Bioresource Technology 145 (2013) 10–16

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Bioresource Technology

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Batch and semi-continuous anaerobic digestion of food waste in a dualsolid–liquid system

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.03.030

⇑ Corresponding author. Tel.: +86 10 64452756; fax: +86 10 64414268.E-mail address: suhj@mail.buct.edu.cn (H. Su).

Cunsheng Zhang, Haijia Su ⇑, Tianwei TanBeijing Key Lab of Bioprocess Laboratory, Beijing University of Chemical Technology, Beijing 100029, PR China

h i g h l i g h t s

� Anaerobic digestion in a dual solid–liquid (ADSL) system was examined.� The ADSL system overcame several disadvantages of single-step digestion.� The methane production was enhanced by 13.6% in ADSL system with oil separation.� The optimum C/N and lower oil content explain the improved methane production.

a r t i c l e i n f o

Article history:Available online 15 March 2013

Keywords:Anaerobic digestionBiogasWaste oilMethaneFood waste

a b s t r a c t

To avoid the inhibition from both of waste oil and high concentrations of cationic elements, anaerobicdigestion of food waste in a dual solid–liquid (ADSL) system was examined in this present paper. Resultsfrom batch test indicated that a higher methane yield could be obtained in the ADSL system. The methaneyield of food solid waste (FSW), food liquid waste (FLW) and raw food waste (RFW) were 643, 659 and581 mL/g-VS, respectively. In semi-continuous anaerobic digestion, the optimum organic loading rates(OLR) for FSW, FLW and RFW were 9, 4 and 7 g-VS/L/d, respectively. The total methane production ofRFW and ADSL systems, based on 1 kg-VSRFW, were 405 and 460 L, respectively, indicating that the meth-ane production increased by 13.6% in the ADSL system. The optimum C/N ratio, redistribution of metalelement and lower content of waste oil in FSW explain the higher methane production.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction the optimum ratio for anaerobic digestion (Zhang et al., 2013). Sec-

The disposal of food waste (FW) has caused severe environmen-tal pollution in many countries due to the increasing discharge ofFW (Zhang et al., 2011). To tackle the domestic waste crisis, anew regulation was issued by the Chinese government. Non-resi-dential buildings are required to pay $4/ton for disposing of FWand $12/ton for other refuse after 2012, an increase by nearly400% against the previous disposal charges (Zheng, 2012).

Among the current approaches (e.g., incineration, landfill, com-post and anaerobic digestion) for FW disposal, anaerobic digestionis a proven approach for organic waste treatment (Appels et al.,2011). However, it can be affected by various key parameters suchas the C/N ratio, HRT and characteristics of the feedstock (Izumiet al., 2010; El-Mashad and Zhang, 2010; Li et al., 2009; Choet al., 2013; Liu et al., 2009). Among of these parameters, the C/Nratio is one of the important parameters in anaerobic digestion. Re-cent studies reported that anaerobic digestion could be operatedeffectively at a C/N range of 15–20 (Zhang et al., 2013; Kumaret al., 2010). However, for FW, the C/N was sometimes outside

ondly, various cationic elements, including Na+, K+ and others, arerequired for microbial growth: a concentration of e.g., 350 mg Na+/L is considered to be the optimum growth condition for hydro-genotrophic methanogens. But the bacterial activity can also be re-strained by high concentrations of these cations, e.g., moderateinhibition could be caused at concentrations of 3.5–5.5 g Na+/L(Appels et al., 2008). Thirdly, lipids can also be toxic to anaerobicorganisms. The 18-C long-chain fatty acids (LCFAs) such as oleicand stearic acids inhibit the microbial activity at concentrations1.0 g/L (Appels et al., 2008).

Generally, FW includes two parts: a fraction of food solid waste(FSW) and liquid fraction. The solid waste contains complex organ-ic materials, e.g., rice, vegetables, meat and so on. By contrast, theliquid fraction is composed of food liquid waste (FLW) and wasteoil (Heo et al., 2011). Heo et al. (2011) also pointed out that theFLW mainly contains mineral salts, soluble organic materials andsmall particulate matter. The waste oil however mainly comprisesanimal and vegetable derived oils, and is about 5 g/L in FW (Kim,2010). Due to the high content of organic substrates, FW is consid-ered to be a source of energy not waste (Heo et al., 2011; Li et al.,2013). Nevertheless, various drawbacks, such as lower methaneyield and longer digestion time, were observed when FW is directly

Table 1Characteristics of RFW, FSW, FLW, waste oil and inoculums.

Feedstock RFW FSW FLW Wasteoil

Inoculums

pH 4.2 (0.2) 4.4 (0.2) 4.1(0.1)

– 7.5 (0.1)

TS (%, w.b.) 23.1(0.3)

25.7(0.3) 6.8(0.8)

95 (0.1) 3.0 (0.2)

VS (%, w.b.) 21.0(0.3)

23.4(0.3)

5.2(0.4)

94 (0.1) 1.2 (0.4)

VS composition (%a,d.b.)

100 80.9 6.0 13.1 –

Oil (g/L, w.b.) 4.6 (0.5) 0.7 (0.2) 0.2(0.1)

797 (1) –

TOC (%, d.b.) 56.3(1.1)

51.8(2.0)

72.5(0.2)

73.6(1.2)

27.3 (1.2)

TON (%, d.b.) 2.3 (0.3) 2.9 (0.2) 1.3(0.3)

– 22.8 (0.3)

C/N 24.5(1.1)

17.9(0.9)

55.8(1.5)

– 1.2 (0.1)

Na+ (%, d.b.) 3.45(0.2)

2.8 (0.1) 14.7(0.3)

– 0.2 (0.1)

K+ (%, d.b.) 2.30(0.04)

2.1 (0.1) 9.1(0.3)

– 0.2 (0.1)

Mg2+ (%, d.b.) 0.16(0.01)

0.17(0.1)

0.4(0.1)

– 0.2 (0.2)

Ca2+ (%, d.b.) 0.4(0.01)

0.4(0.02)

0.7(0.1)

– 0.3 (0.1)

Fe3+ (ppm, w.b.) 100 (23) 106 (13) 90 (18) – 0.1 (0.1)Mn2+ (ppm, w.b.) 110 (95) 129 (56) 50 (28) – <0.1Zn2+ (ppm, w.b.) 160 (30) 192 (46) 78 (23) – 0.2 (0.3)

C. Zhang et al. / Bioresource Technology 145 (2013) 10–16 11

digested. Therefore, pretreatment (e.g., ultrasonication, heat, acidand physical pretreatment) on FW was conducted to overcomethe deficiencies (Elbeshbishy et al., 2011; Bernstad et al., 2012).However, existing anaerobic technology of FW, without oil re-moval, still resulted in low methane yield, and even in a clottedoil phase in the treatment plants (Heo et al., 2011). It is thereforeimportant to develop an alternative technology to counteract theadverse factors for methane production improvement in the anaer-obic digestion.

The novelty of the present study involves the use of anaerobicdigestion in a dual solid–liquid (ADSL) system, with the aim ofavoiding the inhibition from both waste oil and high concentra-tions of cationic elements. The ADSL system involves the use oftwo digesters for FSW and FLW digestion, whilst the waste oil isseparated from the liquid phase prior to digestion. Several disad-vantages of single-step digestion were overcome, and the totalmethane production was improved in the ADSL system.

To examine the performance of anaerobic digestion of FSW andFLW, batch tests were firstly conducted. The methane yield of FSWand FLW was higher than that of RFW in a batch test, as discussedin Section 3.1. Since the treating capacity of the anaerobic digesterin batch test was lower, the semi-continuous anaerobic digestionwas further investigated. The objectives of this study were: (i)determine the optimum conditions for FSW and FLW in ADSL sys-tem; (ii) identify the key parameters affecting the improved meth-ane production of ADSL system.

a Based on VS of RFW.

2. Methods

2.1. Feedstock and inoculums

The inoculums were collected from a 20-L single-stage continu-ously stirred tank reactor (CSTR) which had been used for anaero-bic digestion of FW for more than one year in the lab. FW wasprovided by the canteen of the Beijing University of Chemical Tech-nology. Each organic substrate was stored in the fridge at a temper-ature of �20 �C. To avoid the influence of thawing on the anaerobicdigestion, each feedstock thawed slowly in the room temperaturebefore feeding. The characteristics of feedstock and inoculumsare shown in Table 1. Average values and deviations of triplicatemeasurements are included.

2.2. Flowchart of ADSL and RFW systems

The flowcharts of the ADSL and raw food waste (RFW) systemsare shown in Fig. 1. In the ADSL system, solid and liquid fractionswere firstly separated by a sieve with 2 � 2 mm lattice. The coarserimpurities of the solid fraction, such as big bones and plastics, weresubsequently manually removed. The organic substrates of the so-lid fraction were milled into small particles (<3 mm) by a mill(SS3300, Waste King in USA). The liquid fraction was stored in abottle for one hour. Thereafter the FLW was obtained throughoil–water separation. Thirdly, FSW and FLW were degraded inFSW and FLW digesters, respectively. In the RFW system, the impu-rities were separated out firstly, and then the RFW was ground intosmall particles (<3 mm) using the same mill as FSW. The RFW wasdirectly fed into RFW digester without any other pretreatment. Thecharacteristics of RFW, FSW, FLW and waste oil were shown in Ta-ble 1. The fatty acid composition of waste oil is shown in Table 2.

2.3. Experimental set-up and procedure

2.3.1. Batch testAnaerobic digestion was carried out in 1-L glass cylinder digest-

ers at mesophilic conditions (35 ± 1 �C, Appels et al., 2008). The ac-

tive volume of each digester was 0.8 L. The inoculums andfeedstock were mixed before being added into digester. All digest-ers were tightly closed with a rubber septa and a screw cap. To as-sure anaerobic conditions, the head space was purged with inertgas (N2) for five minutes. Each digester was shake-mixed manuallyonce a day (Zhang et al., 2007). The organic load of each streamwas set at 8 g-VS/L to keep the concentration of organic substratesin each digester identical. All experiments were performed induplicate and average results are reported.

2.3.2. Semi-continuous anaerobic digestionThe active digestion volume and temperature in the semi-con-

tinuous tests were the same as in batch tests. Table 3 shows theexperimental conditions of the semi-continuous anaerobic diges-tion. The OLR was designed as: 6, 8, 12 and 16 g-VS/L/d for RFWand FSW, and 3, 6, 9 and 12 g-VS/L/d for FLW, respectively, todetermine the optimum OLR of each stream. The correspondingHRT was shown in Table 3. The feedstock of each digester wasfed daily at 10 a.m. All digesters were tightly closed with a rubbersepta and a screw cap. To assure anaerobic conditions, the headspace was purged with inert gas (N2) for five minutes. Before eachdischarge and after feeding, the mixture of sludge and food wastein digesters was fully mixed.

2.4. Biogas and VFA measurements

Biogas was collected by water displacement method. The dailybiogas volume was calculated and transformed into the volume atSTP condition. Biogas samples were analyzed by gas chromatogra-phy (GC – 2014C, Shimadzu in Japan) to determine the CH4 con-tent. The GC applied a thermal conductivity detector (TCD) and astainless steel column of TDX – 01 (packed with carbon molecularsieve, 2 m � 3 mm) for biogas measurements. Argon was used ascarrier gas at pressure of 0.3 MPa and a flow rate of 25 mL/min.The temperatures of injection port, column and TCD were 160,160 and 180 �C, respectively.

Fig. 1. Flowcharts of RFW and ADSL systems.

Table 2Fatty acid composition in waste oil.

Fatty acid Molecular formula Mass composition (%)

Lauric acid C12:0 4.3Palmitoleic acid C16:0 9.2Stearic acid C18:0 7.1Oleic acid C18:1 39.3Linoleic acid C18:2 33.4Linolenic acid C18:3 2.9Eicosaenoic acid C20:0 1.5Eicosanoids C20:1 1.2

Table 3Experimental conditions and results in semi-continuous tests.

Digestion Feedstock OLR(g-VS/L/d)

HRT(d)

VFA(g/L)

InitialpH

FinalpH

Ammonia(mg/L)

S1 FSW 6 31.2 3.3 7.4 7.2 578S2 FSW 8 23.4 5.6 7.3 7.1 676S3 FSW 12 15.6 7.3 7.2 6.9 735S4 FSW 16 11.7 11.6 7.3 6.8 568L1 FLW 3 13.9 2.7 7.4 7.5 412L2 FLW 6 6.9 4.8 7.4 7.2 456L3 FLW 8 4.6 8.2 7.3 6.8 367L4 FLW 12 3.5 14.5 7.3 4.9 189R1 RFW 6 28.0 4.6 7.5 7.3 513R2 RFW 8 21.0 7.8 7.3 7.0 635R3 RFW 12 14.0 14.3 7.4 4.7 221R4 RFW 16 10.5 15.4 7.4 4.5 135

12 C. Zhang et al. / Bioresource Technology 145 (2013) 10–16

The VFAs of the effluent were also detected by GC (GC – 2014C,Shimadzu in Japan), using a hydrogen flame ionization detector(FID) and a capillary column (DB-WAX, 30 m � 0.32 mm, Agilent).Nitrogen was used as carrier gas at a pressure of 0.4 MPa. The flowrates of nitrogen, hydrogen and air were 50, 30 and 500 mL/min,respectively. The temperatures of the injection port and FID wereboth 250 �C. The temperature programming of the column was:95 �C during 2 min; increase to 210 �C at a rate of 30 �C/min; keepat 210 �C for 2 min; increase to 230 �C at a rate of 30 �C/min; andmaintain at 230 �C during 3.5 min.

2.5. Chemical analysis

The total carbon was determined by a TOC analyzer (TOC-V, Shi-madzu in Japan) and the total nitrogen was determined by KjeltecNitrogen Analyzer. The oils were extracted by n-hexane, and de-tected by GC (Li et al., 2010). Fat was measured by Soxhlet extrac-tion. The metal elements including K, Na Mg, Ca, Fe, Mn and Znwere analyzed before digestion by AAS (Varian SpectrAA55 – B,Palo Alto, USA). The pH, TS and VS were measured according tothe standard methods (APHA, 1998).

3. Results and discussion

3.1. Batch tests

The characteristics of the feedstock influence the performanceof anaerobic digestion. Fig. 2 shows the cumulative methane pro-duction and the daily methane production rates of RFW, FSWand FLW in batch tests. Fig. 2A indicates that the cumulative meth-ane productions of both FSW and FLW, after 17 d of digestion, werehigher than that of RFW. The total methane production of FSW,FLW and RFW were 643, 659 and 581 mL/g-VS, respectively, indi-cating that the methane yield was improved when food waste wasdigested in ADSL system. Fig. 2B shows the daily methane produc-tion rate in batch tests. Results indicated that the methane produc-tion rates increase and then decrease rapidly at the initial 2 d ofdigestion. During the initial 2 d, the methane production rate ofFLW was higher than the rates of FSW and RFW. No significant dif-ference on the methane production rate of the three substrates wasobserved in the following digestion periods.

The corresponding VFA is shown in Fig. 3. Fig. 3A indicated thatthe total VFA increased and then decreased rapidly at the initial2 d. The highest VFA concentration was obtained in FLW digester.From Fig. 3B–D, VFAs are mainly composed of acetate, propionateand butyrate. Obviously, the acetate played a dominant role duringanaerobic digestion. Veeken and Hamelers (1999) reported thatVFA are the main intermediary products in biowaste digestion. Itwas concluded that the higher VFA concentration result in thehigher methane production rate in FLW digester.

The FLW contains soluble materials and small particulate mat-ter, while RFW and FSW mainly comprise insoluble organic mate-

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Fig. 2. Cumulative methane yield (A), daily methane production rate (B) of RFW,FSW and FLW in batch tests.

C. Zhang et al. / Bioresource Technology 145 (2013) 10–16 13

rials and high molecular weight compounds (Heo et al., 2011; Ap-pels et al., 2008). During the anaerobic digestion process, solubleorganic materials could be microbiologically degraded rapidly.However, insoluble organic materials and high molecular weightcompounds such as polysaccharides and proteins are firstly hydro-lyzed into soluble organic substrates. Moreover, the hydrolysis stepis considered to be rate limiting step (Appels et al., 2008; Veekenand Hamelers, 1999). It is therefore concluded that the physical–chemical characteristics of each stream was the main reason forthe difference on the methane production rate and VFA concentra-tion during the initial 2 d of digestion.

3.2. Semi-continuous anaerobic digestion

3.2.1. Methane yieldFig. 4 shows the specific and mean methane yield of FSW, FLW

and RFW during semi-continuous anaerobic digestion. Fig. 4Ashows that anaerobic digestion experienced a transient period dur-ing the initial 5 d. During the transient period, the specific methaneyield increased gradually at the OLRs of 6–12 g-VS/L/d; neverthe-less, it decreased at the OLR of 16 g-VS/L/d. After the transient per-iod, a significant difference on the methane yield was observed atdifferent OLR. From Fig. 4D, an increase on the mean methane yieldwas observed with the OLR increase to 9 g-VS/L/d. However, themean methane yield decreased gradually when the OLR was abovethe threshold. Therefore, the OLR of 9 g-VS/L/d was recommendedfor FSW in semi-continuous anaerobic digestion. Fig. 4B and Cshow that a significant difference on the methane yield at different

OLR was also observed in FLW and RFW digesters. An increase onOLR resulted in lower methane yield, and even the failure of anaer-obic digestion. Results of Fig. 4B and C indicated that the criticalOLR for FLW and RFW were 6 and 8 g-VS/L/d, respectively.Fig. 4D also reveals that the methane yield decreased rapidly withthe increase of OLR. Experimental results indicated that lower OLRshould be applied in FLW and RFW digester to obtain higher meth-ane yield. However, lower OLR are corresponding to lower treat-ment capacity of anaerobic digester. Taking the methane yieldand the treatment capacity of anaerobic digester into consider-ation, the optimum OLR for FLW and RFW were determined tobe 4 and 7 g-VS/L/d, respectively. The corresponding methaneyields of RFW, FSW and FLW, at the optimum condition, were405, 540 and 390 mL/g-VS (Table 4), respectively. The results showthat the methane yield of FSW was obviously enhanced in semi-continuous digestion. From Table 1, significant differences on theC/N ratio, trace elements and oil content were observed. The per-formance of anaerobic digestion could be explained by the charac-teristics of feedstock.

Microbes need several nutrient elements for their growth aswell as biogas production, whilst the nutrient balance was also re-quired for anaerobic digestion (Li et al., 2009). The C/N is one of theimportant parameters influencing anaerobic digestion of FW(Zhang et al., 2011; Wu et al., 2010). In general, a C/N range of20–30 was considered to be appropriate for anaerobic digestion.However, recent studies found that the optimum C/N ratio is inthe range of 15–20. Kumar et al. (2010) reported that anaerobicdigestion of FW and green waste could be effectively at the C/N ra-tio of 19.6. Zhu (2007) found that organic substrates could be effec-tively digested at lower initial C/N ratio (20) than at higher C/Nratio (25). From Table 1, The C/N ratio of RFW, FSW and FLW were24.5, 17.9 and 55.8, respectively. Obviously, the C/N ratios of bothRFW and FLW were higher than the optimum C/N ratio reportedpreviously. The higher C/N ratio might be the reason for the lowermethane yield of RFW and FLW. By contrast, the C/N ratio of FSWwas in the range of 15–20. Therefore, the optimum C/N ratio ex-plains the higher methane yield of FSW.

Besides nutrient elements, trace elements were also required byanaerobic microorganisms, because many enzymes and co-en-zymes depend on a minimal amount of certain traces of metalsfor their activation and activity. Zhang et al. (2011) found that add-ing trace elements through co-digestion of FW and cattle manurecould enhance the digestion performance. The concentration oftrace elements is shown in Table 1. Most of trace elements in-creased in FSW because metal elements such as Mg2+ and Fe3+

mainly occurred in high molecular substrates (e.g., protein) whichare in a solid state (e.g., meat and cooked eggs) in FSW. By contrast,the soluble inorganic salts of K+ and Na+ dissolved in the liquidfraction, resulting in high concentration of K+ and Na+ in FLW.The redistribution of metal element might be another reason forthe difference of methane yield of FSW and FLW.

3.2.2. VFA and ammoniaAnaerobic digestion of organic substrate is a complex process

which depends on the coordinated activity of a complex microbialassociation to convert organic substrates into biogas (CO2 andCH4). All the reactions (e.g., hydrolysis, acidogenesis, acetogenesisand methanogenesis) occur simultaneously in single stage diges-ter. Usually, it is considered that hydrolysis is the rate-limiting step(Appels et al., 2008). However, the rates of acidogenesis, acetogen-esis will be higher than the rate of methanogenesis at a high organ-ic load, resulting in VFA accumulation (Nagao et al., 2012). Resultsfrom Table 3 indicated that VFA increase with the increase of OLR,resulting in acidification when the OLR was too high. In a FSW di-gester, the VFA was 11.6 g/L at OLR of 16 g-VS/L/d, correspondingto a final pH of 6.8. However, severe acidification occurred in both

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Fig. 3. Variation of the total VFA (A), acetate (B), propionate (C) and butyrate (D) in batch test.

14 C. Zhang et al. / Bioresource Technology 145 (2013) 10–16

FLW and RFW digesters: the VFA at high OLR increased to 14.5 g/L(L4) and 15.4 g/L (R4), respectively. The corresponding pH de-creased to the value below 5. According to Ma (2005), the optimumpH range for methanogens is 6.8–7.2. It was concluded that theactivity of methanogens was inhibited at such lower pH value,resulting in lower rate of methanogenesis. Thus the methanogene-sis might be the rate-limiting step during biogas production pro-cess at high OLR, contrary to the common hydrolysis ratelimitation. Wan et al. (2011) pointed out that self-recovery wasslow once the acidification occurred in the digester, and nearly40 d were required for full recovery. Therefore, the VFA accumula-tion and the corresponding pH drop might be the main reason forlower methane production at high OLR.

During the biodegradation process, a buffer played an impor-tant role in maintaining the stable environment of digester. Theammonia which was produced during the degradation process ofprotein and amino acids, neutralized acidic materials (e.g., acetate)(Zhang et al., 2013) to weaken to acidification. As shown in Table 4,the corresponding ammonia concentration of the FSW digester, atthe OLR of 6, 8, 12 and 16 g-VS/L/d, were 578, 676, 735 and568 mg/L, respectively, whilst being lower in other digesters (e.g.,the ammonia concentration was 456 mg/L in L2, as shown in Ta-ble 3). It was considered that the buffer capacity of the FSW diges-ter was higher than that of FLW and RFW digesters. From Table 1,the total nitrogen of RFW, FSW and FLW, on dry basis, were 2.3%,2.9% and 1.3%, respectively. The results indicated that the totalnitrogen of FSW was the highest, resulting in the high ammoniaconcentration in FSW digester. Thus the high buffer capacity is rec-ognized as the reasons for the stable condition of FSW digester.

3.2.3. LCFAsThe 18-C LCFAs are another parameter affecting anaerobic

digestion efficiency. They are formed during the degradation offat and lipids and are further reduced to acetate and hydrogenthrough b-oxidation by proton-reducing acetogens. However, theycan be inhibitory to several essential reactions, e.g., degradation ofLCFAs and methanogenesis, due to their toxicity to both syntrophicacetogens and methanogens (Wan et al., 2011). The inhibitioncould also be caused by the limitation of nutrient transport to cellsbecause LCFAs could be adsorbed on the microbial surfaces (Pere-ira et al., 2005). The oil content of RFW, on wet basis, was 4.6 g/L.Moreover, Table 2 shows the waste oil separated from liquid frac-tion is mainly composed of 18-C LCFAs (e.g., oleic acid and linoleicacid). LCFAs such as oleic and stearic acids are inhibitory at concen-trations of 1.0 g/L (Appels et al., 2008). Inhibition from fatty acidsmay have occurred in the RFW system. However, the oil contentsof FSW and FLW were both 0.2 g/L, indicating that most of thewaste oil had been removed from feedstock in the ADSL system.Thus the inhibition caused by LCFAs may have been alleviated inthe ADSL system due to the lower content of waste oil in FSWand FLW. It is therefore concluded that waste oil removal in theADSL system improved the performance of anaerobic digestion.

3.2.4. Comparison of ADSL and RFW systemsBased on 1 kg-VS of RFW, the methane yield and methane pro-

duction of ADSL and RFW systems were compared, as shown in Ta-ble 4. According to the VS composition shown in Table 1, thecorresponding VS of FSW and FSW, based on 1 kg-VS RFW, were809 g and 60 g, respectively. At the optimum OLR, as discussed in

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Fig. 4. The specific methane yield of FSW (A), FLW (B) and RFW (C) and mean methane yield (D) at different OLR during semi-continuous anaerobic digestion.

Table 4Comparison of methane yield and methane production of ADSL and RFW systems.

Item RFW FSW FLW

VSa (g) 1000 809 60Optimum OLR (g-VS/L/d) 7 9 4Methane yield (mL/g-VS) 405 540 390Methane production (L) 405 437 23Systems RFW system ADSL systemSystem methane production (L) 405 460System methane yield (mL/g-VS) 405 529

a Based on 1 kg-VS of RFW.

C. Zhang et al. / Bioresource Technology 145 (2013) 10–16 15

Section 3.2.1, the total methane production of RFW, FSW and FLWwere 405, 437 and 23 L, respectively. Thus 405 L methane could beproduced in the RFW system, and 460 L methane could be obtainedin the ADSL system. The total methane production was improvedby 13.6% in the ADSL system during semi-continuous digestion.

High methane production has been widely reported by the pre-vious researchers. Nagao et al. (2012) obtained a high methaneyield of 455 mL/g-VS at the OLR of 9.2 g-VS/L/d in a CSTR. Linet al. (2011) found that a higher methane yield of 490 mL/g-VScould be obtained by co-digestion food with fruit and vegetablewaste in a 4-L lab-scale continuous stirred-tank reactor. As shownin Table 4, the methane yield of ADSL system was 529 mL/g-VS. Incomparison of the methane yields reported, the methane yield ofthe ADSL system was higher. The results indicated that anaerobicdigestion performance treating with FW could be improved inADSL system.

Despite lower methane yield and lower optimum OLR was ob-tained in FLW digester, it was worth to mention that the VS com-position of FLW in RFW is only 6%, while FSW account for 80.9% ofRFW, meaning that most of the organic substrates (FSW) could beeffectively transformed into biogas. Although the waste oil was notsuggested for anaerobic digestion, it could be used as crude mate-rial in bio-diesel industrialization (Heo et al., 2011).

4. Conclusions

To prevent the inhibition from both of waste oil and high con-centrations of cationic elements, the ADSL system was proposed.Results from semi-continuous digestion indicated that the opti-mum OLR for FSW, FLW and RFW were 9, 4 and 7 g-VS/L/d, respec-tively. The optimum C/N ratio for anaerobic digestion is 17.9. Basedon 1 kg-VSRFW, a methane production of 460 L was obtained in theADSL system. Compared with RFW system, the total methane pro-duction was enhanced by 13.6%. The optimum C/N, redistributionof metal element and lower oil content might be the main reasonsfor the improved methane production.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (21076009), the Program for New Century Excel-lent Talents in University (NCET-100212) and the Project-sponsored by SRF for ROCS, SEM (LXJJ2012-001).

16 C. Zhang et al. / Bioresource Technology 145 (2013) 10–16

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