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International Biodeterioration & Biodegradation 64 (2010) 601e608
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International Biodeterioration & Biodegradation
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Effects of particle size on anaerobic digestion of food waste
Kouichi Izumi a,*, Yu-ki Okishio b, Norio Nagao a, Chiaki Niwa a,c, Shuichi Yamamoto a, Tatsuki Toda a
aDepartment of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Tangi-cho, Hachioji, Tokyo 192-8577, Japanb School of Public and Environmental Affairs, Indiana University, 1315 East Tenth Street, Bloomington, IN 47405, USAc Institute of Technology, Shimizu Co., Etchujima, Koutou-ku, Tokyo 135-8530, Japan
a r t i c l e i n f o
Article history:Received 3 February 2010Received in revised form21 June 2010Accepted 22 June 2010Available online 13 August 2010
Keywords:Anaerobic digestionMethaneParticle sizePretreatmentFood wasteVolatile fatty acids
* Corresponding author. Tel.: þ81 42 691 9455; faxE-mail address: [email protected] (K. Izumi).
0964-8305/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.ibiod.2010.06.013
a b s t r a c t
The objective of this study was to investigate the effects of particle size reduction and solubilization onbiogas production from food waste (FW). To clarify the effects of volatile fatty acids (VFAs) in thedigestion process, the relationship between particle size and VFA accumulation was investigated indetail. For this purpose, substrates of various particle sizes were prepared by bead milling to supporthydrolysis. Batch anaerobic digestion experiments were carried out using these pretreated substrates atmesophilic temperature for a period of 16 days. The results of pretreatment showed that the meanparticle size (MPS) of substrates ground with a bead mill decreased from 0.843 to 0.391 mm, andsolubilization accounted for approximately 40% of the total chemical oxygen demand (total COD) forgrinding pretreatment by bead milling. Anaerobic digestion batch experiments revealed that MPSreduced by bead milling at 1000 rpm improved methane yield by 28% compared with disposer treat-ment. Moreover, this may have increased microbial degradation during the VFA production process withincreasing total number of revolutions (operation time � revolutions per minute). However, excessivereduction of the particle size of the substrate resulted in VFA accumulation, decreased methaneproduction, and decreased solubilization in the anaerobic digestion process. These results suggest thatoptimized reduction of the particle size of the substrate in conjunction with optimized microbial growthcould improve the methane yield in anaerobic digestion processes.
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1. Introduction
In Japan, an ever-increasing amount of food waste (FW) is gener-ated, owing to population growth and rising living standards. Atpresent, the primary method for the disposal of FW in Japanis incineration. Because FW has over 80% water content, extra fuel isneeded to treat FW by direct incineration (Sawayama et al., 1997;Zhang et al., 2005). Anaerobic digestion has been widely studied asan alternative method for the treatment of organic waste such assewage sludge and manure. Since FW has the advantage of highorganic content compared with traditional substrates, anaerobicdigestion is considered to be a feasible alternativemethod to decreasetreatmentcostsandrecover renewableenergy inthe formofmethane.
Anaerobic digestion is amethod of FW treatment that can utilizethe biological processes of many classes of bacteria and generallyconsists of four steps: hydrolysis, acidogenesis, acetogenesis, andmethanogenesis (Speece, 1996; Xu et al., 2002; Li, 2004; Metcalfand Eddy Inc., 2004). In the case of methane fermentation of
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solid organic materials such as FW and waste activated sludge, themethane yield is significantly affected by the mass transfer in eachbiological step, as well as by food availability (Eastman andFerguson, 1981; Li and Noike, 1992). In particular, the hydrolysisand acidogenesis stages are considered to be rate-limiting steps inthe process, since these two steps directly affect both the masstransfer and the food availability in the process (Gomec et al.,2002). To reduce the impact of these rate-limiting steps, pretreat-ment of organic solids with heat (Kim et al., 2003), pressure(Engelhart et al., 1999), ultrasonic irradiation (Shimizu et al., 1992),or mechanical grinding (Nah et al., 2000) is required. These treat-ments accelerate the solubilization (hydrolysis and acidogenesis) ofthe substrate and reduce the particle size of the FW, subsequentlyimproving the anaerobic digestion.
Pretreatment to reduce particle size has two effects: first, if thesubstrate has a high fiber content and low degradability, commi-nution of the substrate increases gas production; second, it can leadto more rapid digestion (Palmowski and Muller, 1999). Smallerparticles increase the surface area available to the microorganisms,resulting in increased food availability to bacteria; thus, anaerobicbiodegradability increases (Mshandete et al., 2006). Sharma et al.(1988) reported the effects of particle size on agricultural and
Table 1Composition of food waste used in this study; the wet weight percentages werebased on Izawa et al. (2001).
Largeclassification
Percentage(% wet weight)
Content in each category(% wet weight)
Rice 4.0 RiceNoodle 2.5 Buckwheat (50.0), spagetti (50.0)Bread 1.7 Bread (100.0)Tea leaves 8.0 Coffee (50.0), oolong (50.0)Vegetables 53.6 Carrot (10.5), spinach (10.1), tomato (8.5),
radish (7.5), cabbage (6.0), onion (5.5),pumpkin (5.0), cucumber (4.5),komatsuna (4.0), Chinese cabbage (2.2),green pepper (2.2), broccoli (2.1),others (31.9)
Fruits 24.8 Citrus (40.5), apple (26.0), banana (8.0),strawberry (4.7), processed fruits (2.5),others (18.3)
Fish 2.7 Saury (100.0)Meat 2.2 Bacon (35.5), chicken (26.5), beef (17.4),
mince (17.0), duck (3.6)Egg shell 0.5 Egg shell (100.0)
Table 2Characteristics of food wastes before grinding and seed sludge.
Parameter Food wastes Seed sludge
Total solids: TS (% wet weight) 16.5 2.9Total volatile solids: TVS (% wet weight) 15.5 1.9TVS/TS (e) 0.94 0.66pH (e) 6.7 7.7Salinity (g l�1) 0.35 0.39Total COD (g-CODcr l�1) 250.5 19.1Soluble COD (g-CODcr l�1) e 0.8Total organic carbon (% dry weight) 42.3 32.4Total organic nitrogen (% dry weight) 3.2 5.6
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608602
forest residues used as feedstock for biogas generation throughanaerobic digestion in batch digesters at 37 �C. Out of five particlesizes (0.088, 0.40, 1.0, 6.0, and 30.0 mm), the maximum quantity ofbiogas was produced from 0.088-mm to 0.40-mm particles. Kimet al. (2000) reported the effects of particle size on anaerobicthermophilic digestion in FW treatment. The maximum substrateutilization rate coefficient doubled with a decrease in the averageparticle size from 2.14 to 1.02 mm, indicating that particle size isone of the most important factors in anaerobic FW digestion.
Smaller particle size increases the surface area available to themicroorganisms. On the other hand, it is possible that it acceleratesthe hydrolysis and acidogenesis steps as well as the production ofsoluble organic materials such as volatile fatty acids (VFAs),resulting in excessively high organic loading in the anaerobicdigestion reactor. The early stages of the anaerobic solubilizationprocess, especially the hydrolysis and acidogenesis steps, aresignificantly affected by physicochemical conditions such astemperature and pH rather than by the effects of biological factors(Cha et al., 1992; Komemoto et al., 2009). In such an overloadingoperation, the imbalance in production and consumption of VFAsleads to accumulation of VFAs and decreased pH, resulting ininhibition of the biogas production rate (Griffin et al., 1998; Kimet al., 2003; Hori et al., 2006; Ward et al., 2008).
Ahring et al. (1995) reported that the accumulation of acetateoccurred faster than that of any other VFA after organic overloadingand that butyrate was a good indicator of organic overloading ina continuous stirred tank reactor. Ahring et al. (1995) also showedthat VFAs at concentrations of 100e200 mM reduced the overallmethane production rate in batch experiments. The inhibition ofmethane production rate by organic overloading leads to problemsof delayed digestion and inefficient treatment in batch operation(Neves et al., 2004). In the case of continuous operation, thereduction of the chemical oxygen demand (COD) removal efficiencyhas been observed as a result of organic overloading (Kim et al.,2002). Therefore, the effects of particle size reduction asa method of pretreatment on the anaerobic digestion processshould be evaluated in order to avoid organic overloading.
A bead mill is a grinding device used to reduce the particle sizeof fine particles and microalgae in pretreatment. This device isexpected to provide smaller particles and higher solubilizationcompared with traditional pretreatment methods and to enablehigher methane production rates. The objective of this study was toinvestigate the effects of particle size on FW soluble COD andmethane production; specifically, particle size was modified bybead milling (BM). Moreover, to clarify the effects of VFAs on thedigestion process, the relationship between particle size and VFAaccumulation was investigated in detail.
2. Materials and methods
2.1. Preparation of FW and seed sludge
The FW used in this study was based on the standard wasteprepared by Izawa et al. (2001), which is based on the compositionof actual food waste in Japan (Table 1). The chemical characteristicsof the collected FW are shown in Table 2. The FW was groundto reduce the particle size by combining two methods: grindingwith a household disposer (Anaheim, KDF55JK, USA: DP) and witha bead mill (Aimex, RMB, Japan). First, 1 l of FW and 1 l of Milli-Q�
water weremixed and groundwith a DP (I-DP), as shown in Table 3.Next, 150 ml of I-DP was ground by BM in the presence of 150 ml ofMilli-Q� water and 300 ml of glass beads (Aimex, 0.71e1.00 mm).Six types of pretreated substrates were prepared by changing thetotal number of revolutions of the BM between 300 and 40,000revolutions by varying the operation time and number of
revolutions per minute (II-300, III-1000, IV-1000, V-4000, VI-20000, and VII-40000; Table 3).
Mesophilic anaerobic sewage sludge was collected from theHokubuSludgeTreatmentCenter,Yokohama, Japan, in two20-l tanks.The chemical characteristics of this seed sludge are shown in Table 2.
2.2. Anaerobic digestion
Experiments were carried out using various pretreatedsubstrates (I-DP, II-300, III-1000, IV-1000, V-4000, VI-20000,and VII-40000) in mesophilic anaerobic digestion. Eight batchexperiments including a control experiment were conducted induplicate in 2-l glass reactors (working volume: 1200ml) at a meanmesophilic temperature of 37 � 1 �C. Materials were added to thereactor in the following sequence: (1) seeded sludge, (2) substrate(10 g-COD l�1 total reactor volume) and (3) sufficient Milli-Q�
water for a total liquid volume of 1200 ml. The reactors were sealedwith silicone stoppers, and a 1-l aluminum gas pack (GL Sciences,AAK-2) was attached for biogas collection. The batch reactors werekept under constant agitation of 80 rpm using a shaker (Taitec,NR-150, Japan) for a period of 16 days. The 10 sampling points takenfor the 16-day experiment were as follows: 0 h, 12 h, D1, D1.5, D2,D2.5, D3, D5, D10, and D16. Depending on the biogas productionrate, the collection interval varied from once every few hours at thestart of the experiment to once every several days. Means ofduplicates are shown in each table and figure.
2.3. Experimental parameters and analytical methods
Total solids (TS), total volatile solids (TVS), pH, total organiccarbon (TOC), total organic nitrogen (TON), total chemical oxygen
Table 3Operation conditions and characteristics of experiment in this study.
Exp. No. Grinding methods Pretreatment MPSa (mm) Solubilityb (%) SD
Total revolutions (rpm � min) Revolution (rpm) Time (min)
I-DP Disposer e e e 0.888 28.1 �0.6II-300 þ Beads mill 300 300 1 0.843 39.0 �3.3III-1000 þ Beads mill 1000 500 2 0.718 37.3 �2.6IV-1000 þ Beads mill 1000 2000 0.5 0.715 42.1 �3.5V-4000 þ Beads mill 4000 2000 2 0.508 41.4 �1.0VI-20000 þ Beads mill 20,000 2000 10 0.391 39.7 �0.7VII-40000 þ Beads mill 40,000 2000 20 0.393 40.3 �1.0
a Mean particle size.b Solubility was calculated as soluble COD/total COD after filtration through 0.45-mm membrane.
0
0.2
0.4
0.6
0.8
1.0
0 10000 20000 30000 40000
Total revolution (min rpm)
Mea
n pa
rtic
le s
ize
(mm
)
A
0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20 25
Operation time at 2000 rpm (min)
B
I-DP
Beads mill
Fig. 1. Effect of grinding process on mean particle size (MPS). (A) Total revolutionsdenote grinding condition by bead milling. (B) At 2000 revolutions.
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608 603
demand (total COD), soluble chemical oxygen demand (solubleCOD), long-chain fatty acids (LCFAs), volatile fatty acids (VFAs),and particle size distribution of the pretreated substrate anddigestion samples were measured. The TS, TVS, TOC, TON, LCFA,pH, and total COD of pretreated substrates and digestion sampleswere measured before filtration with a combusted 0.45-mm glassfilter (Advantec, GC-50). We measured TS and TVS according tosewage analysis methods of the Japan Sewage Works Association(1997). The pH of all batch reactors was measured using a pHmeter (Horiba, B-212). By catalytic oxidation, TOC and TON weremeasured on an elemental analysis instrument (Fisons, EA1108).At the start of the experiment, LCFAs (oleate and stearate) in theloaded substrate were analyzed using the off-line tetramethy-lammonium hydroxide (TMAH)eGCeMS method. The GCeMSanalyses were carried out on an Agilent 6890N-GC/5973MSDsystem equipped with a DB-5MS capillary column (Agilent,30 m � 0.25 mm; film thickness: 0.25 mm). Chromatographicseparation was achieved by using the following temperatureprogram: 60 �C (1 min isothermal), raised at 6 �C min�1 to 310 �C(20 min isothermal). Helium was the carrier gas at 1.90 ml min�1;the injector temperature was 300 �C in spilt injection mode for1 min. Mass spectra were obtained in El mode (70 eV), scanning inthe range of m/z 50e750. Compound identification was based oncomparison of mass spectra with the NIST02 and Wiley7n librarydatabases, published spectra, and purchased compounds used asstandards. The values of total COD, soluble COD, and VFAconcentration of the resultant filtrate were quantified in accor-dance with standard methods of the American Public HealthAssociation (APHA, 1998). The VFAs (acetic acid, propionic acid, n-butyric acid, i-butyric acid, n-valeric acid, i-valeric acid) weremeasured on a gas chromatograph (Shimadzu, GC-9A, Japan)equipped with a packed column (Shincarbon A) and flame ioni-zation detector. The column temperature was maintained at140 �C. The temperature at the injector and detector was main-tained at 200 �C. Helium was used at the carrier gas at a flow rateof 50 ml min�1. The particle size of the substrate was measuredusing a laser particle size analyzer (Beckman Coulter, LS320, USA)after filtration through a 2.0-mm mesh (Nonaka Rikaki, TestingSieve: JIS Z 8801). Particle sizes over 2.0 mm were measured at105 �C for 4 h. The mean particle size (MPS) was calculatedaccording to the method of Izawa et al. (2001). The volume ofbiogas was quantified by the water displacement method. Carbondioxide and methane were monitored on a gas chromatograph(Shimadzu, GC-2014AT, Japan) equipped with a packed column(Shincarbon ST) and thermal conductivity detector. The injectorand detector temperatures were maintained at 120 �C and 260 �C,respectively. The column temperature was gradually increasedfrom 40 �C to 250 �C. Helium was used as the carrier gas at a flowrate of 40 ml min�1. The volumes of accumulated methaneand carbon dioxide were corrected for standard conditions oftemperature (0 �C) and pressure (1 atm).
3. Results and discussion
3.1. Effects of particle size and solubilizationon grinding pretreatment
The mean particle size (MPS) of I-DP was 0.888 mm, which wasthe largest of all the pretreated substrates (Table 3 and Fig. 1A). TheMPS of substrates pretreated by BM decreased exponentially from0.843 to 0.391 mm as the total number of revolutions increased to20,000, and did not decrease any further at 40,000 total revolu-tions. The particle size reduction by BM pretreatment of 20,000total revolutions was 50% greater than that of DP pretreatment.Comparing pretreatments III-1000 and IV-1000 (Table 3), the sizereductionwas enhanced for IV-1000, which had a higher number ofrevolutions per minute in comparison with III-1000, which hada longer operation time. Izawa et al. (2001) have reported that the
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608604
particle size reduction of FW increased with increasing totalnumber of revolutions. Furthermore, increasing the number ofrevolutions per minute was more effective at reducing particle sizethan increasing the operation time, even at the same total numberof revolutions. At the maximum number of revolutions (2000 rpm)used in this study, MPS decreased exponentially with increasingoperation time up to 10 min, after which there was no furtherdecrease in MPS (Fig. 1B). These results indicate that particle sizedecreased as the number of revolutions per minute increased, andthat long operation times over 10 min were not effective atreducing MPS. Thus, to reduce the size of FW, the optimal condi-tions for pretreatment by BM were around 2000 rpm and 10 min.
The COD solubility of substrates pretreated by I-DP was 28.1%(Table 3). The average COD solubility of substrates pretreated by BMwas 39.8% regardless of the total number of revolutions, and thisvalue was 42% higher than I-DP. The solubility of FW treated bydifferent grinding methods, as measured by Izawa et al. (2001), wasabout 30%, regardless of the total number of revolutions andoperation time; this solubility was lower than that for the BMpretreatment in this study. This result is possibly attributable to thedifferent grinding devices, which have differing efficacy of sizereduction. Grinding the substrate with rotary crushing hammers,DP treatment produces particle sizes in the millimeter range(Takezaki et al., 2001). On the other hand, BM is used for dispersingagglomerated particles and producing small particles in the sub-micrometer range (Inkyo et al., 2006). As a result, the solubility ofII-300 was found to be 1.39 times higher than that of I-DP in spite oftheir similar MPS. These results clearly show that BM pretreatmenteffectively promoted solubilization even at a low total number ofrevolutions. On the other hand, a high total number of revolutionswere effective at reducing particle size. Therefore, grindingpretreatment by BM would be an effective method for solubiliza-tion and particle size reduction of FW.
3.2. Effect of particle size on pH and biogas production
The time course of pH variation in mesophilic anaerobic diges-tion of FW is shown in Fig. 2. The initial pH value in the conditionsof I-DP, II-300, III-1000, IV-1000, V-4000, VI-20000, and VII-40000were 7.5, 7.3, 7.3, 7.2, 7.2, 7.2, and 7.2, respectively. In all conditions,sharp drops in pH to around 6.9 were observed on the first day. ThepH values in all experiments gradually increased and reachedapproximately 7.5 by the end of the experimental period. The pH ofthe reactor running with only seed sludge (control) was around 7.8throughout the experimental period. The pH value of VII-40000 in
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
0 2 4 6 8 10 12 14 16 18
Digestion time (day)
pH
Control
III-1000
IV-1000
VII-40000
I-DP
II-300
V-4000
VI-20000
Fig. 2. Time course of pH in mesophilic anaerobic digestion of food waste.
the early phase was lower than that of the other conditions. Thelower pH value of VII-40000 suggests the possibility that excessiveparticle size reduction of the substrate accelerated hydrolysis andacidogenesis in the early stage of anaerobic digestion, resulting inaccumulation of VFAs.
Cumulative biogas production increased with digestion time(Fig. 3). Considerable increases in biogas production were observedfor the initial 6 days in all experimental conditions. Cumulativebiogas production differed among I-DP, II-300, III-1000, IV-1000,V-4000, VI-20000, and VII-40000, and reached, respectively, 375,439, 503, 455, 470, 455, and 404 ml g-total COD�1 at the end ofexperiment. In the case of the seed sludge (control), biogas was notdetected throughout the 16 days (data not shown). The cumulativebiogas production of III-1000 reached the highest value of503 ml g-total COD�1, which was 34% higher than that of I-DP. Theaverage concentrations of methane gas were similar among I-DP,II-300, III-1000, IV-1000, V-4000, VI-20000, and VII-40000, andwere 66.9, 62.1, 64.0, 65.8, 65.5, 65.7, and 62.9%, respectively.
The methane production rate increased with decreasing MPS topeak at around 330 ml g-total COD�1, and then decreased withmore intense pretreatment beyond the maximum value (Fig. 4).Methane production rate increased 28% when the MPS of FW wasdecreased from 0.888 to 0.718mmby BM pretreatment. Mshandeteet al. (2006) reported that the methane production rate increasedby 22% when the fibers were cut to a size of 2 mm compared withuntreated fibers (100mm). Smaller MPSmakes a larger surface areaavailable to the microorganisms, resulting in an increased methaneproduction rate. The results obtained in this study possibly indicatethat particle size reduction increased substrate utilization formethane fermentation, thereby enhancing the biogas productionrate. These results also suggest that BM pretreatment was effectivefor methane production rate even at short operation times. It isthought that the grinding pretreatment by BM was effective forimproving methane production efficiency to a certain extent,beyond which methanogenesis was inhibited. In BM pretreatment,the MPS decreased with increasing operation time and totalnumber of revolutions. However, the solubility values were similarat about 40%, regardless of the difference in the number of revo-lutions per minute and operation time. In anaerobic digestionexperiments using substrates pretreated by BM, high methaneproduction rates were observed for the smaller MPS substrates.This suggests that the availability of the particulate substrate asfood for the microorganisms increased with increasing surface areaof the substrate in the digestion period. However, a low methaneproduction rate for VII-40000 was observed in spite of its
0
100
200
300
400
500
600
0 2 4 6 8 10 12 14 16 18Digestion time (day)
Cum
ulat
ive
biog
as p
rodu
ctio
n(m
Lg-
tota
l CO
D-1
)
I-DP
II-300
III-1000
IV-1000
V-4000
VI-20000
VII-40000
Fig. 3. Time course of cumulative biogas production in mesophilic anaerobic digestionof food waste.
Cum
ulat
ive
met
hane
pro
duct
ion
(mL
g-to
tal C
OD
-1)
0
220
240
260
280
300
320
340
0 0.2 0.4 0.6 0.8 1.0
Mean particle size (mm)
V-4000 VI-20000 VII-40000
I-DP II-300 III-1000 IV-1000
1.2
Fig. 4. Relationship between methane production and mean particle size of pretreatedsubstrate for each pretreatment condition.
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608 605
maximum degree of pretreatment, which is possibly the result oforganic overloading due to excessive size reduction.
3.3. Evaluation of inhibitors in anaerobic digestion
Although MPS was smallest for the VII-40000 substrate, cumu-lative methane production was the lowest at 254 ml g-total COD�1
(Fig. 4). Generally, methanogenesis is inhibited by lower pH values,which result from accumulation of VFAs (Boone and Xum, 1987;Griffin et al., 1998), accumulation of NH4
þ (Gallert and Winter,1997), accumulation of LCFAs such as oleate and stearate(Angelidaki and Ahring,1992; Masse et al., 2002), production of H2Sfrom protein materials (Koster et al., 1986), and accumulation ofpersistent substrates (Moen et al., 1997). To evaluate the effects ofinhibition factors such as NH4
þ, LCFAs, H2S, and VFAs, the concen-tration of each inhibition factorwasmeasured and calculated. Theseconcentrations were compared with the corresponding inhibitionlevels reported in the literature (Table 4). The theoretical maximumconcentrations of NH4
þ and H2S were calculated under theassumption that all nitrogen and sulfur initially present in theloaded substrate were converted in the batch reactor to NH4
þ andH2S, respectively. The oleate and stearate in the FW substrate weredirectly measured, and each VFA in the digested sludge was alsomeasured. It is unlikely that process inhibition was caused by theaccumulation of persistent organic materials (Moen et al., 1997),because the substrates were loaded only once at the start of thebatch experiment. Hansen et al. (1998) have reported that processinhibition occurs at the NH4
þ concentration of 6000 mg l�1 in mes-ophilic anaerobic digestion of swine manure. In the literature, thelevels of H2S required for inhibition ofmethane production vary; thereported IC50 values are 90 mg H2S l�1 at pH 7.8e8.0 and250 mg H2S l�1 at pH 6.4e7.2 (Koster et al., 1986). In the present
Table 4The concentration of inhibitory substances in this study and each inhibition level in the
Inhibitors Inhibitor concentrationin this study a (mg l�1)
Inhibition levelb (mg l�1)
NH4eNþa 210 6000H2Sb 11 90e250
LCFAOleate 39 100e200Stearate 10 500
Total VFAb 4200e5600 6000Propionic acid 930e1300 3000
a Calculated maximum values.b Total VFA was calculated as sum of each VFA.
study, the theoretical maximum concentrations of NH4þ and H2S
were 210mg N l�1 and 11mg H2S l�1, respectively. These low valuessuggest that process inhibition byNH4
þ andH2S did not occur duringthe batch experiments. The oleate and stearate levels required forinhibition of methane production have been reported to be100e200 mg l�1 and 500 mg l�1, respectively (Angelidaki andAhring, 1992). The maximum concentrations of oleate and stea-rate were 39 mg l�1 and 10mg l�1 in the present experiment. Thesevalues were less than 40% of the inhibition levels for both LCFAs.These low LCFA concentrations suggest that no experimentalconditions were substantially inhibited by LCFAs. In contrast, a hightotal VFA concentration of 5600 mg l�1 was observed in theVII-40000 condition. This high concentrationwas equivalent to 96%of the VFA inhibition level reported by Siegert and Banks (2005).Therefore, the high VFA concentration possibly inhibits the diges-tion process in the VII-4000 condition.
The relationship between the rate of pH change per day duringthe early phase of the experiment (from 1 to 3 days) and cumulativemethane production is shown in Fig. 5. This relationship was similarto the relationship between MPS and cumulative methane produc-tion. The methane production increased with increasing rate of pHchange to peak at around 0.2 per day, and then decreased thereafter.The rate of pH change in VII-40000 was the lowest among allconditions as the increase in pH was most gradual in this condition.This result suggests that a low recovery rate from decreased pH forthe substrate with the smallest MPS reduced microbial activity,causing a decrease inmethane production. The pH of VII-40000 waslow compared to that of other pretreatment conditions.
In this study, smaller MPS resulted in an increased methaneproduction rate due to the increased surface area available to themicroorganisms. On the other hand, excessive size reduction of thesubstrate caused low pH, resulting in a decreased methaneproduction rate. Neves et al. (2004) reported that the waste/seedsludge ratio is an important parameter in batch high-solid anaer-obic digestion processes, and the use of a large seed sludge amountin a batch process allows for successful digestion without pHadjustment. In this study, it was thought that optimization of thewaste/seed sludge ratio could further improve methane produc-tion, even for substrates with excessively small MPS. For larger MPSvalues (>0.7 mm) where the rate of pH change is high, themicroorganisms are not limited by low pH. However, the surfacearea available to the microorganisms is smaller, leading to theirstarvation. Therefore, the anaerobic biodegradability is not expec-ted to be enhanced, even if the waste/seed sludge ratio is increased.
3.4. Effect of particle size on the concentrationand characteristics of VFAs
As mentioned above, the decline of pH is caused by the accu-mulation of VFAs. In the present study, VFA concentrations increased
previous studies.
Inhibitor concentration/Inhibitorlevel a/b (%)
Reference
3.5 Hansen et al. (1998)4.2e12 Koster et al. (1986)
20e39 Angelidaki and Ahring (1992)2.0 Angelidaki and Ahring (1992)
70e93 Siegert and Banks (2005)31e43 Boone and Xum (1987)
0
1000
2000
3000
4000
5000
6000
VFA
sco
ncen
trat
ion(
mg
L-1
)
I-DP
0
1000
2000
3000
4000
5000
6000II-300
0
1000
2000
3000
4000
5000
6000III-1000
Digesti
0
1000
2000
3000
4000
5000
6000
0 2 4 6 8 10 12 14 16 18
IV-1000
Acetic acid Propionic acid
Fig. 6. Variation of VFA production as a function of
Cum
ulat
ive
met
hane
pro
duct
ion
(mL
g-to
tal C
OD
-1)
220
240
260
280
300
320
340
0 0.1 0.2 0.3 0.4
Rate of pH change at early phase
0V-4000 VI-20000 VII-40000
I-DP II-300 III-1000 IV-1000
Fig. 5. Relationship between methane production and rate of pH change in the initialphase for each pretreatment condition.
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608606
from the start of the experiments to day 2 (Fig. 6), and themaximumtotal VFA concentration for I-DP, II-300, III-1000, IV-1000, V-4000,VI-20000, and VII-40000 was 4330, 4210, 4270, 4750, 4500, 5030,and5600mg l�1, respectively. Acetic acid, butyric acid, andpropionicacid accounted for most of the total VFA concentration. Generally,macromolecular organic compounds such as carbohydrates andlipids are decomposed and transformed to VFAs and alcohol viabiochemical reactions such ashydrolysis andbeta-oxidation (Speece,1996). Themicroorganisms that producemethane gas consume onlyacetic acid and hydrogen. Other VFAs such as butyric acid aretransformed into acetic acid, and then acetic acid is converted tomethane and carbon dioxide by methanogenic bacteria (Mata-Alvarez, 2003). In this study, all VFA concentrations reacheda maximum value 1e3 days from the start of the experiment andthen sharply decreased within 6 days, with the exception of pro-pionic acid (Fig. 7). Propionic acid,which is known to degrade slowly,was also consumed within 16 days. The maximum values of aceticacid and propionic acid generated at the initial stages of the
on time (day)
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Fig. 7. Variation of VFA production as a function of digestion time for production of different VFAs.
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608 607
experiments increased with decreasing MPS (Fig. 8). Maximumbutyric acid production decreased with decreasing MPS andcomprised the main component in the maximum total VFAs withvalues of 48.4, 63.4, 60.8 55.7, 40.2, 37.0, and 31.7% in the I-DP, II-300,III-1000, IV-1000, V-4000, VI-20000, and VII-40000 treatments,respectively (Figs. 7 and 8). These results indicate that the differencein MPS values affected the production and composition of VFAs.
The maximum cumulative concentration of VFAs varied withMPS. Acetic acid and propionic acid increased while butyric aciddecreased with decreasing MPS. The amount of seed sludge in allconditions was fixed in this study. Therefore, the difference inproduction of VFAs may have been caused by the increased surfacearea of the substrate and solubilization of intracellular macromol-ecules due to mechanical breakage by grinding.
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Fig. 8. Relationship between maximum VFA concentration and mean particle size inmesophilic anaerobic digestion of food waste.
These VFAs are converted by methanogenic bacteria to methaneand carbon dioxide (Thiele and Zeikus, 1988). The formation andconsumption of some VFAs, such as propionic and butyric acid,demonstrate the conversion to acetic acid by acetogenic bacteria;acetic acid is considered to be the major precursor of methane.Shimizu et al. (1992) reported that the VFA concentration of wastedactivated sludge pretreated by ultrasonic pretreatment was higherthan that of the non-pretreated control. Comparing the solubility ofhigh-molecular-weight biological polymers with the VFA produc-tion rate of pretreated wasted activated sludge, the VFA productionrate was higher (Shimizu et al., 1992). These observations indicatethat the conversion of VFAs from pretreated wasted activatedsludge to methane was the rate-limiting step. In the present study,excessive reduction of the MPS caused VFA accumulation, resultingin decreased methane production. On the other hand, smaller MPSimproved acetic acid accumulation and led to increased methaneproduction. Smaller MPS values also increased the solubility in theanaerobic digestion process.
4. Conclusions
The following conclusions can be drawn from this studyregarding the effects of particle size and solubilization withpretreatment by BM in anaerobic digestion processes.
1. Grinding pretreatment by BM resulted in 30% higher solubili-zation than pretreatment with a DP, even at 300 revolutions,and yielded a particle size that was 57% smaller at 20,000revolutions.
2. Smaller MPS improved the methane yield by 28% at 1000revolutions compared with I-DP, and enhanced VFA productionwith an increasing total number of revolutions.
3. Excessive size reduction of the substrate caused VFA accumu-lation, resulting in decreased methane production anddecreased solubility in the anaerobic digestion process.
K. Izumi et al. / International Biodeterioration & Biodegradation 64 (2010) 601e608608
4. Optimizing the size reduction of the substrate as well as themicrobial growth could potentially improve the methane yieldof anaerobic digestion processes.
These results suggest that size reduction by BM pretreatmenteffectively promoted solubilization and increased methane yield inthe anaerobic digestion process. However, the energy required forgrinding the FW was not measured, and should be evaluated fromthe point of view of energy balance. In this experiment, we inves-tigated the effects of size reduction as pretreatment in the batchanaerobic digestion process. The conclusions reached in this studyapply to small-scale batch operations, and further investigation isnecessary to relate these findings to the operation of full-scalecontinuous digestion processes. Furthermore, we plan to investi-gate the effects of size reduction using persistent organic substratessuch as seaweed and phytoplankton in a future study. The effects ofthe seed sludge concentration and the composition of the bacterialcommunity are also of interest.
Acknowledgements
This work was supported by the University-Industry JointResearch Project for Private Universities and matching funds fromthe Ministry of Education, Culture, Sports, Science and Technology,Japan, MEXT, 2004e2008. We are grateful to the Hokubu SludgeTreatment Center, Yokohama, Japan, for preparation of the seedsludge.
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