Alkaline-mechanical pretreatment process for enhanced anaerobic digestion of thickened waste activated sludge with a novel crushing device: Performance evaluation and economic analysis.

Bioresource Technology xxx (2014) xxx–xxx

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Alkaline-mechanical pretreatment process for enhanced anaerobic digestion of thickened waste activated sludge with a novel crushing device: Performance evaluation and economic analysis Si-Kyung Cho a, Hyun-Jun Ju b, Jeong-Gyu Lee c, Sang-Hyoun Kim b,⇑ a b c

Department of Civil and Environmental Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea Department of Environmental Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712-714, Republic of Korea 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 2014 Received in revised form 24 March 2014 Accepted 25 March 2014 Available online xxxx Keywords: Waste activated sludge Alkaline-mechanical pretreatment Anaerobic digestion Co-metabolism Economic 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. The authors examined the performance and economic analysis of an alkaline-mechanical process with a novel mechanical crushing device for thickened WAS pretreatment. The pretreatment at 40 g TS/L, pH 13, and 90 min reaction time achieved 64% of solubilization efficiency and 8.3 times higher CH4 yield than the control. In addition, a synergistic CH4 yield enhancement was observed when the pretreated and raw WAS were used together as feedstock, and the greatest synergy was observed at a volumetric mixture ratio of 50:50. Economic estimates indicate that up to 22% of WAS treatment costs would be saved by the installation of the suggested process. The experimental results clearly indicate that the alkalinemechanical process would be highly effective and economically feasible for the AD of thickened WAS. Ó 2014 Published by Elsevier Ltd.

1. Introduction Waste activated sludge (WAS) is a highly putrescible residue collected from the second clarifiers of wastewater treatment plants (WWTPs). Since it contains easily decomposable organics, hazardous heavy metals, and pathogens, it should be properly treated prior to final disposal (Appels et al., 2008). However, WAS handling is one of the most difficult and expensive problems since it accounts for 30–40% of capital costs and 50% of operating costs of WWTPs (Wilson and Novak, 2009). Among the various disposal methods, anaerobic digestion (AD) consisting of hydrolysis, acidogenesis, acetogenisis, and methanogen-

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

esis is the most traditional and widely adopted method for WAS treatment and stabilization due to the following advantages: (1) reduction of the waste volume, (2) generation of energy-rich gas in the form of methane (CH4), and (3) production of a nutrient-containing final product (Mata-Alvarez et al., 2000). However, its application is hindered by the requirement of long retention time (20–50 days) and low biodegradability (3% of total solid (TS)) has rarely been reported, even though the TS range of WAS is highly recommendable for the stable operation and economic feasibility of an AD plant. In alkaline-mechanical pretreatments with higher performance than alkaline-thermo pretreatments, microwave and ultrasound have been considered the most powerful and effective mechanical devices (Liu et al., 2008). Using microwave, two times higher volatile fatty acids accumulation was obtained from WAS at a specific energy input of 28,800 kJ/kg TS and fermentation time of 72 h as compared to that of the control (Yang et al., 2013). In addition, 205% higher CH4 yield was achieved at 135 °C with 10 min holding time, and the addition of 20 meq NaOH/L from thickened WAS (Jang and Ahn, 2013) while 17% enhanced CH4 yield obtained at 170 °C with 1 min holding time and the addition of 0.05 g NaOH/g SS from thickened WAS (Chi et al., 2011). After alkaline-ultrasonic pretreatment, solubilization efficiency of WAS enhanced up to 70% and a significant increase in CH4 yield, from 82 to 127 ml CH4/g CODadded at pH 9 with 7000 kJ/kg TS ultrasonication, was reported (Kim et al., 2010). In addition, significant enhancements of both solubilization efficiency and biodegradability of WAS were achieved along with significantly reduced hydraulic retention time for stable operation after alkaline-ultrasonic pretreatment (Sahinkaya and Sevimli, 2013). Despite the obvious effectiveness of alkaline-microwave and alkaline-ultrasonic pretreatment on the AD of thickened WAS, the application of both pretreatments on the actual field has been limited due to the lack of economic feasibility caused by the huge energy consumptions of the mechanical devices (Sahinkaya and Sevimli, 2013). Thus, development of an effective and economic mechanical device for pretreatment is required. In this study, an alkaline-mechanical process using a novel mechanical crushing device was suggested as an economically feasible pretreatment for thickened WAS. The authors evaluated the performance of the pretreatment, solubilization efficiency, and CH4 yield at various TS content (30–50 g/L), pH (11–13), and reaction time (30–90 min). The operating conditions for pretreatment solubilization efficiency were optimized by response surface methodology (RSM) after consideration of CH4 yield. In addition, economic analysis 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 in a local WWTP at G city. It was thickened with a centrifuge at 4000 rpm to TS 5.5%. Thickened WAS was then kept at 4 °C to avoid

unintended microbial reactions. The characteristics of thickened WAS were as follows: TS (55 g/L), volatile solid (VS) (40 g/L), total suspended 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 an alkaline reactor (310 mm height with 240 mm diameter). WAS and a predetermined amount of alkaline agent (NaOH) were injected into the alkaline reactor, and the WAS was circulated through the crushing device equipped with four cutting blades (41.5 mm diameter with 3.5 mm width) rotating at 2500 rpm and tangential velocity of 5430 m/s for a designated reaction time. The centrifugal force by the rotation enabled the circulation at a flow rate of 100–200 L/h without an additional pump. The experimental conditions (TS concentration, pH, and reaction time) were as shown in Table 1, which is explained in detail in Section 2.4. The solubilization efficiency of each pretreatment condition was evaluated according to Eq. (1) below:

Solubilizaion efficiency ð%Þ ¼

ðVSSin VSSout Þ 100; VSSin

ð1Þ

where VSSin is the concentration of WAS before pretreatment while VSSout is the concentration of WAS after pretreatment. 2.3. Biological methane potential (BMP) assay The effect of sludge pretreatment on digestibility was investigated by biological methane potential assays. In all the cases, the batch experiment was performed in duplicate. The seed sludge was taken from a mesophilic anaerobic digester located in G city. pH, alkalinity, concentrations of TS, VS, TSS, and VSS of the seed sludge 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 with 150 mL working volume, where 30 mL of the seed sludge was inoculated to each bottle. The bottle was added with a predetermined amount of pretreated WAS for 2 g TS/L as the initial feed concentration of the BMP assay and was then supplemented with 79.5 mg of NH4Cl, 75 mg of KH2PO4, 11.25 mg of CaCl22H2O, 15 mg of MgCl26H2O, 3 mg of FeCl24H2O, and 600 mg of NaHCO3. Each bottle was then filled to 150 mL with distilled water, and the pH was adjusted to 7.0–7.5 using either 1 M HCl or 1 M KOH. Subsequently, the headspace of the bottle was flushed with N2 gas for 1 min, and the bottle was tightly sealed using open-top screw caps with rubber septa. The bottle was incubated in a shaking incubator at 35 °C. Then, a series of the batch experiments were performed to assay the 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 for batch experiment were those same as those for the BMP assay of the pretreated WAS. Biogas production and its constituents were monitored periodically. The headspace pressure was maintained below 2 atm in all the cases. CH4 production was calculated from the headspace measurements of gas composition and the total volume of biogas produced at each time interval using the mass balance Eq. (2):

V H;i ¼ V H;i1 þ C H;i ðV G;i V G;i1 Þ þ V H ðC H;i C H;i1 Þ;

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 = total biogas volumes in the current and previous time intervals, CH,i

Please cite this article in press as: Cho, S.-K., et al. Alkaline-mechanical pretreatment process for enhanced anaerobic digestion of thickened waste activated sludge with a novel crushing device: Performance evaluation and economic analysis. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/ j.biortech.2014.03.138

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S.-K. Cho et al. / Bioresource Technology xxx (2014) xxx–xxx Table 1 Experimental design matrix for alkaline-mechanical pretreatment. Run No.

1 2 3 4 5 6 7 8 9 10 11 12 13

Coded factor

Uncoded factor

TS (X1)

pH (X2)

Reaction time (X3)

TS (g/L)

pH

Reaction time (min)

1 0 1 1 1 1 1 0 0 0 1 1 0

0 1 1 1 0 0 1 1 1 1 0 1 0

1 1 0 0 1 1 0 1 1 1 1 0 0

30 40 50 30 50 30 50 40 40 40 50 30 40

12 13 11 13 12 12 13 13 11 11 12 11 12

90 90 60 60 30 30 60 30 90 30 90 60 60

and CH,i 1 = the fractions of methane gas in the headspace of the bottle measured using gas chromatography in the current and previous intervals, and VH = the total volume of headspace in the reactor. A modified Gompertz equation was employed to describe the cumulative CH4 production in the BMP tests:

equipped with a thermal conductivity detector (TCD) and a 0.9 m 3.2 mm stainless steel column packed with a Porapak Q mesh 80/100 with helium as the carrier gas. The temperatures of the injector, detector, and column were maintained at 80, 90, and 50 °C, respectively.

0 R e MðtÞ ¼ P exp exp ðk tÞ þ 1 ; P

3. Results and discussion

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 surface methodology A statistical approach is an efficient way to simultaneously optimize 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 reaction time (30–90 min) were selected as the independent variables, and the target surface response was the solubilization efficiency and CH4 yield. Three independent variables were converted to coded values for computational convenience: the upper limit of a factor to +1, the center level to 0, and the lower limit to 1. The matrix for the optimization of solubilization efficiency is presented in Table 1. In order to correlate the response to the independent variables, the response was fitted using a polynomial quadratic equation. To predict the optimal conditions, a second order polynomial model was employed, as shown below:

Y ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b1 b2 X 1 X 2 þ b1 b3 X 1 X 3 þ b2 b3 X 2 X 3 þ b11 X 21 þ b22 X 22 þ b33 X 23 ; where Y indicates the predicted response, X1, X2, and X3 are independent variables, b0 is the offset term, b1, b2, and b3 are linear coefficients, b11, b22 and b33 are quadratic coefficients, and b12, b13, and b23 are the interaction coefficients. The p-values of the parameter estimation were used to validate the model, and only less than 0.05 of p-value indicated significant model terms. 2.5. Analytical methods The concentrations of the TS, VS, TSS, VSS, alkalinity and COD were measured according to standard methods (APHA, 1998). The measured biogas production was adjusted to a standard temperature (0 °C) and pressure (760 mmHg) (STP). The CH4 gas content was analyzed via gas chromatography (GC, SRI 310)

3.1. Effects of alkaline-mechanical pretreatment on solubilization efficiency and CH4 yield The effects of the suggested process on the solubilization efficiency of WAS were summarized in Table 2. VSS of WAS were decreased by the pretreatment in all cases, and the highest solubilization efficiency (64%) was obtained under the conditions of 40 g TS/L, pH 13, and 90 min reaction time. The results could be attributed to the combination of swelling effects and mechanical shear forces representing that the swollen microbial cell after alkaline pretreatment was easily solubilized by shear forces induced by the crushing device, thereby enabling such a huge enhancement of solubilization efficiency. In addition, as the propagation of acoustic wave and microwave, which is the main driving force of ultrasonic and microwave pretreatment, would be limited at high TS content due to reduced mass transfer rate. On the other hand, the physical shear force generated by cutting blades should be less affected resulting in up to 64% of solubilization efficiency at 40 g TS/L. Table 2 also shows the effects of alkaline-mechanical pretreatment on the CH4 yield of WAS. Enhanced CH4 yield was observed after 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) was achieved at 40 g TS/L, pH 13, and 90 min of reaction time. Even though the highest solubilization efficiency and CH4 yield were achieved at the same pretreatment conditions, inconformity between solubilization efficiency and CH4 yield was observed in some cases. In particular, during Runs 1, 4 and 7, the similar (55–56%) solubilization efficiency resulted in significantly different CH4 yields (46–205 mL/g TS), implying that different pretreatment conditions (TS content, pH, reaction time) generated other influential factors to CH4 yield besides the solubilization efficiency. This could have resulted from the production of recalcitrant soluble organics or toxic/inhibitory intermediates such as melanoidines, furfural, and hydroxymethylfurfural (HMF) being reportedly prone to 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 predictive model, an analysis of variance (ANOVA) test was carried out and


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

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

1 2 3 4 5 6 7 8 9 10 11 12 13 Control

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)

30 40 50 30 50 30 50 40 40 40 50 30 40 55

12 13 11 13 12 12 13 13 11 11 12 11 12 –

90 90 60 60 30 30 60 30 90 30 90 60 60 –

20.3 27.7 35.7 20.3 37.0 20.3 35.7 27.7 27.7 27.7 35.7 20.3 27.7 –

9.2 10.1 20.6 9 16.9 11 16.2 11.5 14.2 14 14 10 13.1 –

55 64 42 56 53 46 55 58 49 50 61 51 53 –

205 233 86 125 96 123 46 115 148 160 92 126 150 25

the results were summarized in Table 3. The model F-value of 9.86 implies that the model was significant, and a value of ‘Prob > F0 less than 0.05 indicates that the model terms were significant. The ANOVA results imply that X1, X3, X2X3, and X 1 2 were significant model terms for CH4 yield in this study. Although other variables were found to be insignificant (P > 0.05), they cannot be eliminated to support the hierarchy of the model because the coefficient of determination (R2 = 0.967) indicates that this model can explain up to 96.7% variability of the response. By applying regression analysis, CH4 yield results were fitted to a second-order polynomial equation, as shown below:

Y ¼ 1630:50 þ 48:462X 1 þ 204:87X 2 12:5166X 3 0:9750X 1 X 2 0:07166X 1 X 3 þ 1:0833X 2 X 3 0:44625X 1 2 9:6250X 2 2 þ 0:0262X 3 2 ;

this is known to commonly occur at above 10% TS content; however, the much lower hydrolysis rate of WAS compared to other organic substances seemed to trigger the limited microbial reactions in this study, thereby causing a much lower CH4 yield (Appels et al., 2010). In addition, interestingly, at TS content of 30 and 40 g/L, a higher CH4 yield was achieved at extreme pretreatment conditions (pH 13 and reaction time of 90 min) than at moderate conditions (pH 11 and reaction time 30 min) while the opposite result was observed at a TS content of 50 g/L. The experimental results represent that an AD of organic substances having a lower biodegradability seems more easily influenced by the refractory substances than those of higher ones, which matches the expectation that refractory substances will be generated under such extreme conditions (Dwyer et al., 2008). 3.3. Effects of mixture ratio of raw and pretreated WAS on CH4 yield

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 yield of 242 mL/g TS was predicted at the following optimum pretreatment conditions: TS content of 32.86 g/L, pH 13, and reaction time of 90 min. Fig. 1 presents two-dimensional contour plots with one variable being kept constant at its optimum condition (TS content of 32.86 g/L, pH 13, and reaction time of 90 min) with variation of the other two variables within the experimental range. CH4 yield increased with an increase in the pH and reaction time, as shown in Fig 1(b), while CH4 yield decreased above the optimum TS content, as shown in Fig. 1(a) and (c). At TS content of 50 g/L, much lower CH4 yield was achieved than that of CH4 yield at TS content of 30 and 40 g/L at the same pH and reaction time regardless of solubilization efficiency, as shown in Fig. 2. This lower yield could be ascribed to the retarded enzymatic reaction and microbial activity under higher TS content (Abbassi-Guendouz et al., 2012). In fact,

Table 4 presents CH4 production results of various mixture ratios of raw and pretreated WAS. The synergistic effect was calculated by differences between the measured CH4 yield and the estimated CH4 yield based on the mixture ratio and the CH4 yields of raw and 100% pretreated WAS. Interestingly, synergistic CH4 yield enhancement was observed from all the cases, and the biggest synergistic effect was obtained at the mixture ratio of 50:50 on a volume basis. This synergy could be presumably explained by the co-metabolism defined as ‘‘a microbial transformation of a compound by microorganisms which are unable to use it as energy or carbon source’’ (Angelidaki and Ahring, 1997). In addition, Furukawa (2000) stated that the main co-metabolic reactions involve the microbial enzymes. Remarkable enhancements of the protease activity along with the hydrolysis rate of WAS after the addition of carbohydrates were reported by Feng et al. (2009); thus, an increase of easily biodegradable substances by a mixture

Table 3 ANOVA results of CH4 yield. Source

Sum of squares

Degree of freedom

Mean square

F-value

P-value

Model X1 X2 X3 X1X2 X1X3 X2X3 X12

28774.44 8385.125 0.125 4232 380.25 1849 4225 4551.75

9 1 1 1 1 1 1 1

3197.16 8385.125 0.125 4232 380.25 1849 4225 4551.75

9.855105 25.84678 0.000385 13.04495 1.172104 5.699461 13.02338 14.03057

0.0429 0.0147 0.9856 0.0365 0.3582 0.0970 0.0365 0.0332

X22

211.75

1

211.75

0.65271

0.4783

X32

1275.75

1

1275.75

3.932443

0.1416



5

Fig. 1. Two-dimensional contour plots for CH4 yield: (a) fixed retention time at optimum 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 and enhance the hydrolysis of mixed WAS in this study, thereby enhancing CH4 yield synergistically. Nzila (2013) expressed

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


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S.-K. Cho et al. / Bioresource Technology xxx (2014) xxx–xxx Table 4 CH4 yield from AD of different mixture ratios of raw and pretreated WAS. The pretreatment condition was 40 g TS/L, pH 13 and 90 min of reaction time. Mixture ratio (%) (V:V) Raw WAS

Pretreated WAS

0 20 40 50 60 80 100

100 80 60 50 40 20 0

CH4 yield (mL/ g TS)

Estimated CH4 yielda (mL/ g TS)

Synergistic effect (mL/ g TS)

233 222 215 210 157 111 35

– 193 154 134 114 72 –

– 29 61 76 43 39 –

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

hydrocarbons, aliphatic, and aromatic polychlorinated pollutants, reportedly detected in WAS, are known to be rarely used as growth-substrates by microorganisms (Barret et al., 2010). However, these substances can be biodegraded by microorganisms that can use them as non-growth substrates; thus, biodegradation of pollutants and recalcitrant substances was described as a fortuitous event by Nzila (2013) since non-growth substrates do not provide a source of energy. In this study, originally-contained pollutants and generated recalcitrant substances during pretreatment seemed to be further biodegraded as a result of fortuitous events, thereby enhancing CH4 yield synergistically (Delgadillo-mirquez et al., 2011). Despite 50:50 on a volume basis was the best mixture ratio in this study for the synergistic effect, it should be noticed that the best condition would be dependent on the pretreatment method, intensity/severity and co-substrates characteristics, since the co-metabolism would be highly affected by the content of easily biodegradable organic matter and inhibitory substances generated by pretreatment. In addition, lag period depends on the acclimation period of microorganisms to a proper substrate and environmental conditions (Lay et al., 1997), thus it could be affected by the co-digestion of raw and pretreated WAS. However, unlike the expectations, no meaningful changes (0.3–0.7 day) were observed in this study. Similarly, no relationship between lag period and hydrogen yield was reported by Guo et al. (2008) in the various pretreatment conditions, thus effect of co-digestion and/or pretreatment on the lag period seems to contradictory and needs to be further investigated.

operating costs in the upgraded situation, upgraded situations were divided into I and II based on single- and co-digestion of pretreated WAS with raw WAS (situation I: single-digestion of 100% pretreated WAS, situation II: co-digestion of pretreated WAS with raw WAS). In the upgraded situation II, a mixture ratio of 50% pretreated WAS with 50% raw was chosen for the following economic analysis since it led to the highest synergistic enhancement. After considerations of all the aforementioned conditions, experimental results and assumptions for economic analysis were summarized in Table 5. In these analyses, the US dollar ($) was used as the unit of currency, and the results are summarized in Table 6. In the baseline situation, the WAS treatment costs were determined to be $711,977/year. Twelve and twenty-two percent of the WAS treatment costs were determined to be saved after application of the alkaline-mechanical pretreatment at virtually upgraded situations I and II, respectively, representing that suggested process was found to be economically feasible. The increased operating costs due to the requirement of additional electricity and alkali agent were compensated by increased revenues due to the increased CH4 yield after installation of the suggested pretreatment facility. In addition, the payback period, defined as the length of time for the value of an investment to equal the capital costs, was calculated according to Eq. (4) below (Apul and Sanin, 2010). Payback period ¼

capital costs ½ðrevenues costsÞupgraded ðrevenues costsÞbaseline

ð4Þ 3.4. Economic analysis To investigate the economic feasibility of the suggested pretreatment process, an economic analysis was conducted using a simple cost calculation with consideration of the baseline and improved situation based on the experimental results obtained this study. Although a maximum CH4 yield of 242 mL/g TS was predicted at TS content of 32.86 g/L, a pH of 13, and a reaction time of 90 min, 40 g TS/L at the same pH and reaction time provided a slightly lower CH4 yield (233 mL/g TS) and was chosen for the economic analysis since higher TS content is highly recommended for the stable operation and economic feasibility of an AD plant. Initial investments and operating costs of piping, thickening, AD, dewatering, and other sludge treatment procedures were not included in accounts since both cases used the same plant (Apul and Sanin, 2010; Dhar et al., 2012); thus, the main expenses of the baseline situation were sludge cake disposal while additional operating costs including electricity (used for the crushing device and pumps) and chemicals (used for alkali pretreatment) together with sludge cake disposal costs were the main expenses of the upgraded case. To enhance the economic feasibility by reducing additional

Table 5 Experimental results and assumptions for economic analysis. Experimental results of CH4 yield at 40 g TS/L CH4 yield of raw WAS (mL/g TS) CH4 yield of 100% pretreated WAS (mL/g TS) CH4 yield of mixture with raw and pretreated WAS (V:V = 50:50) Assumptions Population equivalent of a conventional WWTP Mass of thickened WAS (m3/d) CHPa efficiency for electricity generation CHP efficiency for waste heat recovery Heat requirement for temperature maintenance in digester (kcal/d) Heat loss from digester (kcal/d) Sludge cake disposal cost ($/ton) Electricity price ($/kWh) Waste heat price ($/Mcal) a

35 mL/g TS 233 mL/g TS 210 mL/g TS

300,000 people 200 m3/d 30% 50% 208,333 (kcal/ d) 1,985,570 (kcal/d) $53/ton $0.106 $0.07

CHP = combined heat and power.


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S.-K. Cho et al. / Bioresource Technology xxx (2014) xxx–xxx Table 6 Summary 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 ($/year) Electricity ($/year) Waste heat ($/year)

3788 25,879 22,091

247,189 172,279 74,910

218,915 155,273 63,642

Operation cost ($/year) Additional electricity ($/year) Alkali agent ($/year) Disposal cost ($/year)

715,765 – – 715,765

876,252 23,137 425,590 427,525

694,583 11,568 238,080 444,935

Total cost ($/year) (=revenue operation cost)

711,977

629,063

475,668

Cost save ($/year) Payback period (year)

– –

82,914 6.71

236,309 2.35

As a result, the suggested pretreatment facility will break even after 6.71 and 2.35 years in upgraded situations I and II, respectively; therefore, the co-digestion of pretreated WAS with raw WAS is highly recommendable for the economic operation of the suggested process. In addition, the performance and economic feasibility of the suggested full scale process will be depending on a wide range of factors, thus, demonstration and long operation of the suggested process should be required in further as well as economic comparison with other pretreatment methods without any assumptions. 4. Conclusions WAS pretreatment by an alkaline-mechanical process using a novel crushing device was investigated through a series of batch tests designed by RSM. The pretreatment improved CH4 yield 8.3 times and saved 22% of WAS treatment costs. The authors propose feeding an anaerobic digester on the pretreated WAS with raw WAS at a volumetric mixture ratio of 50:50 because doing so reduces the pretreatment costs and leads to synergistic effects on CH4 yield. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MOST) (2011-0014666). References Abbassi-Guendouz, A., Brockmann, D., Trably, E., Dumas, C., Delgenès, J.P., Steyer, J.P., Escudié, R., 2012. Total solids content drives high solid anaerobic digestion via mass transfer limitation. Bioresour. Technol. 111, 55–61. Angelidaki, I., Ahring, B.K., 1997. Codigestion of olive oil mill wastewaters with manure, household waste or sewage sludge. Biodegradation 8, 221– 226. APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Baltimore, pp. 2, 57– 59. Appels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34 (6), 755–781. Appels, L., Degreve, J., Van der Bruggen, B., Van Impre, J., Dewil, R., 2010. Influence of low temperature thermal pre-treatment on sludge solubilizaiton, heavy metal release and anaerobic digestion. Bioresour. Technol. 101, 5743–5748. Apul, O.G., Sanin, F.D., 2010. Ultrasonic pretreatment and subsequent anaerobic digestion under different operational conditions. Bioresour. Technol. 101, 8984–8992. Barret, M., Carrere, H., Delgadillo, L., Patureau, D., 2010. PAH fate during the anaerobic digestion of contaminated sludge: do bioavailability and/or cometabolism limit their biodegradation? Water Res. 44, 3797–3806.

Bougrier, C., Delgenès, J.-P., Carrère, H., 2006. Combination of thermal treatments and anaerobic digestion to reduce sewage sludge quantity and improve biogas yield. Process Saf. Environ. Prot. 4 (4), 280–284. Carrere, H., Dumas, C., Battimelli, A., Batstonce, D.J., Delgenes, J.P., Steyer, J.P., Ferrer, I., 2010. Pretreatment methods to improve sludge anaerobic degradability: a review. J. Hazard. Mater. 183 (1–3), 1–15. Chi, Y., Li, Y., Fei, X., Wang, S., Yuan, H., 2011. Enhancement of thermophilic anaerobic digestion of thickened waste activated sludge by combined microwave and alkaline pretreatment. J. Environ. Sci. 23 (8), 1257– 1265. Cho, S.K., Shin, H.S., Kim, D.H., 2012. Waste activated sludge hydrolysis during ultrasonication: two-step disintegration. Bioresour. Technol. 121, 480– 483. Delgadillo-mirquez, L., Lardon, L., Steyer, J.-P., Patureau, D., 2011. A new dynamic model for bioavilability and cometabolism of micropollutants during anerobic digestion. Water Res. 45, 4511–4521. Dhar, B.R., Nakhla, G., Ray, M.B., 2012. Techno-economic evaluation of ultrasound and thermal pretreatments for enhanced anaerobic digestion of municipal waste activated sludge. Waste Manage. 32, 542–549. Dwyer, J., Starrenburg, D., Tait, S., Barr, K., Batstone, D., Lant, P., 2008. Decreasing activated sludge thermal hydrolysis reduces product color, without decreasing degradability. Water Res. 42, 4699–4709. Feng, L., Chen, Y., Zheng, X., 2009. Enhancement of waste activated sludge protein conversion and volatile fatty acids accumulation during waste activated sludge anaerobic fermentation by cabohydrate substrate addition: the effect of pH. Environ. Sci. Technol. 43, 4373–4380. Furukawa, K., 2000. Engineering dioxygenases for efficient degradation of environmental pollutants. Curr. Opin. Biotechnol. 11, 244–249. Guo, L., Li, X.M., Bo, X., Yang, Q., Zeng, G.M., Liao, D.X., Liu, J.J., 2008. Impacts of sterilization, microwave and ultrasonication pretreatment on hydrogen producing using waste sludge. Bioresour. Technol. 99, 3651–3658. Jang, J.H., Ahn, J.H., 2013. Effect of microwave pretreatment in presence of NaOH on mesophilic anaerobic digestion of thickened waste activated sludge. Bioresour. Technol. 131, 437–442. Kim, D.H., Cho, S.K., Lee, M.K., Kim, M.S., 2013b. Increased solubilization of excess sludge does not always result in enhanced anaerobic digestion efficiency. Bioresour. Technol. 143, 660–664. Kim, D.H., Jeong, E., Oh, S.E., Shin, H.S., 2010. Combined (alkaline + ultrasound) pretreatment effect on sewage sludge disintegration. Water Res. 4, 3093– 3100. Kim, J., Yu, Y., Lee, C., 2013a. Thermo-alkaline pretreatment of waste activated sludge at low-temperature: effects on sludge disintegration, methane production, and methanogen community structure. Bioresour. Technol. 144, 194–201. Lay, J.J., Li, Y.Y., Noike, T., 1997. Influence of pH and moisture content on the methane production in high-solids sludge digestion. Water Res. 31 (6), 1518– 1524. Liu, X., Liu, H., Chen, J., Du, G., Chen, J., 2008. Enhancement of solubilization and acidification of waste activated sludge by pretreatment. Waste Manage. 28, 2614–2622. Mata-Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour. Technol. 74 (1), 3–16. Nzila, A., 2013. Update on the cometabolism of organic pollutants by bacteria. Environ. Pollut. 178, 474–482. Park, N.D., Helle, S.S., Thring, R.W., 2012. Combined alkaline and ultrasound pretreatment of thickened pulp mill waste activated sludge for improved anaerobic digestion. Biomass Bioenergy 46, 750–756. Rani, R.U., Kumar, S.A., Kaliappan, S., Yeom, I.T., Banu, J.R., 2012. Low temperature thermo-chemical pretreatment of diary waste activated sludge for anaerobic digestion process. Bioresour. Technol. 103, 415–424.


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Sahinkaya, S., Sevimli, M.F., 2013. Synergistic effects of sono-alkaline pretreatment on anaerobic biodegradability of waste activated sludge. J. Ind. Eng. Chem. 19 (1), 197–206. Wilson, C.A., Novak, J.T., 2009. Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Res. 43, 4489–4498.

Yang, Q., Yi, J., Luo, K., Jing, X., Li, X., Liu, Y., Zeng, G., 2013. Improving disintegration and acidification of waste activated sludge by combined alkaline and microwave pretreatment. Process Saf. Environ. Prot. 91, 521–526.


aerobic digestion for enhanced sludge stabilization: comparison of the process performance for mixed and waste sludge [corrected].

Enhanced waste activated sludge digestion using a submerged anaerobic dynamic membrane bioreactor: performance, sludge characteristics and microbial community.

Enhanced anaerobic digestion of waste activated sludge digestion by the addition of zero valent iron.

Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment.

Hybrid alkali-hydrodynamic disintegration of waste-activated sludge before two-stage anaerobic digestion process.

Enhanced high-solids anaerobic digestion of waste activated sludge by the addition of scrap iron.

Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge.

Treatment of waste activated sludge together with agro-waste by anaerobic digestion: focus on effluent quality.

Winery waste recycling through anaerobic co-digestion with waste activated sludge.

Kinetic characterization of thermophilic and mesophilic anaerobic digestion for coffee grounds and waste activated sludge.

Mesophilic batch anaerobic co-digestion of fruit-juice industrial waste and municipal waste sludge: process and cost-benefit analysis.

Anaerobic digestion of yard waste with hydrothermal pretreatment.

Digestion and dewatering characteristics of waste activated sludge treated by an anaerobic biofilm system.

Microbial community related to lysozyme digestion process for boosting waste activated sludge biodegradability.

Enhanced anaerobic digestion of food waste by thermal and ozonation pretreatment methods.

Biodegradability of wastewater and activated sludge organics in anaerobic digestion.

Evaluation of the anaerobic co-digestion of sewage sludge and tomato waste at mesophilic temperature.

Enhanced dewaterability of waste-activated sludge by combined cationic polyacrylamide and magnetic field pretreatment.

Effects of temperature and organic loading rate on the performance and microbial community of anaerobic co-digestion of waste activated sludge and food waste.

Enhanced digestion of waste activated sludge using microbial electrolysis cells at ambient temperature.

Process performance of high-solids batch anaerobic digestion of sewage sludge.

A novel process for anaerobic composting of municipal solid waste.

Thermophilic anaerobic digestion of coffee grounds with and without waste activated sludge as co-substrate using a submerged AnMBR: system amendments and membrane performance.

Start-up of an anaerobic dynamic membrane digester for waste activated sludge digestion: temporal variations in microbial communities.