456 Chiang Mai J. Sci. 2015; 42(2)

Chiang Mai J. Sci. 2015; 42(2) : 456-468http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Biogas Production from Poultry Slaughter House andFood Processing Wastes by Microwave ThermalPretreatmentNatchari Chuchat [a] and Wanwisa Skolpap*[a,b][a] Department of Chemical Engineering, Faculty of Engineering, Thammasat University (Rangsit campus),

Pathumthani 12120,Thailand.[b] National Center of Excellence for Environmental and Hazardous Waste Management, Thammasat University, Pathumthani 12120, Thailand.*Author for correspondence; e-mail: swanwisa@engr.tu.ac.th

Received: 11 December 2012Accepted: 7 February 2015

ABSTRACTDomestic waste activated sludge (WAS) produced from wastewater treatment

process, non hazardous organic waste can reach up to 550,000 ton/y by 2012. For instance,poultry slaughter house and food processing plant generates lots of sludge waste 20,000 tonper year. To minimize the sludge waste disposal and to further transform organic matter intobiogas, sludge anaerobic digestion is one of the most potential treatment processes. This studyaimed to accelerate hydrolysis period by microwave-assisted pretreatment for improvementof biogas production from the WAS. Effect of mixing ratios between primary and anaerobicsludge, temperature and vacuum degree using microwave (MW) irradiation on biogas productionimprovement were studied in anaerobic batch reactors under mesophilic condition for 25days. Experimental results showed that vacuum MW pretreatment at 65oC and sludge mixtureratio of primary to secondary sludge at 60:40 was the most suitable condition for biogasproduction. The average methane yield was achieved 5,256 g CH4/kg dried solid and 91%CH4 content which was about 1.3-fold higher than non-MW pretreatment. The acceleratedhydrolysis of organic matters in WAS was hydrolyzed 3-fold faster than non-MW pretreatment.In comparisons between MW and non- MW pretreating experiments, average %COD removal,%glucose consumption and %VS production rate during hydrolysis period were decreasedby 25.6%, 47.5% and 82.27%, respectively. A vacuum degree of MW thermal pretreatmenthad a direct effect on the mesophilic anaerobic biodegradability of WAS by damaging activatedsludge floc structures and cell membranes as indicated by the solubilization of particulateCODs, glucose concentration and volatile solid amount.

Keywords: biogas, microwave thermal pretreatment, waste activated sludge, biodegradability,anaerobic digestion

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1. INTRODUCTIONSludge represents the major solid waste

from biological wastewater treatmentprocesses. Sludge production is increasingwhereas disposal routes tend to be reducing[1]. In Thailand, 50% of non hazardousorganic waste activated sludge (WAS)produced from wastewater treatmentprocess is disposed to landfill [2]. Poultryslaughter house and food processing plantannually generates lots of sludge whichresults to 50% of the total operating cost ofwaste water treatment [3]. Capital andoperating costs in sewage treatment forprimary and secondary waste sludge dependon digestibility and dewaterability. Sludgedigestion generates pathogens and odourproblems associated with residual putresciblematter pertaining to world climate changeproblem due to green house gas emissionfrom sludge treatment process. It is thusessential to minimize sludge quantity andsimultaneously produce renewable energy.One of the most promising processes isanaerobic digestion or methanizationpertaining to sludge stabilization by convertinga part of its organic matters into biogas(60-70%vol of methane, CH4) considered asa renewable energy source [1].

Biogas typically refers to gas producedby the anaerobically biodegradation oforganic matters containing in biomass,manure or sewage, municipal waste, greenwaste and energy crops [4]. The maincomponents of biogas are methane andcarbon dioxide. Anaerobic degradation ofparticulate materials and macro-moleculesoccurs in four steps: hydrolysis, acidogenesis,aceto-genesis, and methanogenesis. In sludgedigestion, hydrolysis is the rate-limitingstep [5] because of its low biodegradabilityof extracellular polymeric substances (EPS)and the other non-biodegradable components[6]. To improve digestion efficiency, the most

logical approach is achieved by disruptionof bacterial or microbial cells in the sludge[7]. Disruption of bacteria in sludge maybe performed mechanically, ultrasonically,chemically, or thermally [8]. Mechanicalpretreatment is highly effective; however, it iscomplicated and expensive [9]. Sonicationcan disrupt 70-100% of sludge cells, butthis approach is energy-intensive [9]. Chemicaland thermo-chemical pretreatments areefficient [10], but they require extreme reactionconditions and commonly require the use ofspecialized chemicals. Thermal treatmentprior to anaerobic digestion has beenexamined as a potential approach forhydrolysis promotion [9]. Conventional lowtemperature thermal treatment requires alonger contact time than high-temperaturetreatment [11]. All pretreatment methodscan disrupt the EPS and divalent cationnetwork, enhancing the rate and extentof WAS biodegradability before WAS istransferred to the digesters. Consequently,an effective and economical pretreatmentmethod is essential. One possible approach isto use microwave (MW) irradiation [7].Recent studies suggested significant potentialof the use of MW irradiation in theenvironmental engineering area [13] followingits application in the telecommunicationsand food-industries. Initial MW pretreatmentstudies on WAS focused on the effect ofthermal and athermal effects of microwaveradiation on enhancement of anaerobicdigestion, dewaterability [15, 16], pathogendestruction [17]. The term “athermal effect”,for microwave, generally relates to an effectthat is not associated with an increasedtemperature [13], while “thermal effect”relates to the process that generates heat as aresult of the absorption of the microwaveenergy by water or organic complexes markedby either constant or induced polarization.

458 Chiang Mai J. Sci. 2015; 42(2)

The microwave energy is transformed intoheat derived from the internal resistance ofrotation [15]. Alternatively, industrial use ofmicrowave heating in chemical reactions isbecoming popular mainly due to a dramaticreduction in reaction times [12]. In theelectromagnetic spectrum, MW irradiationoccurs in wavelengths of 1 mm-1 m atcorresponding frequencies of 300 GHz-300MHz, respectively. Heating applicationsgenerally use a frequency of 2,450 MHzwith a wavelength of 12.24 cm and energyof 1.02 × 10 5 eV [12].

The advantages of microwaves over theconventional heating (CH) method in theseapplications include rapid heating, pathogendestruction, ease of control, compactness ofthe microwave generator [14], significantwaste volume reduction, processing timereduction, savings in energy consumption[14] and generating environmentally safesludge [13]. Pino-Jelcicet reported thatmicrowave could achieve higher WAS flocand cell destruction and release extracellularpolymeric substances (EPS) and intracellularmaterials into the soluble phase comparedto CH [15]. Eskicioglu [12] also reported theexistence of the microwave athermal effecton WAS solubilization and concomitantimprovement in volatile solid destructionand biogas production. Park [17] observedthat the power, temperature, and total solidconcentration significantly affected thesolubilization degree of waste-activatedsludge. Wojciechowska [18] observed theincrease of organic matter (biochemicaloxygen demand, BOD) and nutrients(nitrogenous and phosphorus) concentrationin the sludge liquor under microwavepretreatment. It was found that microwavewas additionally capable of decomposingcomplex chemical compounds andtransforming them into simple compoundsthat could be easily decomposed by

microorganisms. Zielinski [16] found thatmicrowave radiation could affect the structureand function of bacterial communitiesindependent of thermal effects.Liquefactionstage of anaerobic digestion, begins withbacterial hydrolysis to break down insolubleorganic polymers such as carbohydrates,acidogenic bacteria and acetogenic bacteriathen converts resulting organic acids intoacetic acid, along with ammonia, hydrogen,and carbon dioxide, respectively. In sludgedigestion, hydrolysis is a significantrate-limiting step of anaerobic digestionand require to improve. It was hypothesizedthat MW-irradiation would disrupt thecomplex WAS floc structure as CH doseand as well as enhance solubilization ofWAS particulate into the soluble phase.In this work, the effects of mixing ratiosof the primary and secondary sludge,pretreatment temperature, and vacuum ofMW irradiation on hydrolysis periodacceleration of biogas production werestudied under batch anaerobic mesophiliccondition. This research examined theinfluence of anaerobic biodegradability ofWAS by measurement of the solubilizationof particulate CODs (%CODs removal),glucose (%glucose consumption) and solid(%increasing solid production) representingdamage of activated sludge floc structure andcell membranes during fermentation period.Moreover, the improvement of methanecontent and yield were also discussed.

2. MATERIAL AND METHODS2.1 Sludge Sample

Two types of sludge from poultryslaughter house and food processing plantwere chosen as raw materials for biogasproduction. The first main chemical sludgesample was taken from the dissolved airfloatation (DAF) tank of wastewatertreatment plant at Charoen Pokphand

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Foods Public Company Limited, NakhonRatchasima, Thailand, generating 70-80 tonsof sludge per day. The second digestedsludge sample for substrate usage wasobtained from anaerobic wastewatertreatment lagoon of the same companygroup, Minburi, Thailand. The collectedsludge was stored at 4 °C before mixingand MW pretreatment.

2.2 Experimental Sample PreparationThe seven experimental sample were

prepared under conditions as shown inTable 1. There were three major studiedparameters for biogas productionimprovement such as WAS mixture ratio,MW pretreatment, and a degree of vacuum

MW. Firstly, the WAS mixtures of chemicalsludge and digested sludge in reactors 1,2, and 3 were prepared in a volume ratioof 100:0, 50:50, and 60:40, respectively.Amongst these various WAS ratio mixtures,CH4 yield obtained from reactor 3 achievedthe highest. Secondly, the WAS ratio mixturesof 60:40 were then applied for vacuumMW pretreatment under various pretreatingtemperatures for 30 minutes in reactors 4,5, and 6. Finally, a degree of vacuum MWwas studied at pretreating condition of95 °C for 30 min with a comparisonbetween the pretreated WAS mixture undernon-vacuum MW in reactor 7 and thepretreated WAS mixture under vacuumMW in reactor 5.

Following the experimental samplepreparation in Table 1, reactors 4 to 7 withthermal pretreatment for 30 minutes atsludge mixing ratio 60:40 and reactor 7 withnon-vacuum microwave pretreatment usedat 95 °C were suggested by the study ofEskicioglu [12].

2.3 Thermal and Microwave PretreatmentIn the study of the microwave effects

on waste activated sludge, a lab-scale industrialvacuum microwave oven was applied as

shown in Figure 1 [20]. It was equipped with800W magnetron operating at a frequencyof 2,450 MHz fre-quency. The dimensionsof the wave guide were 55mm × 110mm ×210mm and induced MW irradiation inputfrom side of vacuum vessel. In pretreatmentprocedure, each 1,000 g of WAS sample wastransferred to a 1 L plastic MW resistantcontainer into a stainless steel AISI 304vacuum vessel (240mm × 500mm × 4mm;maximum pressure at 20 mbar). The depthof the sludge irradiated was maintained at

Table 1. Experimental sample preparation.

Reactor

1234567

Volume ratio of mixture sludge

100 : 050 : 5060 : 4060 : 4060 : 4060 : 4060 : 40

MW pretreating conditionMW system

---

MW *MW *MW *MW**

Temperature/period---

65°C / 30 min95°C / 30 min120°C / 30 min95°C / 30 min

MW* = vacuum Microwave Pre-treatmentMW** = Non- vacuum Microwave Pre-treatment

460 Chiang Mai J. Sci. 2015; 42(2)

about 17 mm suggested by the study ofHong et al. [21]. The vacuum microwavewas operated at 800 Watt and conductedvacuum degree pressure of 95 kPa. Themicrowave pretreatment temperaturesstudied of 65 °C , 95 °C and 120 °C wereselected because the greatest improvementin biogas production were observed in

Figure 1. Vacuum microwave system used for accelerated sludge pretreatment.1. Controller System 2. AC Transformer 3. Magnetron 4. Vacuum Vessel 5. Signal Lighting6. Condensate Tank 7. Vacuum Pump 8. Safety Cut 9. Electronic Circuit 10. Drainage tank

these temperature range for 30 minutepretreatment period [12]. The microwavepretreated sludge was then cooled to roomtemperature then weighed before mixingand further fermentation. The vacuummicrowave system used for acceleratedsludge pretreatment is shown in Figure 1.

2.4 Anaerobic DigestionEach seven experiment as shown in

Table 1 was carried out in a 40 L mesophilicbatch anaerobic sludge reactor at roomtemperature for 25 days of fermentation assuggested by the previous study [4]. Eachbatch reactor was connected to a 5 L gasstorage tank applied at the gas outlet on topof the reactor. After the sludge samplewere mixed and then added into the reactor,N2 purge was applied for air removal beforefermentation [19, 22]. WAS samples weremixed and collected daily for analysis fromthe bottom taps of each digester.

2.5 Analytical MethodsTotal phosphate was analyzed

photometrically as Vanadate-molybdate-

complex at the wavelength of % transmissionat the wavelength of 420 nm (SHIMADZUUV 1201, UV-vis spectrophotometer;Kyoto, Japan).

The COD was analyzed according toMicro-COD by sealed digestion at 150 °Cfor 2 h and then measured at the wavelengthof absorbability of 600 nm.

The ammomia-nitrogen was measuredby using total nitrogen test kit (HACH;Loveland, USA) based on per sulfatedigestion method at the wavelength ofabsorbability of 410 nm.

The total carbohydrate was analyzedusing phenol-sulphuric acid method [24] andwas determined in glucose equivalent gramper litre.

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Total solids, volatile solids, and fixedsolids were determined by evaporation andusing furnace. To obtain total matterconcentration samples were heated at105 °C for 24 h for water evaporation.Samples were then heated at 550 °C for 2 hto determine mineral matter concentration.

pH, volatile fatty acid and alkalinity weremeasured with a pH probe measurement(HORIBA, pH meter F-21; Kyoto, Japan).

Methane, carbon dioxide and air wereevaluated by injecting sample gas into gaschromatography (SHIMADZU GC-8A;Kyoto, Japan). Helium gas was used ascarrier gas, injection temperature at 150 °Cand column temperature at 120 °C. Theresult obtained from gas chromatographywas used in estimation of biogas compositionusing response factor analysis [4]. The responsefactor is calculated by Eq. (1) based on themixture of methane, carbon dioxide, and air.

% Response factor = (1)

Biogas volume was measured by waterreplacement method.

3. RESULTS AND DISCUSSIONThe effect of MW thermal pretreatment

on enhancement of bio-digestibility andbiogas yield during hydrolysis period wereinvestigated by varying mixture sludge ratio,temperature and vacuum condition in batchanaerobic system. The results of the measuredparameters were discussed as follows:

3.1 Characteristics of Untreated WASRaw untreated 100% chemical waste

activated sludge in reactor 1 containinghighly potential organic matters compositionas shown by high values of CODs, solid(TDS, VS, FS), phosphate, glucose andammonia-nitrogen concentrations were439,000 mg/l, 3.7g, 2.05g, 1.65g, 64.65

% weight% area

mg/l, 33.33 g/l, 6.37 mg/l, respectively.The preliminary results expressed that theraw untreated 100% chemical waste activatedsludge in reactor 1 had high conversion ofpotential organic substances into solubleform and biogas after anaerobic fermentation.

3.2 Effect of Mixed WAS Ratios on WASDisintegration and Anaerobic Digestion

The WAS mixture volume ratios ofchemical sludge and digested sludge werevaried as 100:0, 50:50, and 60:40 in reactors1, 2, and 3, respectively. Amongst thesethree experiments the experiment of 60:40WAS mixing ratio in reactor 3 achieved thehighest average daily methane content of90.4% based on the total mass of the mixtureof methane, carbon dioxide, and air, whilethe experiments of 100:0 and 50:0 WASmixture ratio obtained 86.5% and 49.8%,respectively as shown in Figure 2. Therefore,the 60:40 WAS mixing ratio was selectedfor the study of MW pretreatment effecton biogas production.

3.3 Effect of Microwave Pretreatment onWAS Disintegration and AnaerobicDigestion

The 60:40 WAS mixing ratio were MWpretreated at temperatures of 65, 95, and120 °C in reactors 4, 5, and 6, respectively.Accelerated pretreatment time in hydrolysisprocess of experiments 4, 5, and 6 wasobserved from a significant change insoluble concentrations of chemical oxygendemand (COD) and the amount of solids(VS) as shown in Figures 3 and 5, respectively.MW pretreating temperature at 65 °C and95 °C caused insignificant improvement ofmethane content about 1% comparing tothe experiment of 60:40 WAS mixing ratiowithout MW pretreatment (reactor 3) asshown in Figure 2. At MW irradiatedtemperature of 120 °C it showed an adverse

462 Chiang Mai J. Sci. 2015; 42(2)

effect on WAS disintegration and CH4

yield (as shown in Figure 9). Except theexperiment of untreated 100% chemicalsludge, other experiments had similarconcentration profiles of glucose andammonia-nitrogen as shown in Figures 4and 7, respectively.

Figure 2. Daily CH4 content (%v/v) invariation of WAS ratios (Batch 1-3) and MWpretreated WAS mixtures (Batch 4-7).

The Chemical Oxygen Demand (COD)is a measure of the amount of oxygen whichthe chemical is used for biodegradedorganic compounds in water. Consequently,COD removal indicated the increase inamount of biogas generated. Amongstthe first three experiments without MWpretreatment, the initial COD concentrationof untreated 100% chemical sludge (reactor1) was about 4-fold and 2-fold more thanthat of the experiments of 50:50 and 60:40mining ratios, respectively as shown inFigure 3. Additionally, the initial volatilesolid (VS) amount in reactor 1 was about2-fold higher than that of other two untreatedWAS experiments as shown in Figure 5.It indicated that characteristics of rawWAS were favorable to those reported inprevious researches [1, 6, 7, 12, 17] in termsof dissoluble organic matter release. In the

related studies [1, 6, 7, 12, 17, 21], the thermalpretreatment by microwave caused thevalue of soluble COD and positive effecton methane production higher than theuntreated experiment (reactor1). Similar toprevious researches [1, 6, 7, 12, 18, 22],the degree of WAS solubilization on solubleCOD shown in Figure 3 illustrated thepositive effect of MW pretreatment onhydrolysis phase and overall experimentperiod. The highest %CODs removal inreactor 4 (60:40WAS, 65 °C MW irradiated)within hydrolysis process was 34% higherthan control in reactor 1 and otherexperiments. Moreover, soluble COD levelsin all experiments were decreased after25 days due to depletion of soluble organicmatter. Glucose, a carbon source, is one ofthe key parameters in biogas productionresponsible for soluble carbohydrate fromWAS floc disintegration in hydrolysisperiod. The highest glucose consumptionin reactor 4 (MW irradiated (60:40) WASat 65oC) as shown in Figure 4 was 54%higher than control (reactor1) and otherexperiments. At high glucose consumptionrate of anaerobic bio-digestion, it indicatedmore suitable nutrient availability forbiogas production.

Figure 3. Profiles of soluble CODs of sludgedisintegration.

Fermentation Time (day)

Dai

ly m

etha

ne co

nten

t (%

v/v)

Fermentation Time (day)

CO

DS

conc

. (m

g/l)

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untreated 60:40 WAS mixing ratio (reactor3). It was attributed that MW thermalpretreatment directly affected to WASdisintegration and biogas enhancement;however, the degree of thermalpretreatment could destroy activemicroorganisms for biogas production.

Figure 4. Profiles of glucose concentrationof sludge disintegration.

Figure 5. Profiles of volatile solid (VS) ofsludge disintegration.

Solid solubilization shown by the volatilesolid (VS) amount in Figure 5 represents thevalue of solid organic matter transformationinto methane by the decomposition of solidsubstances. During the first 5 days offermentation the initial VS amounts inreactors 2 (untreated 50:50 WAS mixing ratio)and 3 (untreated 60:40 WAS mixing ratio)were increased due to their degradation.Subsequently, VS amount in reactor 3 wasdrastically decreased during day 5-10 dueto biogas formation as shown in Figure 9.However, the VS amount in reactor 2was increased during this period as a resultof insignificant change in biogas formationas shown in Figure 9. In comparisons ofreactors 4, 5, and 6 with reactor 3, it wasobserved that MW pretreatment had adirect effect on the increase of VS duringhydrolysis period. MW irradiated pretreatmentat 95 °C (reactor 5) showed the highestincrease rate of VS amount for enhancingorganic matter solubilization while itsproduced biogas yield at the value of 5,152 gCH4/kg dried solid was 1.3-fold lowerthan that of reactor 4 with 65 °C MWirradiated. At the increase of pretreatingtemperature to 120 °C (reactor 6), % CH4

content was about 1.7-fold lower than

3.4 Effect of Vacuum Microwave Degreeon WAS Disintegration and AnaerobicDigestion

Non-vacuum microwave pretreatment(in reactor 7) did not affect to WASdisintegration and biogas improvement,observed from lowest CH4 yield shownin Figure 9 and non-affected hydrolysisacceleration illustrated in Figures 3, 4 and5, respectively.

3.5 Batch Anaerobic DigestionBiodegradability and potential of MW

irradiated WAS on biogas productionwere studied in duplicate batch mesophilic(at 35± 2 °C) condition. The pre-treatmentwith vacuum MW irradiation at 65 °C(reactor 4) enhanced average biogas yieldwas 5, 256 gCH4/kg dried solid thatwas 1.5-fold higher than the control(reactor1). In addition, the result of reactor

Fermentation Time (day)

Glu

cose

conc

. (g/

l)

Fermentation Time (day)

VS

(g)

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4 showed that, as compared to the control(reactor 1) the hydrolysis period occurredahead about 1-7 days, and the average%CH4 content was increased about 1.3-fold.It was similar to the reported result thatthe thermal, thermal-alkaline, and alkaline,and electrochemical pretreatment ofsludge for biogas production enhancedthe soluble COD and VFA yields comparingto those without pretreatment [25].

The ratio of VFA:Alkalinity, ammonia-nitrogen illustrated for anaerobic efficiencyare shown in Figures 6 and 7, respectively.Moreover, VFA:Alkalinity is practicallya major parameter for biogas productionas shown in Figure 6. The average VFAto alkalinity of 0.76 obtained from reactor4 corresponded to the reportedoptimum ratio of VFA to alkalinity of0.7 [26].The ratio of volatile fatty acid(VFA): Alkalinity as shown in Figure 6indicates the balance of the anaerobicdigestion more precisely, thus consideredas the suitable condition for methaneproduction by microorganisms. The highratio of VFA: Alkalinity representsbasic condition and vice versa. Atlow value of VFA:Alkalinity ratio, it isconsidered as the suitable conditionfor bacteria to produce methane. Theratio of VFA:Alkalinity is less than0.4 indicating that the system is highbuffer (http://www.nstda.or.th/en/) [22].Figure 6 shows that the average ratioof VFA:Alkalinity in all anaerobic digestionexperiments had the value in the rangeof 0.5 and 1.5. During the first periodof fermentation the VFA:Alkalinityvalues were higher than the suitableoperating range afterwards the systemwas self-adjusted into the stable condition.

Figure 6. Profiles of VFA:Alkalinity ratio inall anaerobic digestion experiments.

Figure 7. Profiles of pH in all anaerobicdigestion experiments.

Fermentation Time (day)

VFA

: Alk

alini

ty

Fermentation Time (day)

pH

In reactor 1 (untreated 100% chemicalsludge) pH was approximately in the rangeof 6.5 and 7 as shown in Figure 7. It wasconsidered as the suitable operating rangeof 6.5-7.5 [22]. After thermal pretreatment,the pH value was decreased due to thedegradation of microbial cell by hightemperature causing the release of intracellularsubstances [25]. At high temperature, therates of hydrolysis and acidification ofthese organic molecules were rapid, as a resultof reduction of the pH value of system [25].In other experiments the daily measurement

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of pH were similar; in which, the valuewas approximately in the range of 5.5 and6.0 due to the increase in VFA as shownin Figure 6. The pH was lower than 6.0which is in the operability pH range ofmethanogenesis; however, the activityis adversely affected [27]. Under lowpH conditions, VFAs are mostly in thefree form, so the diffusion in cells will behigher. Consequently, the inhibition ofmethane production will increase [27].It suggested that pH control should beconsidered in further studies.

Every experiment showed the decreasingrate of ammonia-nitrogen concentrationduring the first 17 days of fermentationdue to microbial growth. After thatammonia-nitrogen concentration wasincreased due to lysis of microorganismreleasing protein which was served asadditional nitrogen source. The control(reactor1) showed the highest averagenitrogen concentration of 5.42 mg/land 34.3% nitrogen reduction thatrepresented high potential raw materialfor anaerobic digestion. Additionally,MW pretreatment at 65 °C showedthe highest influence on ammonia-nitrogenreduction of 41.5%. The vacuum MWpretreatment showed more viablemicroorganism in anaerobic reactionand insignificant effect of denaturationof nutrients on microbial growth thatwas observed from higher ammonia-nitrogenconcentration in vacuum MW reactorsthan non-vacuum MW (reactor 7) as shownin Figure 8.

Figure 8. Profiles of ammonia-nitrogenconcentration in all anaerobic digestionexperiments.

Figure 9. Profiles of yield of CH4.

Fermentation Time (day)

N-N

H3

(mg/

l)

Fermentation Time (day)

gCH

4/kg

drie

d so

lid

After 3 days of fermentation, CH4 yeildsin reactors 1, 2, and 3 for non-pretreatment(as shown in Figure 9) were increased inthe range of 1.1- and 1.3-fold. For MWpretreating experiments in reactor 4, 5, 6,and 7, the CH4 yield in reactor 4 was increasedabout 1.4-fold while the CH4 yields ofother experiments were decreased in therange of 10 and 60% after 3 days of fermentation.In Figures 2 and 9, it was suggested that

466 Chiang Mai J. Sci. 2015; 42(2)

vacuum MW pretreating temperature of65 °C (reactor 4) was the most suitablecondition for biogas production fromWAS. The highest yield of CH4 achievedwas 5, 256 gCH4/kg dried solid with 91%of CH4 content which was comparable tobiogas yield obtained from advanced wastetreatment technology [23]. Moreover,pure chemical WAS (reactor1) without MW-pretreatment showed suitable characteristicfor raw-material in biogas productionobserved by the slightly less amount ofCH4 yield and %CH4 compared with themost suitable MW pretreated condition(reactor 4).

4. CONCLUSIONThe vacuum microwave pretreatment

on WAS at 800W with 65 °C pretreatmenttemperature and 60:40 WAS ratio enhancedthe solubilization of organic mattersfrom sludge. Hence, CH4 yield duringthe hydrolysis process for breaking downmacro-molecule was improved 1.5-foldhigher than the control as shown bythe increase of CODs, VS solubilization anddecreasing rate of glucose concentration.These results indicated in hydrolysisacceleration achievement. After the hydrolysisperiod, MW pretreated condition improvedfurther biogas production throughout thefermentation period.

ACKNOWLEDGEMENTSThis research was financially supported

by Faculty of Engineering, ThammasatUniversity. The author would like toexpress our appreciation to Assoc. Prof.Nurak Grisdanurak, Prof. PhadungsakRattanadecho, Mr.Witoon Aobrom andsenior researchers for laboratory assistancein this research.

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