wastewater with high fat content after enzymatichydrolysis. Environ Eng Sci 2011;28:299e307.

[24] Li C, Champagne P, Anderson BC. Evaluating and modeling biogas productionfrom municipal fat, oil, and grease and synthetic kitchen waste in anaerobicco-digestions. Bioresour Technol 2011;102:9471e80.

[25] Angelidaki I, Alves M, Bolzonella D, Borzacconi L, Campos JL, Guwy AJ, et al.Defining the biomethane potential (BMP) of solid organic wastes and energycrops: a proposed protocol for batch assays. Water Sci Technol 2009;59:927e34.

[26] Bishop GC, Burns. RT, Shepherd. TA, Moody. LB, Gooch. CA. Evaluation oflaboratory biochemical methane potentials as a predictor of anaerobic dairymanure digester biogas and methane production. In: ASABE InternationalMeeting, Reno, NV; 2009.

[27] APHA, AWWA, WEF. Standard methods for the examination of water andwastewater; 1998. Washington, DC, USA.

[28] Han S-K, Shin H-S. Biohydrogen production by anaerobic fermentation of foodwaste. Int J Hydrogen Energy 2004;29:569e77.

[29] Dong J, Zhao YS, Hong M, Zhang WH. Influence of alkalinity on the stabili-zation of municipal solid waste in anaerobic simulated bioreactor. J HazardMater 2009;163:717e22.

[30] Vanhaandel AC. Influence of the digested cod concentration on the alkalinityrequirement in anaerobic digesters. Water Sci Technol 1994;30:23e34.

[31] de Nardi IR, Fuzi TP, Del Nery V. Performance evaluation and operatingstrategies of dissolved-air flotation system treating poultry slaughterhousewastewater. Resour Conserv Recycl 2008;52:533e44.

[32] Alexandre VMF, Valente AM, Cammarota MC, Freire DMG. Performance ofanaerobic bioreactor treating fish-processing plant wastewater pre-hydrolyzed with a solid enzyme pool. Renew Energy 2011;36:3439e44.

[33] Santos RB, Hart PW, Colson GW, Evers S, Evers D. Commissioning of a biogaspilot plant: from brown grease to paper mill-generated organic wastes. TappiJ 2012;11:73e8.

[34] Perle M, Kimchie S, Shelef G. Some biochemical aspects of the anaerobicdegradation of dairy waste-water. Water Res 1995;29:1549e54.

[35] Basri MF, Yacob S, Hassan MA, Shirai Y, Wakisaka M, Zakaria MR, et al.Improved biogas production from palm oil mill effluent by a scaled-downanaerobic treatment process. World J Microbiol Biotechnol 2010;26:505e14.

[36] Zhou W, Imai T, Ukita M, Li F, Yuasa A. Effect of limited aeration on theanaerobic treatment of evaporator condensate from a sulfite pulp mill. Che-mosphere 2007;66:924e9.

[37] Norrman J, Narbuvold R, Nystrom L. Anaerobic treatability of waste waterfrom pulp and paper industries. Biotechnol Adv 1984;2:329e45.

[38] Yang MI, Edwards EA, Allen DG. Anaerobic treatability and biogas productionpotential of selected in-mill streams. Water Sci Technol 2010;62:2427e34.

[39] Zhang R, El-Mashad HM, Hartman K, Wang F, Liu G, Choate C, et al. Charac-terization of food waste as feedstock for anaerobic digestion. BioresourTechnol 2007;98:929e35.

[40] Lansing S, Martin JF, Botero RB, da Silva TN, da Silva ED. Wastewater trans-formations and fertilizer value when co-digesting differing ratios of swinemanure and used cooking grease in low-cost digesters. Biomass Bioenergy2010;34:1711e20.

[41] Gunaseelan VN. Anaerobic digestion of biomass for methane production: areview. Biomass Bioenergy 1997;13:83e114.

[42] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobic digestion process: areview. Bioresour Technol 2008;99:4044e64.

[43] Eddy MA. Wastewater engineering treatment and reuse. Mcgraw-Hill HigherEducation; 2003.

[44] Bolzonella D, Fatone F, Pavan P, Cecchi F. Anaerobic fermentation of organicmunicipal solid wastes for the production of soluble organic compounds.Indust Eng Chem Res 2005;44:3412e8.

P. Zhang et al. / Renewable Energy 68 (2014) 304e313 313

[45] de la Rubia Á, Pérez M, Sales D, Romero LI. Municipal sludge degradationkinetic in thermophilic CSTR. AIChE J 2006;52:4200e6.

[46] Raposo F, Borja R, Rincon B, Jimenez AM. Assessment of process control pa-rameters in the biochemical methane potential of sunflower oil cake. BiomassBioenergy 2008;32:1235e44.

[47] Rincon B, Banks CJ, Heaven S. Biochemical methane potential of winter wheat(Triticum aestivum L.): influence of growth stage and storage practice. Bio-resour Technol 2010;101:8179e84.

[48] Carrere H, Rafrafi Y, Battimelli A, Torrijos M, Delgenes JP, Motte C. Improvingmethane production during the codigestion of waste-activated sludge andfatty wastewater: impact of thermo-alkaline pretreatment on batch and semi-continuous processes. Chem Eng J 2012;210:404e9.

[49] Xu F, Shi J, Lv W, Yu Z, Li Y. Comparison of different liquid anaerobic digestioneffluents as inocula and nitrogen sources for solid-state batch anaerobicdigestion of corn stover. Waste Manage 2013;33:26e32.

[50] Song Z, Yang G, Han X, Feng Y, Ren G. Optimization of the alkalinepretreatment of rice straw for enhanced methane yield. Biomed Res Int;2013.

[51] Rincon B, Sanchez E, Raposo F, Borja R, Travieso L, Martin MA, et al. Effect ofthe organic loading rate on the performance of anaerobic acidogenicfermentation of two-phase olive mill solid residue. Waste Manage 2008;28:870e7.

[52] Khanal SK. Anaerobic biotechnology for bioenergy production principles andapplications. 1st ed. Hoboken, NJ: Wiley-Blackwell; 2008.

[53] Gray NF. Water technology: an introduction for environmental scientists andengineers. 3rd ed.. Oxford: Elsevier; 2010.

[54] Alley ER. Water quality control handbook. 2nd ed.. Colombus, OH:WEFþMcGraw-Hill; 2007.

[55] Tchobanoglous G, Burton F, Stensel H. Wastewater engineering: treatmentand reuse. 4th ed. Columbus, OH: McGraw-Hill; 2002.