Waste Management 32 (2012) 542–549

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Waste Management

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Techno-economic evaluation of ultrasound and thermal pretreatmentsfor 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 i n f o

Article history:Received 4 March 2011Accepted 11 October 2011Available online 15 November 2011

Keywords:Anaerobic digestionBiogasThermal pretreatmentUltrasound pretreatmentWaste activated sludge

0956-053X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.wasman.2011.10.007

Abbreviations: Cp, specific heat of sludge (kJ/kg �C)ratio (mg of CODsubstrate/mg of VSSanaerobic seed); P, ultrarequirement for heating the sludge (kJ); SCOD, solublespecific energy input (kJ/kg TSS); t, ultrasonic duratioxygen demand (mg/L); tfinal, final temperature otemperature of sludge (�C); TSS, total suspended soli(s L/g TSS); V, volume of sludge sonicated (L); TVFA, toVsl, volume of sludge thermally treated (m3); VSS, volaqsl, density of sludge (kg/m3).⇑ Corresponding author. Tel.: +1 519 661 2111x812

E-mail address: mray@eng.uwo.ca (M.B. Ray).

a b s t r a c t

To enhance the anaerobic digestion of municipal waste-activated sludge (WAS), ultrasound, thermal, andultrasound + thermal (combined) pretreatments were conducted using three ultrasound specific energyinputs (1000, 5000, and 10,000 kJ/kg TSS) and three thermal pretreatment temperatures (50, 70 and90 �C). Prior to anaerobic digestion, combined pretreatments significantly improved volatile suspendedsolid (VSS) reduction by 29–38%. The largest increase in methane production (30%) was observed after30 min of 90 �C pretreatment followed by 10,000 kJ/kg TSS ultrasound pretreatment. Combined pretreat-ments improved the dimethyl sulfide (DMS) removal efficiency by 42–72% but did not show any furtherimprovement in hydrogen sulfide (H2S) removal when compared with ultrasound and thermal pretreat-ments alone. Economic analysis showed that combined pretreatments with 1000 kJ/kg TSS specificenergy and differing thermal pretreatments (50–90 �C) can reduce operating costs by $44–66/ton drysolid when compared to conventional anaerobic digestion without pretreatments.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction disposal. Further, volatile sulfur compounds (VSCs), including

The biological treatment of wastewater produces a large amountof waste-activated sludge (WAS). Although conventional anaerobicdigestion of WAS is a widely used sludge stabilization process, it hassome technical limitations. Digesting WAS anaerobically is difficultrelative to primary sludge as a result of the rate-limiting hydrolysisstep. This is because WAS is composed of diverse microorganismsand organic and inorganic compounds agglomerated together in apolymeric network formed by extracellular polymeric substances(EPSs), including proteins, carbohydrates, lipids and volatile fattyacids (Eskicioglu et al., 2006; Pavlostathis and Gosset, 1986). EPSsstrongly influence the hydrolysis step such that breaking the EPSnetwork prior to anaerobic digestion can enhance anaerobic biode-gradability 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

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; F/M, food to microorganismsonic power (kW); Qs, energyoxygen demand (mg/L); SE,

on (s); TCOD, total chemicalf sludge (�C); tinitial, initialds (mg/L); TTF, time-to-filtertal volatile fatty acid (mg/L);tile suspended solids (mg/L);

73; fax: +1 519 661 3498.

hydrogen sulfide (H2S) and other organosulfur compounds (e.g.,methyl mercaptan, dimethyl sulfide and dimethyl disulfide) in bio-gas may contribute to corrosion in combustion engines (Rasi et al.,2007) and create unpleasant conditions in wastewater treatmentplants.

Various pretreatment techniques, including chemical, thermaland mechanical methods, have been reported to stabilize WASthrough cell disruption, making organics such as protein, carbohy-drate 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 and180 �C (Appels et al., 2010; Climent et al., 2007). Temperaturesabove 200 �C create toxic compounds such as dioxin (Stuckeyand McCarty, 1984). Pretreatments below 100 �C are consideredlow but have been shown to effectively increase biogas productionin anaerobic digestion (Climent et al., 2007; Gavala et al., 2003).While the timespan of thermal pretreatments have ranged from15 to 60 min, treatment time appears to have little effect on anaer-obic digestion relative to temperature (Valo et al., 2004). Sonica-tion in the P20–40 kHz range is widely reported as a mechanicalsludge hydrolysis technique. Studies on WAS pretreatment usingultrasounds 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 iden-tified as a cost-effective tool for enhancing biogas production dur-ing anaerobic digestion (Aldin et al., 2010; Climent et al., 2007;Elbeshbishy et al., 2011).

B.R. Dhar et al. / Waste Management 32 (2012) 542–549 543

Although the effects of thermal and ultrasound sludge pretreat-ments have been studied extensively, no study has yet combinedthese methods to enhance anaerobic digestion of WAS. Moreover,previous studies on the effectiveness of pretreatments concen-trated on improving solids reduction and biogas production, withlimited information on the reduction of protein fractions and corro-sive constituents (e.g., VSCs) in biogas, or on the overall economicviability of the pretreatment process. This study aims to systemat-ically and comprehensively evaluate the impact of combined soni-cation and thermal pretreatments on various sludge parametersduring anaerobic digestion of WAS. The influence of different pre-treatment conditions were evaluated in terms of (a) sludge solubi-lization, (b) biogas production, (c) H2S and DMS concentrations inbiogas, (d) dewaterability of digested sludge, and (e) a detailed eco-nomic assessment based on bench scale experimental data.

2. Material and methods

2.1. Waste activated sludge (WAS)

WAS samples were collected from the Adelaide Pollution ControlPlant in London, Ontario, Canada. After thickening, the sludge wasstored in a cold room at 4 �C. The average characteristics of theWAS used in this study was as follows: total chemical oxygendemand (TCOD): 22,500 ± 1050 mg/L, soluble chemical oxygendemand (SCOD): 1400 ± 60 mg/L, total suspended solid (TSS):20,700 ± 200 mg/L, volatile suspended solid (VSS): 15,500 ± 450mg/L, total volatile fatty acid (TVFA): 156 ± 10 mg/L, particulateprotein: 3300 ± 60 mg/L, soluble protein: 110 ± 10 mg/L, boundprotein: 1300 ± 15 mg/L, soluble carbohydrate: 100 ± 10 mg/L,ammonia: 90 ± 5 mg/L, total nitrogen: 1200 ± 60 mg/L, soluble totalnitrogen: 140 ± 5 mg/L, pH: 6.9–7, and alkalinity: 1300 ± 75 mg asCaCO3/L.

2.2. Pretreatment experiments

A laboratory-scale ultrasound generator (Model VCX-750,750 W, 20 kHz, Sonic and Materials, Connecticut, USA) was usedfor the ultrasound pretreatments (UP). Sludge was pretreated atthree specific energy inputs (1000, 5000, and 10,000 kJ/kg TSS). Foreach experiment, 300 ml of sludge was sonicated in a beaker contin-uously stirred using a magnetic stirrer. The ultrasound probe (ModelCV 33, 2.54 cm diameter, 5 cm length) was immersed into the sludgeat a depth of 3.8 cm. Sonication times of 1, 5, and 10 min corre-sponded to specific energies of 1000, 5000, and 10,000 kJ/kg TSS,respectively. The amplitude of the ultrasound generator was set at35 lm, and sonication pulses were set to 2 s on and 2 s off to main-tain the sludge temperature below 40 �C during the experiments.

Thermal pretreatments (TP) were conducted at three tempera-tures (50 ± 2, 70 ± 2, and 90 ± 2 �C). Approximately 300 mL ofsludge was put in a glass volumetric flask closed with a rubber sep-tum fit with a temperature probe. The flask was placed on a hotstirring plate (Corning Stirrer/Hot plate, Model PC-420, CorningIncorporated, USA) and heated at the set temperature for 30 min.At first, pretreatment experiments were conducted using eithersonication or thermal pretreatments only. Subsequently, the com-bined pretreatment (CP) was conducted by varying both sonicationenergy and temperature. The pretreatment conditions are summa-rized in Table 1. All experiments were conducted in duplicate. Dif-ferences between duplicate measurements were less than 5% for allparameters, and hence average values are reported here.

2.3. Biochemical methane potential (BMP) test

To assess the effect of different pretreatment conditions onWAS anaerobic digestibility, biochemical methane potential

(BMP) tests were conducted on the treated WAS in 150 mL serumvials. The volumes of WAS (substrate) and anaerobic seed of VSS(8000 mg/L) were 50 and 70 mL, respectively, based on a ratio offood (COD of substrate) to microorganism (VSS of anaerobic seed)(F/M) of 2 (mg of CODsubstrate/mg of VSSanaerobic seed). Anaerobicseed was collected from the anaerobic digester at the St. Maryswastewater treatment plant, Ontario, Canada. For the control, un-treated raw WAS (substrate) was used with seed, whereas onlyseed and deionized water were used for the blank BMP tests. Afterpurging with nitrogen, the serum bottles were sealed with rubbersepta and agitated in 37 ± 1 �C shaker (MaxQ 4000, incubator andrefrigerated shaker, Thermo Scientific, Fremont, CA) at 180 rpm.The BMP test was conducted for approximately 28 days until bio-gas production stopped.

2.4. Analytical methods

All water quality parameters were analyzed according to stan-dard methods (APHA, 1998). Soluble parameters were analyzedafter filtering the sludge sample through 0.45 lm membrane fil-ters. HACH analytical vials (Hach Company, Loveland, Colorado,USA) were used to measure chemical oxygen demand (COD), totalnitrogen (TN), soluble total nitrogen (STN), nitrate, nitrite andammonia. Soluble organic nitrogen was calculated by subtractingthe soluble inorganic nitrogen (ammonia + nitrate + nitrite) fromthe soluble total nitrogen. Dewaterability of digested sludge wasmeasured 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 measurethe time required to filter 30 mL of a 60 mL sample through a filterpaper (Whatman No. 1: Cat. No. 1001090, Whatman InternationalLtd., UK).

The concentrations of volatile fatty acids (VFAs) were analyzedusing a gas chromatograph (Model Varian 8500, Varian Inc., Toron-to, Canada) with a flame ionization detector (FID) equipped with afused silica column (30 m � 0.32 mm). Helium was used as the car-rier gas at a flow rate of 5 mL/min. The temperatures of the columnand detector were 110 and 250 �C, respectively. H2S in the biogaswas measured using an Odalog (Model Odalog type I, App-TekInternational Pty Ltd., Brendale 4500, Australia) with a detectionrange of 0–1000 ppm. DMS in the biogas was measured using agas chromatograph (GC 2010, Shimadzu) with a flame photometricdetector (FPD) equipped with a BPX-5 (5% phenyl polysilphenyl-ene-siloxane) capillary column (30 m � 0.25 m i.d.�0.25 lm thick-ness) obtained from SGE (Austin, TX). Helium was used as thecarrier gas at a flow rate of 4 mL/min. The temperatures of the col-umn and injection were 60 and 250 �C, respectively. The tempera-ture of FPD was 250 �C. The flow rates of hydrogen and air were60 and 70 mL/min, respectively.

Protein fractions were determined by micro-bicinchoninic acidprotein assays (Pierce, Rockford, USA). This method, modified byLowry et al. (1951), uses a standard solution of bovine serum albu-min. The details of measurement of the various protein fractionsare provided by Elbeshbishy et al. (2010). The fraction of cell pro-tein was calculated from the difference between particulate andbound protein. Soluble carbohydrate concentration was deter-mined according to the phenol–sulfuric acid method (Webb,1985). For carbohydrate analysis, 0.1 mL of the test sample wasplaced in a test tube followed by the immediate addition of0.1 ml of 5% phenol and 4 mL of concentrated sulfuric acid. Theabsorbance of the sample was measured using a spectrophotome-ter (Varian Cary 50 UV–Vis) at 490 nm.

The daily volume of biogas was measured by releasing the gaspressure in the serum vials using glass syringes (Perfektum; Pop-per & SonsInc., NY, USA) in the 5–100 mL range, allowing it toequilibrate with the atmospheric pressure (Owen et al., 1979).

Table 1Summary of pretreatment conditions.

Set Specific energy input(kJ/kg TSS)

Thermalpretreatmenttemperature (�C)

Actual energyimparted to thesludge (kJ)

1 Controla – – –2 UPb 1 1000 – 43 UP 2 5000 – 194 UP 3 10,000 – 385 TPc 1 – 50 316 TP 2 – 70 567 TP 3 – 90 828 CPd 1 1000 50 359 CP 2 1000 70 6010 CP 3 1000 90 8511 CP 4 5000 50 5012 CP 5 5000 70 7513 CP 6 5000 90 10014 CP 7 10,000 50 6915 CP 8 10,000 70 9416 CP 9 10,000 90 119

a Control = raw untreated WAS.b UP = ultrasound pretreatment.c TP = thermal pretreatment.d CP = combined pretreatment.

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The concentration of methane in the biogas was analyzed using aSRI 310C Gas Chromatograph (Model 310, SRI Instruments,Torrance, CA) equipped with a molecular sieve column (Molesieve5A, mesh 80/100, 182.88 � 0.3175 cm) and a thermal conductivitydetector (TCD). The temperatures of the column and the TCDdetector were 90 and 105 �C, respectively. Argon was used as thecarrier 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 con-centration. The SE (in kJ/kg TSS) was calculated using the followingequation (Bougrier et al., 2005):

SE ¼ P � tV � TSS

ð1Þ

where P is the applied ultrasonic power in kW, t is the ultrasonicduration in seconds, V is the volume of sludge in liters, and TSS isthe total suspended solids concentration in kg/L. Due to dielectricand mechanical power losses through vibration, the actual ultra-sound energy imparted to the liquid is lower than the amount ofenergy applied by the ultrasound device (Kobus and Kusinska,2008). Acoustic power transferred by the ultrasound generator toa liquid medium is eventually converted into heat (Berlan andMason, 1992). The actual energy transferred to the sludge has beencalculated using the thermal method recommended by Raso et al.(1999). The actual energy requirements for heating sludge in thethermal pretreatments were calculated based on the followingequation (Zupancic and Ros, 2003):

Q s ¼ qslVslCpðtfinal � tinitialÞ ð2Þ

where Qs is the heat required to heat the sludge in kJ, qsl is the den-sity 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 theinitial temperature of sludge in �C, and tfinal is the final temperatureof sludge in �C. The actual energy imparted to the sludge among thedifferent pretreatment conditions is shown in Table 1.

For economic assessments, the operating costs of electricity forultrasound pretreatments are calculated based on the specific en-ergy input applied by the ultrasonic device. For thermal pretreat-ments, the net pretreatment cost is calculated from the

difference in the energy required to heat the sludge and the recov-ery of heat from the thermally pretreated sludge with a heat ex-changer. The assumptions for the cost calculations of the thermalpretreatment included the following: (a) initial sludge temperatureof 25 �C, (b) 20% heat loss due to thermal pretreatment equipment,and (c) 80% heat recovery from pretreated sludge. The costs ofdewatering, transportation and landfill were estimated at $250/ton dry solids, while the costs of electricity and natural gas wereestimated at $0.07/kWh and $0.28/m3, respectively (Elbeshbishyet al., 2010). The amount of solids used in for dewatering, transpor-tation and landfill was calculated based on the total solid removalachieved during the pretreatment and anaerobic digestion (BMPtest). The costs of removing H2S from biogas was calculated basedon the estimates reported by Mckinsey Zicari (2003) using a nonr-egenerable KOH-AC bed (USFilter-Westates). Costs per unit of bio-gas purification and absorbent per unit of H2S removed wereestimated as $0.0005/m3 biogas and $12/kg H2S, respectively.

3. Results and discussion

3.1. COD solubilization

Fig. 1(a) shows the impact of different pretreatment conditionson the ratio of SCOD/TCOD. After all pretreatments, the total CODin the pretreated sludge remained almost constant. As expected,all pretreatments caused significant increases in the ratios ofSCOD/TCOD when compared to the control. However, COD solubi-lization was similar between the 50 and 70 �C thermal pretreat-ments and for the 5000 and 10,000 kJ/kg TSS ultrasoundpretreatments, as the actual energy supplied to the system wassimilar. Combined pretreatments showed increased COD solubili-zation when compared to the ultrasound pretreatment alone.Fig. 1(b) shows that the increase in SCOD/TCOD ratio relative tothe control was significantly correlated with the actual energiesimparted to the sludge during the pretreatment.

The solubilization of COD responded similarly to results foundin previous pretreatment studies. Eskicioglu et al. (2006) usedthermal pretreatments of 96 �C and reported �3.6 times higherSCOD of thickened WAS, whereas there was �5 times higher SCODafter 30 min of holding time at 90 �C in the current study. Ivo andJing (2009) used pretreatment temperatures of 50–70 �C and re-ported an increase in the ratio of SCOD/TCOD from 2% to 21% (therewas no difference between the 50 and 70 �C treatments). For theultrasound pretreatment, Bougrier et al. (2005) found an increasein the ratio of SCOD/TCOD in WAS from 4% to 32% when increasingspecific energy input from 0 to 10,000 kJ/kg TSS; in the currentstudy, the same specific energy input (10,000 kJ/kg TSS) increasedthe SCOD/TCOD ratio from 6% to 33%.

3.2. Solids reduction

The influence of different pretreatment conditions on volatilesuspended solid (VSS) removal is shown in Table 2. Because sludgedisintegrated during the ultrasound pretreatment, VSS decreasedby 23%, 28%, and 30% for the 1000, 5000, and 10,000 kJ/kg TSS spe-cific energy inputs, respectively. This indicates that increasing thespecific energy from 5000 to 10,000 kJ/kg TSS did not significantlyimprove the reduction of VSS. Although the SCOD/TCOD ratioincreased with temperature, the reductions of VSS were nearlythe same for the different thermal pretreatments. The significantincrease in the SCOD/TCOD ratio with thermal pretreatmentsmay have resulted from the solubilization of colloidal matters(<0.45 lm), which was not measured separately in the study. Thecombined pretreatments, using different combinations of specificenergies and temperatures, showed slight improvements in VSS

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 thesludge.

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

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

Set VSSreduction(%)

Soluble organicnitrogen (mg/L)

Solublecarbohydrate(mg/L)

TVFA(mg/L)

Control – 50 104 156UP 1 23 72 272 222UP 2 28 222 397 243UP 3 30 514 863 276TP 1 25 223 298 181TP 2 26 297 379 196TP 3 27 596 555 178CP 1 32 406 468 258CP 2 33 356 587 306CP 3 34 425 589 300CP 4 33 462 568 303CP 5 36 436 680 300CP 6 35 498 755 352CP 7 34 536 1113 416CP 8 33 542 1448 452CP 9 36 538 1485 520

3.3. Protein, carbohydrate and VFA release

It is possible that an increase in the SCOD/TCOD ratio that re-sults from the pretreatments might originate from the disruptionof microbial cells in WAS causing the release of various organiccompounds (e.g., carbohydrates, proteins, lipids, and VFAs). In thisstudy, the changes in different protein fractions, as well as the in-crease in soluble carbohydrate and TVFA concentrations, weremeasured in the pretreated sludge samples. The impact of differentpretreatment conditions on TVFA release is shown in Table 2. TVFAsolubilization increased with increasing specific energy input butwas nearly similar among the three thermal pretreatments. Inthe combined pretreatment, a maximum energy input of 119 kJ(CP 9) led to a 230% increase in TVFA concentration, and maximumVSS reduction, when compared to the control.

The soluble carbohydrate concentrations for the different pre-treatments are shown in Table 2. Soluble carbohydrates increasedby 162%, 282%, and 730% for the specific energy inputs of 1000,5000, and 10,000 kJ/kg TSS, respectively. Soluble carbohydrates in-creased by 350%, 264%, and 434% at 50, 70 and 90 �C pretreatmenttemperatures, respectively. In the combined pretreatments, a max-imum of 1400% increase in soluble carbohydrates was found for CP9 (10,000 kJ/kg TSS + 90 �C). Comparing the absolute energy input,UP 3 (38 kJ) was comparable to TP 1 (31 kJ); however, ultrasoundpretreatments provided greater solubilization of carbohydrates,

possibly due to both mechanical and chemical disintegration ofthe particulates.

Protein content in sludge is usually divided into three differentfractions: 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 microbialcell wall, and the soluble protein is in the aqueous phase. Particu-late protein is the combination of cell protein and bound protein.

546 B.R. Dhar et al. / Waste Management 32 (2012) 542–549

Fig. 2 shows the influence of the pretreatments on different proteinfractions. 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 resultsfrom the reduction in cell protein concentrations. Among thermalpretreatments, particulate protein reduction was nearly the same,and the average particulate (cell + bound) and bound proteinreductions were 18% and 28%, respectively. Soluble protein in-creased by approximately 400% for all thermal pretreatments. Asshown in Fig. 2, the combined pretreatments always produced bet-ter results both in terms of bound protein reduction and increasedsoluble protein when compared to ultrasound and thermal pre-treatments alone. However, as with carbohydrate solubilization,ultrasonication with the same level of energy input relative tothe thermal pretreatment provides better solubilization of particu-late protein. Fig. 3(a) shows a significant correlation between cellprotein reduction and VSS reduction (R2 = 0.8071). All pretreat-ments resulted in significant soluble organic nitrogen release intothe aqueous phase when compared to the control (Table 2).

Fig. 2. Impact of different pretreatme

Fig. 3(b) shows a significant correlation between soluble organicnitrogen concentrations and soluble protein concentrations amongpretreatments (R2 = 0.8114). These results suggest that the solubleprotein concentrations correspond with the organic nitrogen solu-bilization. The reduction in particulate protein corresponds withthe increase in soluble protein, suggesting a closure of proteinmass balance.

3.4. Impact on methane potential

The results of the BMP tests are shown in Table 3. Although theSCOD/TCOD ratio increased with increasing pretreatment temper-atures from 50 to 90 �C, the increases in methane production dur-ing anaerobic digestion was nearly the same among thermalpretreatments. Total methane production was also nearly the samefor both 5000 and 10,000 kJ/kg TSS specific energy input pretreat-ments. The combined pretreatments showed further improvementin methane production when compared to thermal pretreatmentalone. Bougrier et al. (2005) reported no significant improvementin methane production when increasing specific energy input from6250 to 9350 kJ/kg TSS in WAS. In the current study, 5000 kJ/kg TSSwas found to be optimum specific energy input for enhanced

nt on different protein fractions.

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

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

B.R. Dhar et al. / Waste Management 32 (2012) 542–549 547

biogas production. In terms of biogas production, the ultrasoundpretreatments were more effective than thermal pretreatments,although better COD solubilization was observed in the thermalpretreatment. Greater solubilization with thermal pretreatmentsresults from the solubilization of colloidal COD, which was not af-fected by ultrasound pretreatments. It is possible that the lowergas production for thermally pretreated sludge is due to the forma-tion of agglomerates as well as increases in particle size followingthermal pretreatments (Bougrier et al., 2005); the opposite occursduring ultrasound pretreatments. The expected methane yield at37 �C (390 ml CH4/gm of TCODremoved) also agrees well with the

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

Set Specific CH4 production(mL CH4/gm VSS)

Increase in total CH4

production (%)Mean CH4 content(%volume)

Average VSCs removalefficiency comparedto the control (%)

TTF (s L/g TSS)

H2S DMS

Control 325 – 50 – – 81UP 1 374 15 52 18 23 76UP 2 391 20 52 21 38 75UP 3 404 24 53 22 38 52TP 1 370 14 50 34 40 48TP 2 386 19 53 35 30 56TP 3 368 13 53 34 59 54CP 1 388 19 53 33 42 60CP 2 386 19 53 34 53 51CP 3 398 23 52 35 54 50CP 4 420 29 55 33 52 56CP 5 410 26 56 36 45 57CP 6 406 25 55 38 56 55CP 7 414 27 55 39 57 61CP 8 410 26 56 41 57 59CP 9 424 30 56 39 72 58

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

Set Pretreatmentcostb ($)

Increase in CH4

production ($)Saving in H2Sremoval cost ($)

Dewatering, transportation and landfill cost Net saving comparedto the controld ($)

Amount of solidsc (ton) Decrease in cost ($)

1 UP 1 20 8 11 0.54 55 542 UP 2 98 10 11 0.51 63 �143 UP 3 196 12 12 0.52 60 �1124 TP 1 24 7 42 0.55 53 785 TP 2 43 9 46 0.54 55 676 TP 3 62 7 45 0.54 55 457 CP 1 44 10 40 0.52 60 668 CP 2 63 9 42 0.51 63 519 CP 3 82 11 42 0.47 73 4410 CP 4 122 14 28 0.51 63 �1711 CP 5 141 13 44 0.48 70 �1412 CP 6 160 12 49 0.45 78 �2113 CP 7 220 14 46 0.46 75 �8514 CP 8 239 13 54 0.43 83 �8915 CP 9 258 15 46 0.42 85 �112

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

cost ($).

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observed yields, which ranged from 339 to 375 CH4/gm ofTCODremoved (results not shown); slight differences are attributedto experimental error.

After 28 days of BMP tests, all pretreated sludge samples pro-duced higher amounts of methane when compared to the control.Although the increase in SCOD due to the pretreatment is expectedto translate into additional biogas, the increase in biogas produc-tion did not show any linear relationships with the CODsolubilization.

3.5. Volatile sulfur compounds in biogas

In anaerobic digestion, hydrogen sulfide (H2S) and differenttypes of organosulfur compounds (mercaptans) are produced fromsulfur-containing proteins and the methylation of sulfide (Higginset al., 2004). In this study, the H2S and DMS concentrations weremeasured in biogas among the different pretreatment conditions.For the control, the average H2S and DMS concentrations in biogaswere 38 ± 1 and 18 ± 1 ppm, respectively. The average H2S andDMS removal efficiencies in biogas compared to the control for dif-ferent pretreatments are shown in Table 3. Thermal pretreatmentshad more effective VSC removal efficiency compared to the ultra-sound pretreatment. In the combined pretreatments, DMS removalefficiencies were higher than both ultrasound and thermalpretreatments alone, while H2S removal efficiencies were higherthan ultrasound pretreatments alone. Although all pretreatmentsshowed significant effects on VSCs removal in biogas when com-pared to the control, VSC removal efficiencies did not show any lin-ear relationship with increasing specific energy input andtemperature. A recent study found that bound protein had a signif-icant impact on the generation of H2S and other organosulfur com-pounds (Higgins et al., 2004). Fig. 4(a) shows that the reduction ofbound protein concentration was significantly correlated with thereduction of H2S and DMS concentrations when compared to thecontrol and that the bound protein concentration reduction de-creased the H2S and DMS concentration in biogas by 6 and3 ppm, respectively. Fig. 4(b) shows that the reduction in VSS con-centrations due to the pretreatments was significantly correlatedwith VSC concentration reductions in biogas. Verma et al. (2006)and Dhar et al. (2011) also showed that sludge containing lowerVSS concentrations had lower VSC generation potential. However,it is expected that enhanced digestion should produce more sulfuremission in biogas. Fig. 4(c) shows that VSS concentration

reductions during digestion is significantly correlated with VSCconcentrations in biogas. As the VSC concentrations were propor-tional to VSS destroyed during digestion, it is plausible that theVSC concentrations in biogas among the pretreatments were loweras a result of the lower absolute mass of VSS destroyed duringdigestion because some VSS was degraded during pretreatment.The sulfur emission during pretreatment could not be experimen-tally measured to close the mass balance.

3.6. Impact on dewaterability

The TTF results for the digested sludges at the end of BMP testare shown in Table 3. TTF values are normalized to TSS concentra-tions of the digested sludge and expressed in units of s L/g TSS. TheTTF values represent how quickly sludge releases its water. Ultra-sound pretreatments at 1000 and 5000 kJ/kg TSS specific energyinputs showed a marginal improvement in dewaterability, while10,000 kJ/kg TSS specific energy input significantly decreased theTTF by 36%. TTF values were nearly the same among temperaturepretreatments (50–90 �C) but lower than the TTF values observedin the ultrasound pretreatments. Combined pretreatments didnot significantly improve the TTF values when compared to thethermal pretreatment alone. These results suggest that differentpretreatments can improve the dewaterability of the digestedsludge relative to the control. The enhanced dewaterability mightbe due to the various biopolymers (proteins and carbohydrates) re-leased through different pretreatments (Novak et al., 2003).

3.7. Economic assessment

Because the cost of sludge management is around 50% of the to-tal operating cost of the wastewater treatment plant (Odegaard,2004), the economic feasibility of a pretreatment process is closelyrelated to the enhancement in methane production and solidsreduction. Although pretreatments give additional benefits, includ-ing volatile sulfur compound reduction and improved sludgedewaterability, retrofitting pretreatment systems to the conven-tional anaerobic digestion process adds extra operating costs.Therefore, an economic evaluation is required to establish the fea-sibility of implementing a costly pretreatment process. Based onthe experimental results obtained in this work, an economicassessment was conducted per ton of solids (TSS) treated withthe anaerobic digestion process. Table 4 shows the summary

B.R. Dhar et al. / Waste Management 32 (2012) 542–549 549

economic assessment results for different pretreatment processeswhen compared to the conventional process or control. Althoughall 15 pretreatment conditions significantly improved biogas pro-duction and solid reduction relative to the control, only seven areeconomically feasible. Ultrasound pretreatment is economicallyfeasible only at specific energy input of 1000 kJ/kg TSS, with anet 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 pretreatmentsare feasible with specific energy inputs of 1000 kJ/kg TSS with all ofthe different thermal pretreatment temperatures (50–90 �C), giv-ing a net savings of $44–66/ton dry solid. In addition to the costsof H2S removal, the reduction of volatile sulfur compound reduc-tions in biogas can give long-term economic benefits by decreasingcorrosion rates and increasing engine life. Improvement in dewa-terability of digested sludge and the optimization of polymer dos-ages in dewatering may slightly impact the overall dewatering,transportation and landfill costs ($250/ton solids). The installationof pretreatment systems will also increase the overall capitalinvestment. However, these factors are not considered in thiseconomic assessment.

4. Conclusions

Ultrasound, thermal and combined pretreatments can reducethe VSS in raw WAS by 22–31%, 25–39%, and 29–38%, respectively,in addition to significant improvements in COD solubilization andthe release of various organic compounds. Pretreatments combin-ing 10,000 kJ/kg TSS specific energy inputs at 90 �C significantlyincreased total methane by 30% and decreased H2S and DMS inbiogas by 39% and 72%, respectively. Based on the economic eval-uation relative to conventional anaerobic digestion, the relativeranking of economically feasible pretreatment process is as fol-lows: TP 1 (thermal at 50 �C) > TP 2 (thermal at 70 �C) > CP 1 (ther-mal at 50 �C + ultrasound at 1000 kJ/kg TSS) > UP 1 (ultrasound at1000 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 + ultra-sound 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.

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