alkaline-mechanical pretreatment process for enhanced anaerobic digestion of thickened waste...

Bioresource Technology xxx (2014) xxx–xxx

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Bioresource Technology

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Alkaline-mechanical pretreatment process for enhanced anaerobicdigestion of thickened waste activated sludge with a novel crushingdevice: Performance evaluation and economic analysis

http://dx.doi.org/10.1016/j.biortech.2014.03.1380960-8524/� 2014 Published by Elsevier Ltd.

⇑ Corresponding author. Tel.: +82 53 850 6691; fax: +82 53 850 6699.E-mail address: [email protected] (S.-H. Kim).

Please cite this article in press as: Cho, S.-K., et al. Alkaline-mechanical pretreatment process for enhanced anaerobic digestion of thickened waste acsludge with a novel crushing device: Performance evaluation and economic analysis. Bioresour. Technol. (2014), http://dx.doi.org/1j.biortech.2014.03.138

Si-Kyung Cho a, Hyun-Jun Ju b, Jeong-Gyu Lee c, Sang-Hyoun Kim b,⇑a Department of Civil and Environmental Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Koreab Department of Environmental Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712-714, Republic of Koreac Willtech, R&DB Center 305, Gyeongsan, Gyeongbuk 712-902, Republic of Korea

h i g h l i g h t s

� An alkaline-mechanical process with a novel mechanical crushing device for WAS.� The pretreatment solubilized 64% of VSS in WAS.� The pretreatment improved CH4 yield 8.3 times and saved 22% of WAS treatment costs.� Feeding pretreated WAS together with raw WAS led to a synergistic effect.� The effectiveness and economic feasibility of this process was clearly demonstrated.

a r t i c l e i n f o

Article history:Received 8 January 2014Received in revised form 24 March 2014Accepted 25 March 2014Available online xxxx

Keywords:Waste activated sludgeAlkaline-mechanical pretreatmentAnaerobic digestionCo-metabolismEconomic analysis

a b s t r a c t

Although various pretreatments have been widely investigated to enhance the anaerobic digestion (AD)of waste activated sludge (WAS), economic feasibility issues have limited real-world applications. Theauthors examined the performance and economic analysis of an alkaline-mechanical process with a novelmechanical crushing device for thickened WAS pretreatment. The pretreatment at 40 g TS/L, pH 13, and90 min reaction time achieved 64% of solubilization efficiency and 8.3 times higher CH4 yield than thecontrol. In addition, a synergistic CH4 yield enhancement was observed when the pretreated and rawWAS were used together as feedstock, and the greatest synergy was observed at a volumetric mixtureratio of 50:50. Economic estimates indicate that up to 22% of WAS treatment costs would be saved bythe installation of the suggested process. The experimental results clearly indicate that the alkaline-mechanical process would be highly effective and economically feasible for the AD of thickened WAS.

� 2014 Published by Elsevier Ltd.

1. Introduction

Waste activated sludge (WAS) is a highly putrescible residuecollected from the second clarifiers of wastewater treatment plants(WWTPs). Since it contains easily decomposable organics, hazard-ous heavy metals, and pathogens, it should be properly treatedprior to final disposal (Appels et al., 2008). However, WAS handlingis one of the most difficult and expensive problems since it ac-counts for 30–40% of capital costs and 50% of operating costs ofWWTPs (Wilson and Novak, 2009).

Among the various disposal methods, anaerobic digestion (AD)consisting of hydrolysis, acidogenesis, acetogenisis, and methanogen-

esis is the most traditional and widely adopted method for WAS treat-ment and stabilization due to the following advantages: (1) reductionof the waste volume, (2) generation of energy-rich gas in the form ofmethane (CH4), and (3) production of a nutrient-containing finalproduct (Mata-Alvarez et al., 2000). However, its application is hin-dered by the requirement of long retention time (20–50 days) andlow biodegradability (<50%), which are mainly caused by the retardedhydrolysis of WAS since microbial cells, cell walls, and membranes inthe WAS are strong barriers that do not easily permit the penetrationof hydrolytic enzymes (Cho et al., 2012). Therefore, in order to en-hance the hydrolysis rate and the digestion performance, various celldisruption methods have been applied.

Physical, chemical, mechanical, and biological pretreatmentshave been reported, each providing improved solubilizationefficiency and subsequent enhanced AD performance based on dif-ferent mechanisms (Carrere et al., 2010). However, the application

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2 S.-K. Cho et al. / Bioresource Technology xxx (2014) xxx–xxx

of a single pretreatment to WAS has been limited due to the inten-sive energy requirements, high chemical costs, high maintenancecosts, and deterioration of sludge characteristics (Kim et al.,2013a). Thus, recently, combinations of each pretreatment havebeen widely studied since the different mechanisms of each pre-treatment could lead to higher solubilization efficiency with lowerenergy consumption through a synergistic effect (Kim et al., 2010).

Among the various combinations, alkaline-thermo and alkaline-mechanical pretreatments have been most widely investigated dueto their simplicity and high efficiency (Kim et al., 2013a). In alka-line-thermo pretreatments, various temperatures, ranging from60 to 270 �C, have been applied to WAS, and 170–200 �C was re-ported as the optimum range (Appels et al., 2010; Bougrier et al.,2006). However, high-temperature pretreatment not only requiresa large amount of energy but also generates the refractory sub-stances at above 180 �C (Rani et al., 2012). The combination oflow-temperature (under 100 �C) with alkaline pretreatment isbeing widely investigated; however, the effectiveness of low-temperature thermo-alkaline pretreatment on the AD of thickenedWAS (>3% of total solid (TS)) has rarely been reported, even thoughthe TS range of WAS is highly recommendable for the stable opera-tion and economic feasibility of an AD plant. In alkaline-mechanicalpretreatments with higher performance than alkaline-thermopretreatments, microwave and ultrasound have been consideredthe most powerful and effective mechanical devices (Liu et al.,2008). Using microwave, two times higher volatile fatty acids accu-mulation was obtained from WAS at a specific energy input of28,800 kJ/kg TS and fermentation time of 72 h as compared to thatof the control (Yang et al., 2013). In addition, 205% higher CH4 yieldwas achieved at 135 �C with 10 min holding time, and the additionof 20 meq NaOH/L from thickened WAS (Jang and Ahn, 2013) while17% enhanced CH4 yield obtained at 170 �C with 1 min holdingtime and the addition of 0.05 g NaOH/g SS from thickened WAS(Chi et al., 2011). After alkaline-ultrasonic pretreatment, solubiliza-tion efficiency of WAS enhanced up to 70% and a significantincrease in CH4 yield, from 82 to 127 ml CH4/g CODadded at pH 9with 7000 kJ/kg TS ultrasonication, was reported (Kim et al.,2010). In addition, significant enhancements of both solubilizationefficiency and biodegradability of WAS were achieved along withsignificantly reduced hydraulic retention time for stable operationafter alkaline-ultrasonic pretreatment (Sahinkaya and Sevimli,2013). Despite the obvious effectiveness of alkaline-microwaveand alkaline-ultrasonic pretreatment on the AD of thickenedWAS, the application of both pretreatments on the actual fieldhas been limited due to the lack of economic feasibility causedby the huge energy consumptions of the mechanical devices(Sahinkaya and Sevimli, 2013). Thus, development of an effectiveand economic mechanical device for pretreatment is required.

In this study, an alkaline-mechanical process using a novelmechanical crushing device was suggested as an economically feasi-ble pretreatment for thickened WAS. The authors evaluated the per-formance of the pretreatment, solubilization efficiency, and CH4

yield at various TS content (30–50 g/L), pH (11–13), and reactiontime (30–90 min). The operating conditions for pretreatment solu-bilization efficiency were optimized by response surface methodol-ogy (RSM) after consideration of CH4 yield. In addition, economicanalysis and the estimation of payback period were investigated.

2. Methods

2.1. Preparation of thickened WAS

WAS for this experiment was taken from the second clarifier ina local WWTP at G city. It was thickened with a centrifuge at4000 rpm to TS 5.5%. Thickened WAS was then kept at 4 �C to avoid

Please cite this article in press as: Cho, S.-K., et al. Alkaline-mechanical pretreatmsludge with a novel crushing device: Performance evaluation and ecoj.biortech.2014.03.138

unintended microbial reactions. The characteristics of thickenedWAS were as follows: TS (55 g/L), volatile solid (VS) (40 g/L), totalsuspended solid (TSS) (50.2 g/L), volatile suspended solid (VSS)(39.3 g/L), and chemical oxygen demand (COD) (50.4 g/L).

2.2. Alkaline-mechanical pretreatment

The pretreatment process consists of a crushing device and analkaline reactor (310 mm height with 240 mm diameter). WASand a predetermined amount of alkaline agent (NaOH) were in-jected into the alkaline reactor, and the WAS was circulatedthrough the crushing device equipped with four cutting blades(41.5 mm diameter with 3.5 mm width) rotating at 2500 rpmand tangential velocity of 5430 m/s for a designated reaction time.The centrifugal force by the rotation enabled the circulation at aflow rate of 100–200 L/h without an additional pump.

The experimental conditions (TS concentration, pH, and reac-tion time) were as shown in Table 1, which is explained in detailin Section 2.4. The solubilization efficiency of each pretreatmentcondition was evaluated according to Eq. (1) below:

Solubilizaion efficiency ð%Þ ¼ ðVSSin � VSSoutÞVSSin

� 100; ð1Þ

where VSSin is the concentration of WAS before pretreatment whileVSSout is the concentration of WAS after pretreatment.

2.3. Biological methane potential (BMP) assay

The effect of sludge pretreatment on digestibility was investi-gated by biological methane potential assays. In all the cases, thebatch experiment was performed in duplicate. The seed sludgewas taken from a mesophilic anaerobic digester located in G city.pH, alkalinity, concentrations of TS, VS, TSS, and VSS of the seedsludge were 7.5, 3140 mg CaCO3/L, 25.7, 16.3, 20.3, 16.5 g/L,respectively.

At first, the BMP assay of WAS pretreated at various conditions,as shown in Table 1, was conducted in 300 mL serum bottles with150 mL working volume, where 30 mL of the seed sludge was inoc-ulated to each bottle. The bottle was added with a predeterminedamount of pretreated WAS for 2 g TS/L as the initial feed concen-tration of the BMP assay and was then supplemented with79.5 mg of NH4Cl, 75 mg of KH2PO4, 11.25 mg of CaCl2�2H2O,15 mg of MgCl2�6H2O, 3 mg of FeCl2�4H2O, and 600 mg of NaHCO3.Each bottle was then filled to 150 mL with distilled water, and thepH was adjusted to 7.0–7.5 using either 1 M HCl or 1 M KOH. Sub-sequently, the headspace of the bottle was flushed with N2 gas for1 min, and the bottle was tightly sealed using open-top screw capswith rubber septa. The bottle was incubated in a shaking incubatorat 35 �C.

Then, a series of the batch experiments were performed to assaythe BMP of the various mixture ratios of raw and pretreated WAS(0:100, 20:80, 40:60, 50:50, 60:40, 80:20, 100:0 as volume basis).The pretreatment conditions for this assay were 40 g TS/L, pH 13,and 90 min reaction time. The other experimental conditions forbatch experiment were those same as those for the BMP assay ofthe pretreated WAS.

Biogas production and its constituents were monitored period-ically. The headspace pressure was maintained below 2 atm in allthe cases. CH4 production was calculated from the headspace mea-surements of gas composition and the total volume of biogas pro-duced at each time interval using the mass balance Eq. (2):

VH;i ¼ VH;i�1 þ CH;iðVG;i � VG;i�1Þ þ VHðCH;i � CH;i�1Þ; ð2Þ

where VH,i and VH,i-1 = cumulative biogas volumes at the current (i)and previous time (i � 1) time intervals, VG,i and VG,i � 1 = totalbiogas volumes in the current and previous time intervals, CH,i

ent process for enhanced anaerobic digestion of thickened waste activatednomic analysis. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/



Table 1Experimental design matrix for alkaline-mechanical pretreatment.

Run No. Coded factor Uncoded factor

TS (X1) pH (X2) Reaction time (X3) TS (g/L) pH Reaction time (min)

1 �1 0 1 30 12 902 0 1 1 40 13 903 1 �1 0 50 11 604 �1 1 0 30 13 605 1 0 �1 50 12 306 �1 0 �1 30 12 307 1 1 0 50 13 608 0 1 �1 40 13 309 0 �1 1 40 11 9010 0 �1 �1 40 11 3011 1 0 1 50 12 9012 �1 �1 0 30 11 6013 0 0 0 40 12 60

S.-K. Cho et al. / Bioresource Technology xxx (2014) xxx–xxx 3

and CH,i � 1 = the fractions of methane gas in the headspace of thebottle measured using gas chromatography in the current and pre-vious intervals, and VH = the total volume of headspace in thereactor.

A modified Gompertz equation was employed to describe thecumulative CH4 production in the BMP tests:

MðtÞ ¼ P � exp � expR0 � e

Pðk� tÞ þ 1

� �� ; ð3Þ

where M(t) = cumulative CH4 production (mL) at cultivation time t(day), P = CH4 production potential (mL), R0 = CH4 production rate(mL/d), k = lag period (day), and e = exp(1) = 2.71828.

2.4. Pretreatment conditions designed by response surfacemethodology

A statistical approach is an efficient way to simultaneously opti-mize the operating factors that are interrelated with each other.Thus, to achieve the optimum solubilization efficiency and CH4

yield, Box–Behnken design with response surface methodology(RSM) was employed. TS content (3–5%), pH (11–13), and reactiontime (30–90 min) were selected as the independent variables, andthe target surface response was the solubilization efficiency andCH4 yield. Three independent variables were converted to codedvalues for computational convenience: the upper limit of a factorto +1, the center level to 0, and the lower limit to �1. The matrixfor the optimization of solubilization efficiency is presented inTable 1. In order to correlate the response to the independentvariables, the response was fitted using a polynomial quadraticequation. To predict the optimal conditions, a second order polyno-mial model was employed, as shown below:

Y ¼ b0 þ b1X1 þ b2X2 þ b3X3 þ b1b2X1X2 þ b1b3X1X3 þ b2b3X2X3

þ b11X21 þ b22X2

2 þ b33X23;

where Y indicates the predicted response, X1, X2, and X3 are inde-pendent variables, b0 is the offset term, b1, b2, and b3 are linear coef-ficients, b11, b22 and b33 are quadratic coefficients, and b12, b13, andb23 are the interaction coefficients. The p-values of the parameterestimation were used to validate the model, and only less than0.05 of p-value indicated significant model terms.

2.5. Analytical methods

The concentrations of the TS, VS, TSS, VSS, alkalinity and CODwere measured according to standard methods (APHA, 1998).The measured biogas production was adjusted to a standardtemperature (0 �C) and pressure (760 mmHg) (STP). The CH4 gascontent was analyzed via gas chromatography (GC, SRI 310)


equipped with a thermal conductivity detector (TCD) and a0.9 m � 3.2 mm stainless steel column packed with a Porapak Qmesh 80/100 with helium as the carrier gas. The temperatures ofthe injector, detector, and column were maintained at 80, 90,and 50 �C, respectively.

3. Results and discussion

3.1. Effects of alkaline-mechanical pretreatment on solubilizationefficiency and CH4 yield

The effects of the suggested process on the solubilization effi-ciency of WAS were summarized in Table 2. VSS of WAS were de-creased by the pretreatment in all cases, and the highestsolubilization efficiency (64%) was obtained under the conditionsof 40 g TS/L, pH 13, and 90 min reaction time. The results could beattributed to the combination of swelling effects and mechanicalshear forces representing that the swollen microbial cell after alka-line pretreatment was easily solubilized by shear forces induced bythe crushing device, thereby enabling such a huge enhancement ofsolubilization efficiency. In addition, as the propagation of acousticwave and microwave, which is the main driving force of ultrasonicand microwave pretreatment, would be limited at high TS contentdue to reduced mass transfer rate. On the other hand, the physicalshear force generated by cutting blades should be less affectedresulting in up to 64% of solubilization efficiency at 40 gTS/L.

Table 2 also shows the effects of alkaline-mechanical pretreat-ment on the CH4 yield of WAS. Enhanced CH4 yield was observedafter pretreatments in all cases, and 8.3 times higher CH4 yield(233 mL/g TS) than the control (25 mL/g TS at 50 g TS/L) wasachieved at 40 g TS/L, pH 13, and 90 min of reaction time. Eventhough the highest solubilization efficiency and CH4 yield wereachieved at the same pretreatment conditions, inconformity be-tween solubilization efficiency and CH4 yield was observed insome cases. In particular, during Runs 1, 4 and 7, the similar(55–56%) solubilization efficiency resulted in significantly differentCH4 yields (46–205 mL/g TS), implying that different pretreatmentconditions (TS content, pH, reaction time) generated other influen-tial factors to CH4 yield besides the solubilization efficiency. Thiscould have resulted from the production of recalcitrant solubleorganics or toxic/inhibitory intermediates such as melanoidines,furfural, and hydroxymethylfurfural (HMF) being reportedly proneto be generated under higher pretreatment intensity (Kim et al.,2013b; Park et al., 2012; Wilson and Novak, 2009).

3.2. RSM results of CH4 yield

To determine the adequacy and significance of a predictivemodel, an analysis of variance (ANOVA) test was carried out and




Table 2Summary of solubilization rate and CH4 yield of WAS after pretreatments.

Run. Pretreatment conditions Experimental results

TS (g/L) pH Reaction time (min) Initial VSS (g/L) Final VSS (g/L) Solubilization efficiency (%) CH4 yield (mL/g TS)

1 30 12 90 20.3 9.2 55 2052 40 13 90 27.7 10.1 64 2333 50 11 60 35.7 20.6 42 864 30 13 60 20.3 9 56 1255 50 12 30 37.0 16.9 53 966 30 12 30 20.3 11 46 1237 50 13 60 35.7 16.2 55 468 40 13 30 27.7 11.5 58 1159 40 11 90 27.7 14.2 49 14810 40 11 30 27.7 14 50 16011 50 12 90 35.7 14 61 9212 30 11 60 20.3 10 51 12613 40 12 60 27.7 13.1 53 150Control 55 – – – – – 25


the results were summarized in Table 3. The model F-value of 9.86implies that the model was significant, and a value of ‘Prob > F0 lessthan 0.05 indicates that the model terms were significant. TheANOVA results imply that X1, X3, X2X3, and X1

2 were significantmodel terms for CH4 yield in this study. Although other variableswere found to be insignificant (P > 0.05), they cannot be eliminatedto support the hierarchy of the model because the coefficient ofdetermination (R2 = 0.967) indicates that this model can explainup to 96.7% variability of the response. By applying regressionanalysis, CH4 yield results were fitted to a second-order polynomialequation, as shown below:

Y ¼ �1630:50þ 48:462X1 þ 204:87X2 � 12:5166X3

� 0:9750X1X2 � 0:07166X1X3 þ 1:0833X2X3 � 0:44625X12

� 9:6250X22 þ 0:0262X3

2;

where Y, X1, X2, and X3 are the CH4 yield (mL/g TS), TS content (g/L),pH, and reaction time (min), respectively. The maximum CH4 yieldof 242 mL/g TS was predicted at the following optimum pretreat-ment conditions: TS content of 32.86 g/L, pH 13, and reaction timeof 90 min.

Fig. 1 presents two-dimensional contour plots with one variablebeing kept constant at its optimum condition (TS content of32.86 g/L, pH 13, and reaction time of 90 min) with variation ofthe other two variables within the experimental range. CH4 yieldincreased with an increase in the pH and reaction time, as shownin Fig 1(b), while CH4 yield decreased above the optimum TS con-tent, as shown in Fig. 1(a) and (c). At TS content of 50 g/L, muchlower CH4 yield was achieved than that of CH4 yield at TS contentof 30 and 40 g/L at the same pH and reaction time regardless of sol-ubilization efficiency, as shown in Fig. 2. This lower yield could beascribed to the retarded enzymatic reaction and microbial activityunder higher TS content (Abbassi-Guendouz et al., 2012). In fact,

Table 3ANOVA results of CH4 yield.

Source Sum of squares Degree of freedom

Model 28774.44 9X1 8385.125 1X2 0.125 1X3 4232 1X1X2 380.25 1X1X3 1849 1X2X3 4225 1X1

2 4551.75 1

X22 211.75 1

X32 1275.75 1


this is known to commonly occur at above 10% TS content; how-ever, the much lower hydrolysis rate of WAS compared to other or-ganic substances seemed to trigger the limited microbial reactionsin this study, thereby causing a much lower CH4 yield (Appelset al., 2010). In addition, interestingly, at TS content of 30 and40 g/L, a higher CH4 yield was achieved at extreme pretreatmentconditions (pH 13 and reaction time of 90 min) than at moderateconditions (pH 11 and reaction time 30 min) while the opposite re-sult was observed at a TS content of 50 g/L. The experimental re-sults represent that an AD of organic substances having a lowerbiodegradability seems more easily influenced by the refractorysubstances than those of higher ones, which matches the expecta-tion that refractory substances will be generated under such ex-treme conditions (Dwyer et al., 2008).

3.3. Effects of mixture ratio of raw and pretreated WAS on CH4 yield

Table 4 presents CH4 production results of various mixture ra-tios of raw and pretreated WAS. The synergistic effect was calcu-lated by differences between the measured CH4 yield and theestimated CH4 yield based on the mixture ratio and the CH4 yieldsof raw and 100% pretreated WAS. Interestingly, synergistic CH4

yield enhancement was observed from all the cases, and the big-gest synergistic effect was obtained at the mixture ratio of 50:50on a volume basis. This synergy could be presumably explainedby the co-metabolism defined as ‘‘a microbial transformation of acompound by microorganisms which are unable to use it as energyor carbon source’’ (Angelidaki and Ahring, 1997). In addition,Furukawa (2000) stated that the main co-metabolic reactions in-volve the microbial enzymes. Remarkable enhancements of theprotease activity along with the hydrolysis rate of WAS after theaddition of carbohydrates were reported by Feng et al. (2009);thus, an increase of easily biodegradable substances by a mixture

Mean square F-value P-value

3197.16 9.855105 0.04298385.125 25.84678 0.01470.125 0.000385 0.98564232 13.04495 0.0365380.25 1.172104 0.35821849 5.699461 0.09704225 13.02338 0.03654551.75 14.03057 0.0332211.75 0.65271 0.47831275.75 3.932443 0.1416




Fig. 1. Two-dimensional contour plots for CH4 yield: (a) fixed retention time atoptimum point of 90 min (b) fixed TS content at optimum point of 32.86 g/L (c)fixed pH at optimum point of 13.

Fig. 2. Two-dimensional contour plots for CH4 yield: (a) fixed TS content at 30 g/L(b) fixed TS content at 40 g/L (c) fixed TS content at 50 g/L.


of pretreated WAS with raw WAS seemed to both accelerate andenhance the hydrolysis of mixed WAS in this study, therebyenhancing CH4 yield synergistically. Nzila (2013) expressed


co-metabolism as ‘‘the biodegradation of two substrates, thegrowth or essential substrate, and the non-growth or fortuitoussubstrate.’’ Many pollutants, such as polycyclic aromatic




Table 4CH4 yield from AD of different mixture ratios of raw and pretreated WAS. The pretreatment condition was40 g TS/L, pH 13 and 90 min of reaction time.

Mixture ratio (%) (V:V) CH4 yield (mL/g TS)

Estimated CH4 yielda (mL/g TS)

Synergistic effect (mL/g TS)

RawWAS

PretreatedWAS

0 100 233 – –20 80 222 193 2940 60 215 154 6150 50 210 134 7660 40 157 114 4380 20 111 72 39

100 0 35 – –

a Estimated CH4 yield was calculated by the mixture ratio and the CH4 yields of raw and 100% pretreatedWAS neglecting synergism.


hydrocarbons, aliphatic, and aromatic polychlorinated pollutants,reportedly detected in WAS, are known to be rarely used asgrowth-substrates by microorganisms (Barret et al., 2010). How-ever, these substances can be biodegraded by microorganisms thatcan use them as non-growth substrates; thus, biodegradation ofpollutants and recalcitrant substances was described as a fortu-itous event by Nzila (2013) since non-growth substrates do notprovide a source of energy. In this study, originally-contained pol-lutants and generated recalcitrant substances during pretreatmentseemed to be further biodegraded as a result of fortuitous events,thereby enhancing CH4 yield synergistically (Delgadillo-mirquezet al., 2011). Despite 50:50 on a volume basis was the best mixtureratio in this study for the synergistic effect, it should be noticedthat the best condition would be dependent on the pretreatmentmethod, intensity/severity and co-substrates characteristics, sincethe co-metabolism would be highly affected by the content of eas-ily biodegradable organic matter and inhibitory substances gener-ated by pretreatment.

In addition, lag period depends on the acclimation period ofmicroorganisms to a proper substrate and environmental condi-tions (Lay et al., 1997), thus it could be affected by the co-digestionof raw and pretreated WAS. However, unlike the expectations, nomeaningful changes (0.3–0.7 day) were observed in this study.Similarly, no relationship between lag period and hydrogen yieldwas reported by Guo et al. (2008) in the various pretreatment con-ditions, thus effect of co-digestion and/or pretreatment on the lagperiod seems to contradictory and needs to be further investigated.

Table 5Experimental results and assumptions for economic analysis.

Experimental results of CH4 yield at 40 g TS/LCH4 yield of raw WAS (mL/g TS) 35 mL/g TSCH4 yield of 100% pretreated WAS (mL/g TS) 233 mL/g TSCH4 yield of mixture with raw and pretreated WAS

(V:V = 50:50)210 mL/g TS

AssumptionsPopulation equivalent of a conventional WWTP 300,000 peopleMass of thickened WAS (m3/d) 200 m3/dCHPa efficiency for electricity generation 30%CHP efficiency for waste heat recovery 50%Heat requirement for temperature maintenance in digester

(kcal/d)208,333 (kcal/d)

Heat loss from digester (kcal/d) 1,985,570(kcal/d)

Sludge cake disposal cost ($/ton) $53/tonElectricity price ($/kWh) $0.106Waste heat price ($/Mcal) $0.07

a CHP = combined heat and power.

3.4. Economic analysis

To investigate the economic feasibility of the suggested pre-treatment process, an economic analysis was conducted using asimple cost calculation with consideration of the baseline and im-proved situation based on the experimental results obtained thisstudy. Although a maximum CH4 yield of 242 mL/g TS was pre-dicted at TS content of 32.86 g/L, a pH of 13, and a reaction timeof 90 min, 40 g TS/L at the same pH and reaction time provided aslightly lower CH4 yield (233 mL/g TS) and was chosen for the eco-nomic analysis since higher TS content is highly recommended forthe stable operation and economic feasibility of an AD plant. Initialinvestments and operating costs of piping, thickening, AD, dewa-tering, and other sludge treatment procedures were not includedin accounts since both cases used the same plant (Apul and Sanin,2010; Dhar et al., 2012); thus, the main expenses of the baselinesituation were sludge cake disposal while additional operatingcosts including electricity (used for the crushing device andpumps) and chemicals (used for alkali pretreatment) together withsludge cake disposal costs were the main expenses of the upgradedcase. To enhance the economic feasibility by reducing additional


operating costs in the upgraded situation, upgraded situationswere divided into I and II based on single- and co-digestion ofpretreated WAS with raw WAS (situation I: single-digestion of100% pretreated WAS, situation II: co-digestion of pretreatedWAS with raw WAS). In the upgraded situation II, a mixtureratio of 50% pretreated WAS with 50% raw was chosen for thefollowing economic analysis since it led to the highest synergisticenhancement.

After considerations of all the aforementioned conditions,experimental results and assumptions for economic analysis weresummarized in Table 5. In these analyses, the US dollar ($) wasused as the unit of currency, and the results are summarized inTable 6. In the baseline situation, the WAS treatment costs weredetermined to be $711,977/year. Twelve and twenty-two percentof the WAS treatment costs were determined to be saved afterapplication of the alkaline-mechanical pretreatment at virtuallyupgraded situations I and II, respectively, representing thatsuggested process was found to be economically feasible. Theincreased operating costs due to the requirement of additionalelectricity and alkali agent were compensated by increasedrevenues due to the increased CH4 yield after installation of thesuggested pretreatment facility. In addition, the payback period,defined as the length of time for the value of an investment toequal the capital costs, was calculated according to Eq. (4) below(Apul and Sanin, 2010).

Payback period¼ capital costs½ðrevenues�costsÞupgraded�ðrevenues�costsÞbaseline�

ð4Þ




Table 6Summary of economic analysis for baseline and upgraded situations.

Baseline situation Upgraded situation I (100% pretreated) Upgraded situation II(50% pretreated + 50% raw)

Capital cost ($) – 556,000 556,000

Revenue ($/year) 3788 247,189 218,915Electricity ($/year) 25,879 172,279 155,273Waste heat ($/year) �22,091 74,910 63,642

Operation cost ($/year) 715,765 876,252 694,583Additional electricity ($/year) – 23,137 11,568Alkali agent ($/year) – 425,590 238,080Disposal cost ($/year) 715,765 427,525 444,935

Total cost ($/year) (=revenue � operation cost) �711,977 �629,063 �475,668

Cost save ($/year) – 82,914 236,309Payback period (year) – 6.71 2.35


As a result, the suggested pretreatment facility will break evenafter 6.71 and 2.35 years in upgraded situations I and II, respec-tively; therefore, the co-digestion of pretreated WAS with rawWAS is highly recommendable for the economic operation of thesuggested process. In addition, the performance and economic fea-sibility of the suggested full scale process will be depending on awide range of factors, thus, demonstration and long operation ofthe suggested process should be required in further as well as eco-nomic comparison with other pretreatment methods without anyassumptions.

4. Conclusions

WAS pretreatment by an alkaline-mechanical process using anovel crushing device was investigated through a series of batchtests designed by RSM. The pretreatment improved CH4 yield 8.3times and saved 22% of WAS treatment costs. The authors proposefeeding an anaerobic digester on the pretreated WAS with rawWAS at a volumetric mixture ratio of 50:50 because doing so re-duces the pretreatment costs and leads to synergistic effects onCH4 yield.

Acknowledgements

This work was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (MOST)(2011-0014666).

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