Techno-economic evaluation of ultrasound and thermal pretreatments for enhanced anaerobic digestion of municipal waste activated sludge

Waste Management 32 (2012) 542–549

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Techno-economic evaluation of ultrasound and thermal pretreatments for enhanced anaerobic digestion of municipal waste activated sludge Bipro Ranjan Dhar, George Nakhla, Madhumita B. Ray ⇑ Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

a r t i c l e

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Article history: Received 4 March 2011 Accepted 11 October 2011 Available online 15 November 2011 Keywords: Anaerobic digestion Biogas Thermal pretreatment Ultrasound pretreatment Waste activated sludge

a b s t r a c t To enhance the anaerobic digestion of municipal waste-activated sludge (WAS), ultrasound, thermal, and ultrasound + thermal (combined) pretreatments were conducted using three ultrasound specific energy inputs (1000, 5000, and 10,000 kJ/kg TSS) and three thermal pretreatment temperatures (50, 70 and 90 °C). Prior to anaerobic digestion, combined pretreatments significantly improved volatile suspended solid (VSS) reduction by 29–38%. The largest increase in methane production (30%) was observed after 30 min of 90 °C pretreatment followed by 10,000 kJ/kg TSS ultrasound pretreatment. Combined pretreatments improved the dimethyl sulfide (DMS) removal efficiency by 42–72% but did not show any further improvement in hydrogen sulfide (H2S) removal when compared with ultrasound and thermal pretreatments alone. Economic analysis showed that combined pretreatments with 1000 kJ/kg TSS specific energy and differing thermal pretreatments (50–90 °C) can reduce operating costs by $44–66/ton dry solid when compared to conventional anaerobic digestion without pretreatments. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The biological treatment of wastewater produces a large amount of waste-activated sludge (WAS). Although conventional anaerobic digestion of WAS is a widely used sludge stabilization process, it has some technical limitations. Digesting WAS anaerobically is difficult relative to primary sludge as a result of the rate-limiting hydrolysis step. This is because WAS is composed of diverse microorganisms and organic and inorganic compounds agglomerated together in a polymeric network formed by extracellular polymeric substances (EPSs), including proteins, carbohydrates, lipids and volatile fatty acids (Eskicioglu et al., 2006; Pavlostathis and Gosset, 1986). EPSs strongly influence the hydrolysis step such that breaking the EPS network prior to anaerobic digestion can enhance anaerobic biodegradability and dewaterability of digested sludge (Park et al., 2004; Neyens and Baeyens, 2003). WAS is difficult to dewater (Xuan et al., 2004), and inefficient dewatering increases the costs of sludge

Abbreviations: Cp, specific heat of sludge (kJ/kg °C); F/M, food to microorganism ratio (mg of CODsubstrate/mg of VSSanaerobic seed); P, ultrasonic power (kW); Qs, energy requirement for heating the sludge (kJ); SCOD, soluble oxygen demand (mg/L); SE, specific energy input (kJ/kg TSS); t, ultrasonic duration (s); TCOD, total chemical oxygen demand (mg/L); tfinal, final temperature of sludge (°C); tinitial, initial temperature of sludge (°C); TSS, total suspended solids (mg/L); TTF, time-to-filter (s L/g TSS); V, volume of sludge sonicated (L); TVFA, total volatile fatty acid (mg/L); Vsl, volume of sludge thermally treated (m3); VSS, volatile suspended solids (mg/L); qsl, density of sludge (kg/m3). ⇑ Corresponding author. Tel.: +1 519 661 2111x81273; fax: +1 519 661 3498. E-mail address: [email protected] (M.B. Ray). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.10.007

disposal. Further, volatile sulfur compounds (VSCs), including hydrogen sulfide (H2S) and other organosulfur compounds (e.g., methyl mercaptan, dimethyl sulfide and dimethyl disulfide) in biogas may contribute to corrosion in combustion engines (Rasi et al., 2007) and create unpleasant conditions in wastewater treatment plants. Various pretreatment techniques, including chemical, thermal and mechanical methods, have been reported to stabilize WAS through cell disruption, making organics such as protein, carbohydrate and volatile fatty acids available for microbial consumption. Several studies have been conducted on thermal pretreatments, with the most common temperatures used being between 50 and 180 °C (Appels et al., 2010; Climent et al., 2007). Temperatures above 200 °C create toxic compounds such as dioxin (Stuckey and McCarty, 1984). Pretreatments below 100 °C are considered low but have been shown to effectively increase biogas production in anaerobic digestion (Climent et al., 2007; Gavala et al., 2003). While the timespan of thermal pretreatments have ranged from 15 to 60 min, treatment time appears to have little effect on anaerobic digestion relative to temperature (Valo et al., 2004). Sonication in the P20–40 kHz range is widely reported as a mechanical sludge hydrolysis technique. Studies on WAS pretreatment using ultrasounds of specific energies ranging from 1000 to 10,000 kJ/ kg TS resulted in increases of biogas production by up to 40% (Khanal et al., 2007). Low ultrasound energy input has been identified as a cost-effective tool for enhancing biogas production during anaerobic digestion (Aldin et al., 2010; Climent et al., 2007; Elbeshbishy et al., 2011).

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Although the effects of thermal and ultrasound sludge pretreatments have been studied extensively, no study has yet combined these methods to enhance anaerobic digestion of WAS. Moreover, previous studies on the effectiveness of pretreatments concentrated on improving solids reduction and biogas production, with limited information on the reduction of protein fractions and corrosive constituents (e.g., VSCs) in biogas, or on the overall economic viability of the pretreatment process. This study aims to systematically and comprehensively evaluate the impact of combined sonication and thermal pretreatments on various sludge parameters during anaerobic digestion of WAS. The influence of different pretreatment conditions were evaluated in terms of (a) sludge solubilization, (b) biogas production, (c) H2S and DMS concentrations in biogas, (d) dewaterability of digested sludge, and (e) a detailed economic assessment based on bench scale experimental data.

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(BMP) tests were conducted on the treated WAS in 150 mL serum vials. The volumes of WAS (substrate) and anaerobic seed of VSS (8000 mg/L) were 50 and 70 mL, respectively, based on a ratio of food (COD of substrate) to microorganism (VSS of anaerobic seed) (F/M) of 2 (mg of CODsubstrate/mg of VSSanaerobic seed). Anaerobic seed was collected from the anaerobic digester at the St. Marys wastewater treatment plant, Ontario, Canada. For the control, untreated raw WAS (substrate) was used with seed, whereas only seed and deionized water were used for the blank BMP tests. After purging with nitrogen, the serum bottles were sealed with rubber septa and agitated in 37 ± 1 °C shaker (MaxQ 4000, incubator and refrigerated shaker, Thermo Scientific, Fremont, CA) at 180 rpm. The BMP test was conducted for approximately 28 days until biogas production stopped. 2.4. Analytical methods

2. Material and methods 2.1. Waste activated sludge (WAS) WAS samples were collected from the Adelaide Pollution Control Plant in London, Ontario, Canada. After thickening, the sludge was stored in a cold room at 4 °C. The average characteristics of the WAS used in this study was as follows: total chemical oxygen demand (TCOD): 22,500 ± 1050 mg/L, soluble chemical oxygen demand (SCOD): 1400 ± 60 mg/L, total suspended solid (TSS): 20,700 ± 200 mg/L, volatile suspended solid (VSS): 15,500 ± 450 mg/L, total volatile fatty acid (TVFA): 156 ± 10 mg/L, particulate protein: 3300 ± 60 mg/L, soluble protein: 110 ± 10 mg/L, bound protein: 1300 ± 15 mg/L, soluble carbohydrate: 100 ± 10 mg/L, ammonia: 90 ± 5 mg/L, total nitrogen: 1200 ± 60 mg/L, soluble total nitrogen: 140 ± 5 mg/L, pH: 6.9–7, and alkalinity: 1300 ± 75 mg as CaCO3/L. 2.2. Pretreatment experiments A laboratory-scale ultrasound generator (Model VCX-750, 750 W, 20 kHz, Sonic and Materials, Connecticut, USA) was used for the ultrasound pretreatments (UP). Sludge was pretreated at three specific energy inputs (1000, 5000, and 10,000 kJ/kg TSS). For each experiment, 300 ml of sludge was sonicated in a beaker continuously stirred using a magnetic stirrer. The ultrasound probe (Model CV 33, 2.54 cm diameter, 5 cm length) was immersed into the sludge at a depth of 3.8 cm. Sonication times of 1, 5, and 10 min corresponded to specific energies of 1000, 5000, and 10,000 kJ/kg TSS, respectively. The amplitude of the ultrasound generator was set at 35 lm, and sonication pulses were set to 2 s on and 2 s off to maintain the sludge temperature below 40 °C during the experiments. Thermal pretreatments (TP) were conducted at three temperatures (50 ± 2, 70 ± 2, and 90 ± 2 °C). Approximately 300 mL of sludge was put in a glass volumetric flask closed with a rubber septum fit with a temperature probe. The flask was placed on a hot stirring plate (Corning Stirrer/Hot plate, Model PC-420, Corning Incorporated, USA) and heated at the set temperature for 30 min. At first, pretreatment experiments were conducted using either sonication or thermal pretreatments only. Subsequently, the combined pretreatment (CP) was conducted by varying both sonication energy and temperature. The pretreatment conditions are summarized in Table 1. All experiments were conducted in duplicate. Differences between duplicate measurements were less than 5% for all parameters, and hence average values are reported here. 2.3. Biochemical methane potential (BMP) test To assess the effect of different pretreatment conditions on WAS anaerobic digestibility, biochemical methane potential

All water quality parameters were analyzed according to standard methods (APHA, 1998). Soluble parameters were analyzed after filtering the sludge sample through 0.45 lm membrane filters. HACH analytical vials (Hach Company, Loveland, Colorado, USA) were used to measure chemical oxygen demand (COD), total nitrogen (TN), soluble total nitrogen (STN), nitrate, nitrite and ammonia. Soluble organic nitrogen was calculated by subtracting the soluble inorganic nitrogen (ammonia + nitrate + nitrite) from the soluble total nitrogen. Dewaterability of digested sludge was measured using the time-to-filter (TTF) method (Method No. 2710 H, APHA, 1998), defined as the time required to filter 50% of the initial sludge volume. A Buchner funnel was used to measure the time required to filter 30 mL of a 60 mL sample through a filter paper (Whatman No. 1: Cat. No. 1001090, Whatman International Ltd., UK). The concentrations of volatile fatty acids (VFAs) were analyzed using a gas chromatograph (Model Varian 8500, Varian Inc., Toronto, Canada) with a flame ionization detector (FID) equipped with a fused silica column (30 m  0.32 mm). Helium was used as the carrier gas at a flow rate of 5 mL/min. The temperatures of the column and detector were 110 and 250 °C, respectively. H2S in the biogas was measured using an Odalog (Model Odalog type I, App-Tek International Pty Ltd., Brendale 4500, Australia) with a detection range of 0–1000 ppm. DMS in the biogas was measured using a gas chromatograph (GC 2010, Shimadzu) with a flame photometric detector (FPD) equipped with a BPX-5 (5% phenyl polysilphenylene-siloxane) capillary column (30 m  0.25 m i.d. 0.25 lm thickness) obtained from SGE (Austin, TX). Helium was used as the carrier gas at a flow rate of 4 mL/min. The temperatures of the column and injection were 60 and 250 °C, respectively. The temperature of FPD was 250 °C. The flow rates of hydrogen and air were 60 and 70 mL/min, respectively. Protein fractions were determined by micro-bicinchoninic acid protein assays (Pierce, Rockford, USA). This method, modified by Lowry et al. (1951), uses a standard solution of bovine serum albumin. The details of measurement of the various protein fractions are provided by Elbeshbishy et al. (2010). The fraction of cell protein was calculated from the difference between particulate and bound protein. Soluble carbohydrate concentration was determined according to the phenol–sulfuric acid method (Webb, 1985). For carbohydrate analysis, 0.1 mL of the test sample was placed in a test tube followed by the immediate addition of 0.1 ml of 5% phenol and 4 mL of concentrated sulfuric acid. The absorbance of the sample was measured using a spectrophotometer (Varian Cary 50 UV–Vis) at 490 nm. The daily volume of biogas was measured by releasing the gas pressure in the serum vials using glass syringes (Perfektum; Popper & SonsInc., NY, USA) in the 5–100 mL range, allowing it to equilibrate with the atmospheric pressure (Owen et al., 1979).

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Table 1 Summary of pretreatment conditions. Set

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a b c d

Controla UPb 1 UP 2 UP 3 TPc 1 TP 2 TP 3 CPd 1 CP 2 CP 3 CP 4 CP 5 CP 6 CP 7 CP 8 CP 9

Specific energy input (kJ/kg TSS)

Thermal pretreatment temperature (°C)

Actual energy imparted to the sludge (kJ)

– 1000 5000 10,000 – – – 1000 1000 1000 5000 5000 5000 10,000 10,000 10,000

– – – – 50 70 90 50 70 90 50 70 90 50 70 90

– 4 19 38 31 56 82 35 60 85 50 75 100 69 94 119

3. Results and discussion

Control = raw untreated WAS. UP = ultrasound pretreatment. TP = thermal pretreatment. CP = combined pretreatment.

3.1. COD solubilization

The concentration of methane in the biogas was analyzed using a SRI 310C Gas Chromatograph (Model 310, SRI Instruments, Torrance, CA) equipped with a molecular sieve column (Molesieve 5A, mesh 80/100, 182.88  0.3175 cm) and a thermal conductivity detector (TCD). The temperatures of the column and the TCD detector were 90 and 105 °C, respectively. Argon was used as the carrier gas at a flow rate of 30 mL/min. 2.5. Energy and economic analysis The specific energy (SE) input is a function of ultrasonic power, ultrasonic duration, and volume of sonicated sludge and TS concentration. The SE (in kJ/kg TSS) was calculated using the following equation (Bougrier et al., 2005):

SE ¼

Pt V  TSS

ð1Þ

where P is the applied ultrasonic power in kW, t is the ultrasonic duration in seconds, V is the volume of sludge in liters, and TSS is the total suspended solids concentration in kg/L. Due to dielectric and mechanical power losses through vibration, the actual ultrasound energy imparted to the liquid is lower than the amount of energy applied by the ultrasound device (Kobus and Kusin´ska, 2008). Acoustic power transferred by the ultrasound generator to a liquid medium is eventually converted into heat (Berlan and Mason, 1992). The actual energy transferred to the sludge has been calculated using the thermal method recommended by Raso et al. (1999). The actual energy requirements for heating sludge in the thermal pretreatments were calculated based on the following equation (Zupancic and Ros, 2003):

Q s ¼ qsl V sl C p ðt final  t initial Þ

difference in the energy required to heat the sludge and the recovery of heat from the thermally pretreated sludge with a heat exchanger. The assumptions for the cost calculations of the thermal pretreatment included the following: (a) initial sludge temperature of 25 °C, (b) 20% heat loss due to thermal pretreatment equipment, and (c) 80% heat recovery from pretreated sludge. The costs of dewatering, transportation and landfill were estimated at $250/ ton dry solids, while the costs of electricity and natural gas were estimated at $0.07/kWh and $0.28/m3, respectively (Elbeshbishy et al., 2010). The amount of solids used in for dewatering, transportation and landfill was calculated based on the total solid removal achieved during the pretreatment and anaerobic digestion (BMP test). The costs of removing H2S from biogas was calculated based on the estimates reported by Mckinsey Zicari (2003) using a nonregenerable KOH-AC bed (USFilter-Westates). Costs per unit of biogas purification and absorbent per unit of H2S removed were estimated as $0.0005/m3 biogas and $12/kg H2S, respectively.

ð2Þ

where Qs is the heat required to heat the sludge in kJ, qsl is the density of sludge in kg/m3, Vsl is the volume of sludge treated in m3, Cp is the specific heat of sludge in kJ/kg °C (4.18 kJ/kg °C), tinitial is the initial temperature of sludge in °C, and tfinal is the final temperature of sludge in °C. The actual energy imparted to the sludge among the different pretreatment conditions is shown in Table 1. For economic assessments, the operating costs of electricity for ultrasound pretreatments are calculated based on the specific energy input applied by the ultrasonic device. For thermal pretreatments, the net pretreatment cost is calculated from the

Fig. 1(a) shows the impact of different pretreatment conditions on the ratio of SCOD/TCOD. After all pretreatments, the total COD in the pretreated sludge remained almost constant. As expected, all pretreatments caused significant increases in the ratios of SCOD/TCOD when compared to the control. However, COD solubilization was similar between the 50 and 70 °C thermal pretreatments and for the 5000 and 10,000 kJ/kg TSS ultrasound pretreatments, as the actual energy supplied to the system was similar. Combined pretreatments showed increased COD solubilization when compared to the ultrasound pretreatment alone. Fig. 1(b) shows that the increase in SCOD/TCOD ratio relative to the control was significantly correlated with the actual energies imparted to the sludge during the pretreatment. The solubilization of COD responded similarly to results found in previous pretreatment studies. Eskicioglu et al. (2006) used thermal pretreatments of 96 °C and reported 3.6 times higher SCOD of thickened WAS, whereas there was 5 times higher SCOD after 30 min of holding time at 90 °C in the current study. Ivo and Jing (2009) used pretreatment temperatures of 50–70 °C and reported an increase in the ratio of SCOD/TCOD from 2% to 21% (there was no difference between the 50 and 70 °C treatments). For the ultrasound pretreatment, Bougrier et al. (2005) found an increase in the ratio of SCOD/TCOD in WAS from 4% to 32% when increasing specific energy input from 0 to 10,000 kJ/kg TSS; in the current study, the same specific energy input (10,000 kJ/kg TSS) increased the SCOD/TCOD ratio from 6% to 33%. 3.2. Solids reduction The influence of different pretreatment conditions on volatile suspended solid (VSS) removal is shown in Table 2. Because sludge disintegrated during the ultrasound pretreatment, VSS decreased by 23%, 28%, and 30% for the 1000, 5000, and 10,000 kJ/kg TSS specific energy inputs, respectively. This indicates that increasing the specific energy from 5000 to 10,000 kJ/kg TSS did not significantly improve the reduction of VSS. Although the SCOD/TCOD ratio increased with temperature, the reductions of VSS were nearly the same for the different thermal pretreatments. The significant increase in the SCOD/TCOD ratio with thermal pretreatments may have resulted from the solubilization of colloidal matters (<0.45 lm), which was not measured separately in the study. The combined pretreatments, using different combinations of specific energies and temperatures, showed slight improvements in VSS

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Fig. 1. (a) Impact of different pretreatments on SCOD/TCOD ratio, (b) Increase in the SCOD/TCOD ratio compared to the control as a function of actual energies imparted to the sludge.

reduction when compared to ultrasound and thermal pretreatments alone; VSS was reduced 32–36% in the single relative to the combined pretreatments.

Table 2 Impact of different pretreatments on VSS reduction, soluble organic nitrogen, soluble carbohydrate, and TVFA concentrations. Set

VSS reduction (%)

Soluble organic nitrogen (mg/L)

Soluble carbohydrate (mg/L)

TVFA (mg/L)

Control UP 1 UP 2 UP 3 TP 1 TP 2 TP 3 CP 1 CP 2 CP 3 CP 4 CP 5 CP 6 CP 7 CP 8 CP 9

– 23 28 30 25 26 27 32 33 34 33 36 35 34 33 36

50 72 222 514 223 297 596 406 356 425 462 436 498 536 542 538

104 272 397 863 298 379 555 468 587 589 568 680 755 1113 1448 1485

156 222 243 276 181 196 178 258 306 300 303 300 352 416 452 520

3.3. Protein, carbohydrate and VFA release It is possible that an increase in the SCOD/TCOD ratio that results from the pretreatments might originate from the disruption of microbial cells in WAS causing the release of various organic compounds (e.g., carbohydrates, proteins, lipids, and VFAs). In this study, the changes in different protein fractions, as well as the increase in soluble carbohydrate and TVFA concentrations, were measured in the pretreated sludge samples. The impact of different pretreatment conditions on TVFA release is shown in Table 2. TVFA solubilization increased with increasing specific energy input but was nearly similar among the three thermal pretreatments. In the combined pretreatment, a maximum energy input of 119 kJ (CP 9) led to a 230% increase in TVFA concentration, and maximum VSS reduction, when compared to the control. The soluble carbohydrate concentrations for the different pretreatments are shown in Table 2. Soluble carbohydrates increased by 162%, 282%, and 730% for the specific energy inputs of 1000, 5000, and 10,000 kJ/kg TSS, respectively. Soluble carbohydrates increased by 350%, 264%, and 434% at 50, 70 and 90 °C pretreatment temperatures, respectively. In the combined pretreatments, a maximum of 1400% increase in soluble carbohydrates was found for CP 9 (10,000 kJ/kg TSS + 90 °C). Comparing the absolute energy input, UP 3 (38 kJ) was comparable to TP 1 (31 kJ); however, ultrasound pretreatments provided greater solubilization of carbohydrates,

possibly due to both mechanical and chemical disintegration of the particulates. Protein content in sludge is usually divided into three different fractions: cell, bound and soluble (Dimock and Morgenroth, 2006). The cell protein represents the fraction inside the microbial cell, the bound protein is the protein loosely attached to the microbial cell wall, and the soluble protein is in the aqueous phase. Particulate protein is the combination of cell protein and bound protein.

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Fig. 2 shows the influence of the pretreatments on different protein fractions. Particulate protein (cell + bound) decreased by 11%, 22%, and 30%, while soluble protein increased by 271%, 568%, and 764% for the specific energy inputs of 1000, 5000, and 10,000 kJ/kg TSS, respectively. However, the average reduction of bound protein (15%) was the same among the different specific energy inputs. This indicates that the observed increase in soluble protein results from the reduction in cell protein concentrations. Among thermal pretreatments, particulate protein reduction was nearly the same, and the average particulate (cell + bound) and bound protein reductions were 18% and 28%, respectively. Soluble protein increased by approximately 400% for all thermal pretreatments. As shown in Fig. 2, the combined pretreatments always produced better results both in terms of bound protein reduction and increased soluble protein when compared to ultrasound and thermal pretreatments alone. However, as with carbohydrate solubilization, ultrasonication with the same level of energy input relative to the thermal pretreatment provides better solubilization of particulate protein. Fig. 3(a) shows a significant correlation between cell protein reduction and VSS reduction (R2 = 0.8071). All pretreatments resulted in significant soluble organic nitrogen release into the aqueous phase when compared to the control (Table 2).

Fig. 3(b) shows a significant correlation between soluble organic nitrogen concentrations and soluble protein concentrations among pretreatments (R2 = 0.8114). These results suggest that the soluble protein concentrations correspond with the organic nitrogen solubilization. The reduction in particulate protein corresponds with the increase in soluble protein, suggesting a closure of protein mass balance. 3.4. Impact on methane potential The results of the BMP tests are shown in Table 3. Although the SCOD/TCOD ratio increased with increasing pretreatment temperatures from 50 to 90 °C, the increases in methane production during anaerobic digestion was nearly the same among thermal pretreatments. Total methane production was also nearly the same for both 5000 and 10,000 kJ/kg TSS specific energy input pretreatments. The combined pretreatments showed further improvement in methane production when compared to thermal pretreatment alone. Bougrier et al. (2005) reported no significant improvement in methane production when increasing specific energy input from 6250 to 9350 kJ/kg TSS in WAS. In the current study, 5000 kJ/kg TSS was found to be optimum specific energy input for enhanced

Fig. 2. Impact of different pretreatment on different protein fractions.

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Fig. 3. Relationship between (a) cell protein and VSS (mg/L) reduction for different pretreatments compared to the control and (b) soluble protein (mg/L) and soluble organic nitrogen (mg/L) concentrations.

biogas production. In terms of biogas production, the ultrasound pretreatments were more effective than thermal pretreatments, although better COD solubilization was observed in the thermal pretreatment. Greater solubilization with thermal pretreatments results from the solubilization of colloidal COD, which was not affected by ultrasound pretreatments. It is possible that the lower gas production for thermally pretreated sludge is due to the formation of agglomerates as well as increases in particle size following thermal pretreatments (Bougrier et al., 2005); the opposite occurs during ultrasound pretreatments. The expected methane yield at 37 °C (390 ml CH4/gm of TCODremoved) also agrees well with the

Fig. 4. Relationship of (a) VSC concentration reductions (ppm) with bound protein reductions (mg/L) during various pretreatments relative to the control, (b) VSC concentration reductions (ppm) with VSS reductions (mg/L) during various pretreatments relative to the control and (c) VSC concentration in biogas (ppm) with VSS reductions (mg/L) during digestion.

Table 3 Summary of biogas production, VSC removal in biogas, and TTF of digested sludge for different pretreatments. Set

Control UP 1 UP 2 UP 3 TP 1 TP 2 TP 3 CP 1 CP 2 CP 3 CP 4 CP 5 CP 6 CP 7 CP 8 CP 9

Specific CH4 production (mL CH4/gm VSS)

325 374 391 404 370 386 368 388 386 398 420 410 406 414 410 424

Increase in total CH4 production (%)

– 15 20 24 14 19 13 19 19 23 29 26 25 27 26 30

Mean CH4 content (%volume)

50 52 52 53 50 53 53 53 53 52 55 56 55 55 56 56

Average VSCs removal efficiency compared to the control (%) H2S

DMS

– 18 21 22 34 35 34 33 34 35 33 36 38 39 41 39

– 23 38 38 40 30 59 42 53 54 52 45 56 57 57 72

TTF (s L/g TSS)

81 76 75 52 48 56 54 60 51 50 56 57 55 61 59 58

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Table 4 Economic assessment for different pretreatment processes compared to the control.a Set

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a b c d

UP 1 UP 2 UP 3 TP 1 TP 2 TP 3 CP 1 CP 2 CP 3 CP 4 CP 5 CP 6 CP 7 CP 8 CP 9

Pretreatment costb ($)

Increase in CH4 production ($)

Saving in H2S removal cost ($)

Dewatering, transportation and landfill cost Amount of solids (ton)

Decrease in cost ($)

20 98 196 24 43 62 44 63 82 122 141 160 220 239 258

8 10 12 7 9 7 10 9 11 14 13 12 14 13 15

11 11 12 42 46 45 40 42 42 28 44 49 46 54 46

0.54 0.51 0.52 0.55 0.54 0.54 0.52 0.51 0.47 0.51 0.48 0.45 0.46 0.43 0.42

55 63 60 53 55 55 60 63 73 63 70 78 75 83 85

c

Net saving compared to the controld ($) 54 14 112 78 67 45 66 51 44 17 14 21 85 89 112

All results are shown for per ton solid treatment compared to the control. Energy input cost for different pretreatment process. Amount of solid after pretreatment and anaerobic digestion. Net saving compared to control = increase in methane ($) + reduction in H2S removal cost + reduction in dewatering, transportation and landfill cost ($)  pretreatment cost ($).

observed yields, which ranged from 339 to 375 CH4/gm of TCODremoved (results not shown); slight differences are attributed to experimental error. After 28 days of BMP tests, all pretreated sludge samples produced higher amounts of methane when compared to the control. Although the increase in SCOD due to the pretreatment is expected to translate into additional biogas, the increase in biogas production did not show any linear relationships with the COD solubilization.

reductions during digestion is significantly correlated with VSC concentrations in biogas. As the VSC concentrations were proportional to VSS destroyed during digestion, it is plausible that the VSC concentrations in biogas among the pretreatments were lower as a result of the lower absolute mass of VSS destroyed during digestion because some VSS was degraded during pretreatment. The sulfur emission during pretreatment could not be experimentally measured to close the mass balance. 3.6. Impact on dewaterability

3.5. Volatile sulfur compounds in biogas In anaerobic digestion, hydrogen sulfide (H2S) and different types of organosulfur compounds (mercaptans) are produced from sulfur-containing proteins and the methylation of sulfide (Higgins et al., 2004). In this study, the H2S and DMS concentrations were measured in biogas among the different pretreatment conditions. For the control, the average H2S and DMS concentrations in biogas were 38 ± 1 and 18 ± 1 ppm, respectively. The average H2S and DMS removal efficiencies in biogas compared to the control for different pretreatments are shown in Table 3. Thermal pretreatments had more effective VSC removal efficiency compared to the ultrasound pretreatment. In the combined pretreatments, DMS removal efficiencies were higher than both ultrasound and thermal pretreatments alone, while H2S removal efficiencies were higher than ultrasound pretreatments alone. Although all pretreatments showed significant effects on VSCs removal in biogas when compared to the control, VSC removal efficiencies did not show any linear relationship with increasing specific energy input and temperature. A recent study found that bound protein had a significant impact on the generation of H2S and other organosulfur compounds (Higgins et al., 2004). Fig. 4(a) shows that the reduction of bound protein concentration was significantly correlated with the reduction of H2S and DMS concentrations when compared to the control and that the bound protein concentration reduction decreased the H2S and DMS concentration in biogas by 6 and 3 ppm, respectively. Fig. 4(b) shows that the reduction in VSS concentrations due to the pretreatments was significantly correlated with VSC concentration reductions in biogas. Verma et al. (2006) and Dhar et al. (2011) also showed that sludge containing lower VSS concentrations had lower VSC generation potential. However, it is expected that enhanced digestion should produce more sulfur emission in biogas. Fig. 4(c) shows that VSS concentration

The TTF results for the digested sludges at the end of BMP test are shown in Table 3. TTF values are normalized to TSS concentrations of the digested sludge and expressed in units of s L/g TSS. The TTF values represent how quickly sludge releases its water. Ultrasound pretreatments at 1000 and 5000 kJ/kg TSS specific energy inputs showed a marginal improvement in dewaterability, while 10,000 kJ/kg TSS specific energy input significantly decreased the TTF by 36%. TTF values were nearly the same among temperature pretreatments (50–90 °C) but lower than the TTF values observed in the ultrasound pretreatments. Combined pretreatments did not significantly improve the TTF values when compared to the thermal pretreatment alone. These results suggest that different pretreatments can improve the dewaterability of the digested sludge relative to the control. The enhanced dewaterability might be due to the various biopolymers (proteins and carbohydrates) released through different pretreatments (Novak et al., 2003). 3.7. Economic assessment Because the cost of sludge management is around 50% of the total operating cost of the wastewater treatment plant (Odegaard, 2004), the economic feasibility of a pretreatment process is closely related to the enhancement in methane production and solids reduction. Although pretreatments give additional benefits, including volatile sulfur compound reduction and improved sludge dewaterability, retrofitting pretreatment systems to the conventional anaerobic digestion process adds extra operating costs. Therefore, an economic evaluation is required to establish the feasibility of implementing a costly pretreatment process. Based on the experimental results obtained in this work, an economic assessment was conducted per ton of solids (TSS) treated with the anaerobic digestion process. Table 4 shows the summary

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economic assessment results for different pretreatment processes when compared to the conventional process or control. Although all 15 pretreatment conditions significantly improved biogas production and solid reduction relative to the control, only seven are economically feasible. Ultrasound pretreatment is economically feasible only at specific energy input of 1000 kJ/kg TSS, with a net savings of $54/ton dry solid. All thermal pretreatments (50– 90 °C) are economically feasible when compared to the control, with net savings of $45–78/ton dry solid. Combined pretreatments are feasible with specific energy inputs of 1000 kJ/kg TSS with all of the different thermal pretreatment temperatures (50–90 °C), giving a net savings of $44–66/ton dry solid. In addition to the costs of H2S removal, the reduction of volatile sulfur compound reductions in biogas can give long-term economic benefits by decreasing corrosion rates and increasing engine life. Improvement in dewaterability of digested sludge and the optimization of polymer dosages in dewatering may slightly impact the overall dewatering, transportation and landfill costs ($250/ton solids). The installation of pretreatment systems will also increase the overall capital investment. However, these factors are not considered in this economic assessment. 4. Conclusions Ultrasound, thermal and combined pretreatments can reduce the VSS in raw WAS by 22–31%, 25–39%, and 29–38%, respectively, in addition to significant improvements in COD solubilization and the release of various organic compounds. Pretreatments combining 10,000 kJ/kg TSS specific energy inputs at 90 °C significantly increased total methane by 30% and decreased H2S and DMS in biogas by 39% and 72%, respectively. Based on the economic evaluation relative to conventional anaerobic digestion, the relative ranking of economically feasible pretreatment process is as follows: TP 1 (thermal at 50 °C) > TP 2 (thermal at 70 °C) > CP 1 (thermal at 50 °C + ultrasound at 1000 kJ/kg TSS) > UP 1 (ultrasound at 1000 kJ/kg TSS) > CP 2 (thermal at 70 °C + ultrasound at 1000 kJ/ kg TSS) > TP 3 (thermal at 90 °C) > CP3 (thermal at 90 °C + ultrasound at 1000 kJ/kg TSS). These processes achieved savings of $44–78/ton dry solid when compared to the control. Acknowledgments The authors would like to acknowledge Trojan Technologies, Inc. and Natural Science and Engineering Research Council (NSERC), Canada for their financial support. References Aldin, S., Elbeshbishy, E., Nakhla, G., Ray, M., 2010. Modeling the effect of sonication on the anaerobic digestion of biosolids. Energy Fuels 24 (9), 4703–4711. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington DC, USA. Appels, L., Degrève, J., Bruggen, B.V.D., Impe, J.V., Dewil, R., 2010. Influence of low temperature thermal pre-treatment on sludge solubilization, heavy metal release and anaerobic digestion. Bioresour. Technol. 101 (15), 5743–5748. Berlan, J., Mason, T.J., 1992. Sonochemistry: from research laboratories to industrial plants. Ultrasonics 30 (4), 203–212. Bougrier, C., Carrère, H., Delgenès, J.P., 2005. Solubilization of waste-activated sludge by ultrasonic treatment. Chem. Eng. J. 106, 163–169.

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