anaerobic codigestion of kitchen waste with cattle manure for
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Anaerobic Codigestion of Kitchen Waste with Cattle Manure forBiogas Production
Rongping Li,†,‡ Shulin Chen,‡ Xiujin Li,†,* Jam Saifullah Lar,† Yanfeng He,† andBaoning Zhu†
Department of EnVironmental Science and Engineering, Beijing UniVersity of Chemical Technology,Beijing 100029, China, Department of Biological Systems Engineering, Washington State UniVersity,
Pullman 99164, WA, USA
ReceiVed October 11, 2008. ReVised Manuscript ReceiVed January 31, 2009
Kitchen waste (KW), cattle manure (CM), and the mixture of KW and CM were anaerobically digested.The performances of single digestion with KW or CM and of codigestion with KW and CM were investigatedand compared. Two loading rates of 10 and 20 g volatile solid (VS) L-1 were used for KW, CM, and theirmixture digestion, respectively. NaOH was used as supplementary for KW, and sulfuric acid pretreatment wasused for CM to explore the effects of alkalinity and acidification on methane production to verify the roles ofcodigestion. Scanning electron microscopy (SEM) was used to analyze the structural changes of CM fibers.The results showed that the codigestion of KW and CM increased methane yield by 44% as compared to thesingle digestion of KW, and the increase could be attributed to the synergistic effect in the codigestion process.A 32% more methane yield was achieved for the KW with NaOH addition than raw KW, which was due toincreased alkalinity and buffering capacity. The methane yield and VS reduction for acid-pretreated CM were116 and 74% higher than raw CM. SEM analysis showed that the structural changes of CM fibers were helpfulfor methane production. The results showed that codigestion could obtain better and stable performances andmight be one of many options for efficient biogas production.
1. Introduction
In China, disposal of municipal solid waste (MSW) is a majorconcern in large cities. Kitchen waste (KW), which is definedas the food residuals generated from restaurants, cafeterias,hotels, and households etc., is a main organic fraction of MSW,accounting for 37-55% of the total MSW. Anaerobic digestionof organic waste to produce biogas has been regarded as anattractive technology of treating waste biomass, for example,agriculture waste, organic fraction of MSW, as well as organicwaste from food industry.1,2 In terms of high biodegradabilityof KW, it is a typical organic waste suitable for anaerobicdigestion.3 Anaerobic digestion can decrease the amount of KWto be handled while producing bioenergy in methane form.
Anaerobic codigestion of various organic wastes for energyproduction has attracted more interests recently. Codigestiontreats two or more substrates with complementary characteristicsin the same digester, for example, codigestion of fruit/vegetablewaste and animal manure4,5 or organic solid waste with sewagesludge. 6-8 The benefits of codigestion included improved biogas
yield, economic advantages derived from the sharing of equip-ment, and easier handling of mixed wastes, etc.9
The better performance of codigestion was attributed to theimprovement of nutrients balance,10 the enhancement of buffercapacity,11 or the positive synergisms, namely, the combinedeffects established in the digestion medium.12 The synergisticeffect obtained in codigestion has been reported. Mshandete etal.13 reported that codigestion of sisal pulp and fish wastes at avolatile solids (VS) ratio of 2:1 gave an increase of 59-94%in methane yield as compared to that obtained from the digestionof pure fractions. Llabres-Luengo and Mata-Alvarez14 found thatthe ultimate methane yield in the codigestion of straw and pigmanure obtained a maximum value of 0.42 L g-1 VS at 43%VS (straw VS/total VS).
KW contains more readily biodegradable compositions andis easily converted to biogas, but it has a low buffer capacityand is easy to acidify. Usually, buffer materials needs to beadded for achieving stable digestion and better efficiency, whena highly biodegradable substrate such as KW is digested alone.
* To whom correspondence should be addressed. Telephone and fax:+86-10-6443-2281; e-mail: [email protected].
† Beijing University of Chemical Technology.‡ Washington State University.(1) De Baere, L. Water Sci. Technol. 2000, 41 (3), 283–290.(2) Pang, Y. Z.; Liu, Y. P.; Li, X. J.; Wang, K. S.; Yuan, H. R. Energy
Fuels 2008, 22, 2761–2766.(3) Zhang, R.; El-Mashad, H. M.; Hartman, K.; Wang, F.; Liu, G.;
Choate, C.; Gamble, P. Bioresourc. Technol. 2007, 98, 929–935.(4) Callaghan, F. J.; Wase, D. A. J.; Thayanithy, K.; Forster, C. F.
Bioresourc. Technol. 1999, 67, 117–122.(5) Viotti, P.; Genova, P. D.; Falcioli, F. Waste Manage. Res. 2004, 22,
115–128.(6) Romano, R. T.; Zhang, R. Bioresourc. Technol. 2008, 99, 631–637.
(7) Sosnowski, P.; Wieczorek, A.; Ledakowicz, S. AdV. EnViron. Res.2003, 7, 609–616.
(8) Davidsson, Å.; Lovstedt, C.; la Cour Jansen, J.; Gruvberger, C.;Aspegren, H. Waste Manage. 2008, 28 (6), 986–992.
(9) Mata-Alvarez, J.; Mace, S.; Llabres, P. Bioresour. Technol. 2000,74, 3–16.
(10) Yen, H. W.; Brune, D. E. Bioresourc. Technol. 2007, 98, 130–134.
(11) Murto, M.; Bjornsson, L.; Mattiasson, B. J. EnViron. Manage. 2004,70, 101–107.
(12) Gelegenis, J.; Georgakakis, D.; Angelidaki, I.; Mavris, V. RenewableEnergy 2007, 32 (13)), 2147–2160.
(13) Mshandete, A.; Kivaisi, A.; Rubindamayugi, M.; Mattiasson, B.Bioresourc. Technol. 2004, 95, 19–24.
(14) Llabres-Luengo, P.; Mata-Alvarez, J. Resources, ConserVation andRecycling 1988, 1, 27–37.
Energy & Fuels 2009, 23, 2225–2228 2225
10.1021/ef8008772 CCC: $40.75 2009 American Chemical SocietyPublished on Web 03/03/2009
For instance, Llabres-Luengo and Mata-Alvarez14 found thatthe methane yield was significantly improved after addingpowered CaCO3 in batch digestion of wheat straw and pigmanure. Cattle manure (CM) contains more fibers and isrelatively hard to be digested, but it has good buffer capacity.CM might need to be pretreated by acids when high digestionperformance is desired.15 Codigestion of KW and CM wouldcombine together the positive characteristics of both feedstocksand could potentially bring better digestion performances.
The objectives of this study were: (1) to compare theanaerobic digestion performances of KW, CM, and theirmixture; and (2) to investigate the roles of alkalinity andacidification in codigestion.
2. Experimental Section
2.1. Feedstock Characteristics. The KW used in this study wascollected from the restaurants at Beijing University of ChemicalTechnology. The KW consisted of fried vegetables, starches, rice,and meat, etc. It was shredded into slurry state by a food grinderafter the bones, chopsticks, plastic bags, and other inorganicresiduals were removed. The CM was collected from Fuhua BeefCenter located in Dachang County in the southeast of Beijing. Ascraping system was used for manure collection, and the manurewas then stacked in open field. The prepared KW and CM werestored in a freezer at -20 °C for later uses. The characteristics ofKW and CM are shown in Table 1.
2.2. Inoculums. The sludge from a swine waste treatment plant(Shunyi, Beijing) was used as the inoculums. It had a pH of 7.6,total solids (TS) of 85.1 g L-1, VS of 49.6 g L-1, and mixed liquidsuspended solids (MLSS) of 69.7 g L-1. Each digester was seededwith 215 mL of sludge to maintain the mixed liquid suspendedsolids (MLSS) at the level of 15 g L-1 in the digester.2
2.3. Batch Laboratory Tests. The methane yields of feedstockwere determined by laboratory-scale anaerobic batch tests describedin Pang et al.2 The tests were performed in 1 L reactors containingsubstrate of 10 or 20 g of VS (Table 2). R1, R2, and R3 wereoperated with KW, CM, and the mixtures of KW and CM at a VSmixing ratio of 1:1, 10,12 respectively. The same loading rate of10 g VS L-1 was applied for R1, R2, and R3. Loading rate of 20 gVS L-1 was applied for R4, R5, and R6. A 0.82 g portion of NaOH,which was the equivalent alkalinity contained in CM, was addedinto R7 to investigate the effect of alkalinity. The CM was pretreatedby 1% sulfuric acid at a pH of 6.0 for 3 days. After acidpretreatment, the whole liquid was fed into R8 for the batchdigestion.
After inoculation, all batch reactors were sparged thoroughly withnitrogen gas to create an anaerobic condition. The prepared digesterswere then placed in shakers (Taicang DHZ-DA, China) for
anaerobic digestion tests at mesophilic temperature (35 ( 1 °C)and 120 rpm shaking speed. Two replicates for each sample weretested: only one for biogas production and another is for pHmeasurement and other analytic measurements.
2.4. Scanning Electron Microscopy (SEM). Structural changesof different types of CM fiber were studied using a SEM (HitachiS-4700, Japan). The dry samples were mounted on double-sidedtape placed on aluminum stubs. A thin layer of gold (15 nm for 10min sputtering) was sputtered onto the mounted sample to reduceelectron-altering effects using a Hummer V sputtering device(Technics, CA). Finally, the gold-coated samples were observedin a SEM with an accelerating voltage of 20 kV.
2.5. Other Analytical Methods. The daily biogas productionwas recorded by water displacement method,2 and the cumulativebiogas volume and methane yield were calculated after correctionat standard temperature and pressure (STP). Methane and carbondioxide in the biogas were measured by gas chromatography (BeifenSP-2100, China) equipped with a 2 m × 3 mm stainless steelcolumn packed with TDX-01 and a thermal conductivity detector.Temperatures of the detector, injector, and oven were 150, 150,and 120 °C, respectively. TS, VS, alkalinity, and pH weredetermined according to the standard methods.16 The lipids wasdetermined by a Soxhlet system at 65 °C with more than 60 timesof circulation using the petroleum ether as extractive reagent. Thesample weights before and after extraction were used to calculatethe lipids content. Total nitrogen (TN) was analyzed with the totalKjeldahl nitrogen analyzer (Foss 2003, Sweden). The samples forTN were digested at 460 °C for 40 min and then titrated usingH2SO4 after the samples cooling down. Protein was calculated bymultiplying TN by 6.25. Carbohydrate was calculated as the fractionof VS remaining after the subtraction of protein and lipids. Thecontent of cellulose, hemicellulose, and lignin were analyzedaccording to the procedure of Van Soest.17 Total chemical oxygendemand (tCOD) and soluble COD (sCOD, the COD of filtratepassing through a 0.45 µm filter) were measured by using the CODanalyzer (HACH, DR/2500, USA). The degree of COD solubili-zation was calculated by the following equation: 18
(15) Liao, W.; Liu, Y.; Liu, C.; Chen, S. Bioresourc. Technol. 2004,94, 33–41.
(16) APHA. Standard Methods for the Examination of Water andWastewater, 20th ed.; American Public Health Association: Washington,DC 1998.
(17) Van Soest, P. J.; Robertson, J. B.; Lewis, B. A. J. Dairy Sci. 1991,74, 3583–3597.
(18) Kim, J; Park, C.; Kim, T.; Lee, M.; Kim, S.; Kim, S.; Lee., J.J. Bioscience Bioengineering 2003, 95 (3), 271–275.
Table 1. Characteristics of Kitchen Waste (KW), Cattle Manure(CM), and Seeding Sludgea
parameter KW CM seeding sludge
TS (%, wb) 246.6 (0.8) 196.3 (1.2) 85.1 (0.3)VS (%, wb) 232 (0.2) 166.4 (0.7) 49.6 (0.8)tCOD (g O2 L-1) 255.7 (11.1) 274.2 (14.0) 60.1 (1.8)pH 5.1 (0.2) 7.8 (0.1) 7.6 (0.1)alkalinity (g CaCO3 L-1) NA 3.4 (0.2) 44.1 (1.0)carbohydrate (%, db) 55.2 (2.1) 60.4 (4.5) NAprotein (%, db) 15.0 (1.3) 21.8 (4.4) NAlipids (%, db) 23.9 (0.8) 2.6 (0.1) NAcellulose (%, db) NA 21.2 (0.1) NAhemicellulose (%, db) NA 30.4 (0.2) NAlignin (%, db) NA 11.6 (0.0) NA
a Note: NA (no analysis), units were based on wet base (wb) or drybase (db). Data are the means of three measurements, and numbers inparentheses are the standard deviations.
Table 2. Experimental Designs for Anaerobic Batch Digestion
reactors feedstockinitial loading
(g VS L-1)
R1 KW 10R2 CM 10R3 KW and CM at a VS Ratio of 1:1 10R4 KW 20R5 CM 20R6 KW and CM at a VS Ratio of 1:1 20R7 KW + 0.82 g of NaOH 10R8 CM pretreated by diluted sulfuric
acid (pH of 6, 3 days)10
Table 3. Results Obtained from Batch Digestion
digesterinitial
pHfinalpH
cumulativemethane yield
(mL, STP)
methaneyield
(mL g-1 VS, STP)VS reduction
(%)
R1 7.3 7.3 3134 313.4 67.7 ( 0.4R2 7.7 7.3 357 35.7 44.6 ( 0.2R3 7.6 7.2 2986 298.6 52.5 ( 0.3R4 7.2 6.2R5 7.7 7.7 750 37.5 44.0 ( 0.3R6 7.5 7.4 6216 310.8 65.8 ( 0.2R7 8.6 7.4 4584 458.4 74.9 ( 0.2R8 7.7 7.4 770 77.0 77.5 ( 0.2
2226 Energy & Fuels, Vol. 23, 2009 Li et al.
3. Results and Discussion
3.1. Methane Production. Table 3 shows the cumulativemethane production (CMP), methane yields, and VS reductionin batch experiments. Among R1-R6, it was found that R1with KW as single substrate at a loading of 10 g VS L-1
achieved the highest methane yield of 313.4 mL g-1 VS. Thedigestion process in R4 with KW failed. Acidification phenom-enon occurred in R4, as indicated by the lower pH values below5.4 from the third day to the 15th day. The reason was thoughtto be the high loading of 20 g VS L-1 employed, leading toacidification and high food to microorganism (F/M, on VS basis)ratio of 1.9 g VS/g VS. For batch digestion, the F/M ratio is animportant parameter.19 According to the result obtained fromChynoweth et al.,20 F/M ratio of 0.5 g VS/g VS gave maximumconversion rate. The higher the F/M ratio is, the lower themethanogenic activity would be. The F/M ratio in R4 was toohigh and caused the failure of digestion.
However, the acidification of KW at the loading of 20 g VSL-1 could be overcome through adding other supplements. Thiswas verified by the successful operation of codigestion of KWand CM in R6. R6 obtained high methane yield of 310.8 mLg-1 VS at the loading of 20 g VS L-1. No irreversibleacidification phenomenon was found. The reason was believedto be the addition of CM, which not only provided morebalanced nutrients for anaerobic bacteria, but also enhanced thebuffering capacity of the digester. The alkalinity of KW andCM was 0 and 3.4 g CaCO3 L-1, respectively. Adding CM toKW increased the buffering capacity and avoided the occurrenceof acidification phenomenon in R6. Llabres-Luengo and Mata-Alvarez14 investigated the effect of alkalinity in batch digestionand found that the methane yield was significantly improvedafter adding powered CaCO3.
R2 and R5 used CM as single substrate but used differentloading. Both obtained similar results. Their methane yields were35.7 and 37.5 mL g-1VS, respectively, which are lower thanother results.21,22 This might be due to that the fibers were notseparated from the CM. As shown in Table 1, lignin, cellulose,and hemicellulose (LCH) accounted for 63.2% of the total drymatter. The fiber content was higher than that of the manureused by other reserachers.15 Fibers are hard to break down dueto the ester bonds of lignin-carbohydrate complexes; thus,pretreatment is usually required prior to anaerobic digestion.23
Liao et al.24 reported that separating fibers from the raw manurecould enhance methane production and methane content ofbiogas.
3.2. Synergistic Effect. R1 and R2 were loaded with 10 gVS of KW and 10 g VS of CM and obtained the CMPs of 3134and 357 mL (Table 3), respectively. For the same amount ofsubstrate of 20 g VS loaded, the CMP in R6 with mixture ofKW and CM reached 6216 mL, which was 44% higher thanthe total CMP of R1 with KW and R2 with CM. This increasewas attributed to the synergistic effect from the codigestion of
KW and CM. The synergistic effect was mainly due to thecomplementary characteristics of KW and CM when codigested,including increased alkalinity, more balanced nutrients, andimproved biodegradability. Gelegenis et al.13 found that biogasproduction rate in the codigestion of whey with diluted poultrymanure was increased by 40% in a continuously stirred tankreactor. Yen and Brune10 reported that codigestion of wastepaper and algal sludge at a VS ratio of 1:1 increased the methaneproduction rate to 1170 ( 75 mL/day, as compared to 573 (28 mL/day of algal sludge digestion alone, and both operatedat 4 g VS/day, 35 °C, and 10 days HRT.
3.3. Effect of Alkalinity on KW Digestion. The alkalinityof the KW used was 0, indicating that the KW had almostno buffering capacity by itself. This could lead to rapidacidification and low methane yield, as found in R4. Theinhibition from rapid acidification could be overcome byeither codigestion of KW with CM or supplementation ofother materials with high alkalinity.25 To verify the effect ofalkalinity on anaerobic digestion of KW, NaOH was used asbuffer supplementary and was added to enhance bufferingcapacity of KW. R7 was compared with R1 for CMP andmethane yield (Table 3). R7 obtained 4584 mL of CMP and458.4 mL g-1 VS of methane yield, which were 32% higherthan R1. The result indicated that NaOH addition wasbeneficial to enhance the methane production of KW diges-tion and also verified the important role of alkalinity in batchdigestion. It has been reported that the methane yield wassignificantly enhanced after increasing the alkalinity byaddition of CaCO3 in batch digestion of wheat straw and pigmanure.14 Although the better outcome in methane yield wasobtained in R7 with NaOH addition, the chemical additionin industrial application could be a costly process whentreating large amounts of waste. The codigestion of KW andCM seems to be more attractive in terms of operation costand from a viewpoint of integrated solid waste treatment.
3.4. Effect of Acidification on CM Digestion. As mentionedabove, low methane yield was obtained for R2 and R5, whichused CM as the single substrate. This might be due to the highproportional recalcitrant fibers contained in the CM (Table 1).Sulfuric acid was used to pretreat CM in order to decomposethe fibers before anaerobic digestion. It was found that consider-able lignocelluloses were degraded after acid pretreatment. Thetotal LCH, cellulose, and hemicellulose contents were reducedby 13.1, 9.4, and 28% (dry basis), respectively, and the
(19) Nallathambi, G. V. Biomass Bioenergy. 1997, 13 (1/2), 83–114.(20) Chynoweth, D. P.; Turick, C. E.; Owens, J. M.; Jerger, D. E.; Peck,
M. W. Biomass Bioenergy 1993, 5 (1), 95–111.(21) Demirbas, A. Energy Sources, Part A, 2006, 28 (1), 71–78.(22) Møller, H. B.; Sommer, S.G. ; Ahring, B. K. Biomass Bioenergy,
2004, 26, 485–495.(23) He, Y.; Pang, Y.; Liu, Y.; Li, X.; Wang, K. Energy Fuels 2008,
22, 2775–2781.(24) Liao, P. H.; Lo, K. V.; Chieng, S. T. Energy Agric. 1984, 3, 61–
69.(25) Neves, L.; Ribeiro, R.; Oliveira, R.; Alves, M. M. Biomass
Bioenergy, 2006, 30, 599–603.
COD solubilization (%) )Soluble COD after pretreatmentTotal COD after pretreatment
× 100 (%)
Figure 1. Comparison on daily methane production of raw CM andpretreated CM.
Energy & Fuels, Vol. 23, 2009 2227
corresponding contents of the LCH, cellulose, and hemicellulosewere decreased from 63.2, 21.2, and 30.4% to 54.9, 19.2, and21.9%, respectively.
The pretreated CM was anaerobically digested. The dailymethane production of raw CM (R2) and pretreated CM (R8)is shown in Figure 1. There were obvious differences in dailymethane production and digestion time between R2 and R8.The methane production of R2 started in the fifth day andreached a peak of 63 mL on the ninth day, whereas R8 startedafter seeding and reached its biggest peak value of 128 mL onthe third day. The methane started generating later but lastedlonger for R2 as compared to R8. The methane yield and VSreduction of R8 were 77 mL g-1 VS and 77.5%, which was116 and 74% higher than R2, respectively. These results impliedthat pretreated CM became more biodegradable and was moreeasily used by microorganisms after acid pretreatment, thusproducing more methane in less time. It could be further verifiedby the degree of COD solubilization of CM before and afteracid pretreatment. The tCOD of raw CM was 274.1 g L-1. ThesCOD of CM samples before and after acid pretreatment were23.8 and 67.1 g L-1, respectively. The COD solubilization afteracid pretreatment was improved from 8.7 to 24.5%. The increaseof COD solubilization indicated the improvement of biodegrad-ability.
To further explore the effect of acidification on fibersdecomposition, the structure of fibers were observed using SEM.The pictures of raw CM, acid-pretreated CM, and CM codi-gested with KW in the fourth day of digestion are shown inFigure 2, panels A-C, respectively. It can be seen that thetexture of the raw CM was regular and smooth, whereas thestructure of acid-pretreated CM and CM codigested with KWwere rough and partially destroyed. A number of small holeswere observed in the fiber samples of acid-pretreated CM andcodigested CM. The diameters of the holes were in the rangeof 3-8 µm. Liao et al.15 reported that the small holes in themanure fibers meant that part of hemicellulose was degradedfrom the backbone of fibers. The structural changes of fibers inacid-pretreated CM and codigested CM implied, from anotherangle, that acid pretreatment or codigestion with KW would
help hydrolysis of CM and contribute to the methanogensisimprovement.
4. Conclusions
KW and CM can be used as feedstock to produce biogasthrough an anaerobic digestion process. They can be eithersingle-digested or codigested. The synergistic effect was foundin the codigestion process, which contributed 44% more methaneproduction. The addition of NaOH increased alkalinity andbuffering capacity and helped produce 32% more methane yield.The acid-pretreated CM achieved 116 and 74% more methaneyield and VS reduction than raw CM, respectively. SEM analysisshowed that the structural changes of fibers in CM were helpfulto methane production. These findings verified the roles ofcodigestion in helping enhance KW buffering capacity andimprove CM solubilization indirectly. The results showed thatcodigestion of KW with CM might be one of options forefficient biogas production and waste treatment.
Acknowledgment. This work was supported by the fund fromthe Hi-Tech Research and Development Program of China (Grantnumber: 2008AA062401). The authors gratefully acknowledge Dr.Liu Guangqing for his valuable suggestions.
NomenclatureCM ) Cattle manureKW ) Kitchen wasteMLSS ) Mixed liquid suspended solidsMSW ) Municipal solid wasteSEM ) Scanning electron microscopysCOD ) Soluble chemical oxygen demandTAN ) Total ammonia nitrogentCOD ) Total CODTN ) Total nitrogenTS ) Total solidsVFA ) Volatile fatty acidsVS ) Volatile solids
EF8008772
Figure 2. SEM of fibers collected from different types of cattle manure (CM): (A) raw CM fiber; (B) diluted sulfuric acid pretreated fiber (pH of6.0, 3 days); and (C) fiber in the CM codigested with KW in the 4th day of digestion.
2228 Energy & Fuels, Vol. 23, 2009 Li et al.