Thermo-oxidative pretreatment of municipal waste activated sludge for volatile sulfur compounds removal and enhanced anaerobic digestion

Chemical Engineering Journal 174 (2011) 166–174

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Thermo-oxidative pretreatment of municipal waste activated sludge for volatile sulfur compounds removal and enhanced anaerobic digestion Bipro Ranjan Dhar a , Elsayed Elbeshbishy b , Hisham Hafez b , George Nakhla a,b , Madhumita B. Ray a,∗ a b

Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada, N6A 5B9 Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario, Canada, N6A 5B9

a r t i c l e

i n f o

Article history: Received 2 May 2011 Received in revised form 18 August 2011 Accepted 24 August 2011 Keywords: Anaerobic digestion Hydrogen sulfide Waste activated sludge Thermo-oxidative pretreatment Volatile sulfur compounds

a b s t r a c t Thermo-oxidative pretreatment of municipal waste activated sludge was conducted using thermal pretreatment at 60 ◦ C in presence of 0.6 mg H2 O2 + 1.5 mg FeCl2 /mg S2− as oxidants with the objective of achieving sludge disintegration for enhancing anaerobic digestion as well as to remove volatile sulfur compounds generation potential in biogas in continuous anaerobic digestion. For the pretreated feed digester, the hydrogen sulfide (H2 S) and dimethyl sulfide (DMS) concentrations in biogas significantly decreased by an average of 75%, and 40%, respectively, while methanethiol (MT) removal efficiency was statistically insignificant compared to the control digester. Compared to the control, overall TSS and VSS removal efficiency were 10% and 11% higher for the pretreated feed digester operated at 10 days solid residence time (SRT), and methane production rate (L CH4 /Day) increased by ∼20%. The simulation results using BioWin® suggest that the thermo-oxidative pretreatment has significantly increased the hydrolysis rate by 30% with higher methane production rate compared to the control digester. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Biological treatment of wastewater produces significant quantities of sludge. Sludge processing and management costs around 50–60% of the overall wastewater treatment costs [1]. Although anaerobic digestion of municipal sludge is a widely accepted sludge stabilization process, conventional anaerobic digestion has several limitations. Anaerobic digestion of waste activated sludge (WAS) is difficult compared to primary sludge due to the rate-limiting hydrolysis step [2]. Besides, the presence of various volatile sulfur compounds (VSCs) such as hydrogen sulfide (H2 S), and other organosulfur compounds (methyl mercaptan, dimethyl sulfide, dimethyl disulfide) in biogas may contribute to corrosion in combustion engines and generate harmful emissions [3]. Additionally, malodor originating from sludge digestion is a nuisance for people working in the wastewater plant.

Abbreviations: AD, anaerobic digestion; AS, activated sludge; DMS, dimethyl sulfide (ppmv ); EPS, extracellular polymeric substance; FeCl2 , ferrous chloride (mg); H2 O2 , hydrogen peroxide (mg); H2 S, hydrogen sulfide (ppmv ); kh , hydrolysis rate constant coefficient (day−1 ); MT, methanethiol or methyl sulfide (ppmv ); S2− , dissolved sulfide (mg/L); S2− S, sulfur as dissolved sulfide (mmol); SO4 2− , dissolved sulfate (mg/L); SO4 2− S, sulfur as dissolved sulfate (mmol); Total-S, total sulfur (mmol); VSC, volatile sulfur compound; WAS, waste activated sludge. ∗ Corresponding author. Tel.: +1 519 661 2111x81273; fax: +1 519 661 3498. E-mail address: [email protected] (M.B. Ray). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.08.070

Dissolved sulfide is one of the major odor precursors in sludge. Dissolved sulfide is not odorous, but during anaerobic digestion dissolved sulfide can leave the liquid phase to biogas as H2 S. Therefore, a reduction in dissolved sulfide loading can reduce the H2 S generation potential of sludge during anaerobic digestion [4]. Although the reaction pathways at elevated temperature due to the thermal pretreatment are still unknown, at room temperature H2 O2 and FeCl2 can react with dissolved sulfide via a number of different pathways. The following reactions are possible [5,6]: H2 S + FeCl2 → FeS ↓ + 2HCl

(1)

H2 S + H2 O2 → S + 2H2 O

(2)

H2 S + 4H2 O2 → H2 SO4 + 4H2 O

(3)

2FeS + 3H2 O2 → 2S + 2Fe(OH)3

(4)

Various pretreatment techniques such as chemical, mechanical and thermal methods have been used to enhance the hydrolysis rate through sludge solubilization, subsequently improving the anaerobic digestion process. Hydrogen peroxide is easier to implement as a pretreatment technique compared to other technologies such as ultrasound and ozone. However, pretreatment using hydrogen peroxide has received less attention [7,8] since H2 O2 oxidation alone is not effective for high concentrations of refractory contaminants due to the low reaction rates at practical H2 O2 concentrations. Different types of catalysts such as transition metal salts (e.g. iron salts), thermal, and ozone can be used to catalyze the activity of H2 O2 [7,9]. Eskicioglu et al. [9] have reported that thermo-oxidative

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treatment using higher H2 O2 dosages (>0.5 g H2 O2 /g TSS) may decrease the methane production rate due to the formation of refractory compounds. Cacho et al. [8] have reported that the use of H2 O2 oxidation (0.1–0.5 g H2 O2 /g VSS) in low temperature thermal pretreatment can enhance the overall solids reduction during anaerobic digestion, but the economic feasibility of using H2 O2 in thermo-oxidative treatment was not very impressive. Although the combination of H2 O2 and iron salts (also known as Fenton’s reagent) has shown significant positive impact on sludge solubilization as well as anaerobic digestion [10], the wastewater to be treated must be acidic (pH 3–5), and therefore, the pH of pretreated wastewater must be adjusted to the optimum pH of 6.5–7.5 before anaerobic digestion. The aforementioned studies confirmed the potential of thermo-oxidative pretreatment as an effective pretreatment technique for anaerobic digestion, although albeit high uneconomical oxidant dosages were used in those studies. Previous study by Dhar et al. [4] has demonstrated that using 0.6 mg H2 O2 + 1.5 mg FeCl2 /mg S2− (0.9 mg H2 O2 + 2.2 mg FeCl2 /g TSS) with mechanical pretreatment has a significant potential for reduction of volatile sulfur compounds in the biogas without any negative impact on methane production. Although many studies were conducted on thermal pretreatment, ranging from 50 to 270 ◦ C, low temperature pretreatment at temperatures below 100 ◦ C is considered effective for increasing anaerobic biogas production [11,12]. Gavala et al. [12] and Wang et al. [13] have reported that the optimum pretreatment temperature for biogas production from WAS is 60–70 ◦ C, while Valo et al. [14] reported that the thermal pretreatment time beyond 30 min did not improve solubilization of thickened WAS. Based on the extensive literature search, no studies could be found on the impact of thermo-oxidative pretreatment on volatile sulfur compounds in anaerobic digestion. In addition, most of the earlier thermo-oxidative studies reported in the literature mostly concentrated on the improvement of solids reduction and biogas production, with very limited information available on the reduction of VSCs in biogas. The objective of this study is to evaluate the impact of thermo-oxidative pretreatment on various volatile sulfur compounds removal in continuous flow anaerobic digestion. Specifically, the impacts of different pretreatment conditions are evaluated in terms of (a) sludge solubilization, (b) biogas production, (c) H2 S, MT and DMS removal in biogas, (d) solids removal and digested sludge quality. In the current study, thermo-oxidative pretreatment of municipal WAS has been done at the optimum low temperature (60 ◦ C) for thermal pretreatment reported in literature [12,13] simultaneously with the optimum oxidant dosage (0.6 mg H2 O2 + 1.5 mg FeCl2 /mg S2− ) reported by Dhar et al. [4]. Furthermore, a model has been developed using BioWin® 3.0 (EnviroSim Associates Ltd., Flamborough, Ontario, Canada) to estimate the hydrolysis rate coefficient (kh ) of anaerobic digestion with thermo-oxidative pretreatment.

2. Materials and methods 2.1. Thermo-oxidative pretreatment Waste activated sludge (WAS) samples for this study were collected from the Adelaide Pollution Control Plant located in London, Ontario, Canada. After thickening, the sludge was stored in a cold room at 4 ◦ C. Pretreatment of sludge was conducted using conventional heating at 60 ± 2 ◦ C with H2 O2 and FeCl2 . The dosages used were based on theoretical requirement of the chemical dosages for 100% dissolved sulfide (S2− ) removal in raw untreated sludge. The theoretical requirements of H2 O2 and FeCl2 to remove 1 mg dissolved sulfide (S2− ) are 0.6 mg and 1.5 mg, respectively [6]. Hydrogen peroxide was added as 50 wt% H2 O2 (HX0630-1, EMD

167

Chemicals Inc., Germany), and iron was added as iron (II) chloride (98% purity, Sigma Aldrich, Oakville, ON, Canada). About 300 mL of sludge was introduced in a glass volumetric flask closed with a rubber septum fitted with a temperature monitoring probe. The volumetric flask was placed on a hot stirred plate (Corning Stirrer/Hot plate, Model PC-420, Corning Incorporated, USA), and heated at the set temperature for 30 min. 2.2. Anaerobic digestion Continuous anaerobic digestion of sludge was carried out using two identical continuously stirred tank reactors (10 L), with a working volume of 7.5 L at a constant mesophilic temperature of 37 ± 1 ◦ C and operated at a solids retention time (SRT) of 10 days. Although, the SRT chosen is somewhat lower than the typical anaerobic digestion SRTs of (15–45 days), our earlier studies indicate that the effect of pretreatment on various sludge parameters is more prevalent on lower SRTs. Based on the reactor’s working volume, the sludge was fed continuously to the reactor at a rate of 0.75 L/day using a pump (Masterflex® Console Drive, Model No. 77521-50, 1-100 RPM, 0.1 HP, Thermo Fisher Scientific Inc, USA). During start-up, the digesters were initially seeded with 7.5 L of anaerobicaly-digested sludge collected from the Guelph wastewater treatment plant (Guelph, Ontario, Canada). Before starting the comparative study with pretreated sludge, for 2 weeks both reactors were fed with raw WAS in order to enrich micro-organisms as well as to confirm the performance of the both reactors. Then, the reactors were separately fed with raw and pretreated WAS. Detailed analysis of both liquid and gas samples commenced after steady-state conditions were reached, approximately after three turnovers of the mean SRT. Meanwhile, methane composition, pH, alkalinity of both reactors were measured twice a week in order to ensure proper functioning of the reactors. 2.3. Analytical methods Samples were analyzed for total suspended solids (TSS), volatile suspended solids (VSS) according to the Standard Methods [15]. Soluble parameters were analyzed after filtering the sludge sample through 0.45 ␮m membrane filters (Sterile membrane filter, Cat. No. 7141104, Whatman Limited, England). HACH vials were used to measure chemical oxygen demand (COD), total nitrogen (TN), soluble total nitrogen (STN), nitrate (NO3 − ), nitrite (NO2 − ) and ammonia nitrogen (NH4 -N). Total Kjeldahl Nitrogen (TKN) was calculated from total nitrogen, nitrate and nitrite (TKN = TN–NO3 − –NO2 − ). As residual H2 O2 interferes with the COD measurement [16], residual hydrogen peroxide concentrations were measured using Quantofix peroxide test strips (Sigma Aldrich Canada, C9322) to ensure the accuracy of the COD measurement. However, the residual hydrogen peroxide concentrations in pretreated WAS samples were either negligible or could not be detected. 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. Total dissolved sulfide (S2− ) was analyzed by the iodometric titration method [15], and dissolved sulfate (SO4 2− ) was analyzed using ion chromatograph (Model Dionex ICS-3000). H2 S in biogas was measured using the Odalog (Model Odalog type I, App-Tek International Pty Ltd, Brendale 4500, Australia) with a detection range of 0–1000 ppm. Methyl mercaptan was measured in biogas using Gastec gas sampling pump (Model GV-100, GASTEC Corporation, Japan) and Gastec detector tubes (No. 70L, measuring

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range 0.1–8 ppm). The DMS in gas sample was measured using a gas chromatograph (GC 2010, Shimadzu) with flame photometric detector (FPD) equipped with BPX-5 column (5% Phenyl Polysilphenylene–siloxane) type capillary column (30 m × 0.25 m i.d. × 0.25 ␮m thickness, SGE, Austin, TX, USA). 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. Methane composition in biogas was analyzed using SRI 310C Gas Chromatograph (Model 310, SRI Instruments, Torrance, CA) equipped with a molecular sieve column (Molesieve 5A, mesh 80/100, 182.88 cm × 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 carrier gas at a flow rate of 30 mL/min. Proteins were determined by micro-bicinchoninic acid protein assay (Pierce, Rockford, USA). This method modified by Lowry et al. [17], uses standard solution of bovine serum albumin. In this study, various protein fractions (particulate, bound and soluble) have been analyzed, as protein is one of the major precursors for volatile sulfur compounds generation. In order to measure various protein fractions, 50 mL samples were centrifuged at 10,000 rpm for 15 min at 5 ◦ C to separate the solids in the sludge. The supernatant was filtered through a 1.5 ␮m glass microfiber filter and the filtrate was analyzed for the soluble protein fraction. Total protein and bound protein fractions were extracted from the suspended solids by using 1 N NaOH solution and phosphate buffer (pH 8, 50 mM), respectively. The solution was mixed using a magnetic stirrer at 1500 rpm for 10 min and 30 min for bound and total protein, respectively, and then centrifuged at 10,000 rpm for 15 min at 5 ◦ C, with the supernatant filtered through a 1.5 ␮m glass microfiber filter, prior to protein analysis. Carbohydrate concentration was determined by the colorimetric method [18]. The absorbance of the sample was measured using a spectrophotometer (Varian Cary 50 UV–Vis) at 490 nm. 2.4. Statistical analysis In order to determine if the experimental results for both digesters were significantly different, the student’s t-test was used to test the null (no difference) hypothesis of quality at a 95% confidence level.

Table 1 Default kinetic parameters for BioWin 3.0. Parameter

Unit

Default

Arrhenius constant

Maximum specific growth rate Substrate half saturation Anoxic growth factor Aerobic decay Anoxic/anaerobic decay Hydrolysis rate (AS) Hydrolysis half sat. (AS) Anoxic hydrolysis factor Anaerobic hydrolysis factor Adsorption rate of colloids Ammonification rate Assimilative nitrate/nitrite reduction rate Fermentation rate Fermentation half sat. Anaerobic growth factor (AS) Hydrolysis rate coefficient (AD) Hydrolysis half sat. (AD)

day−1

3.2

1.029

mg COD/L – day−1 day−1 day−1 – – – L/(mg COD day) L/(mg N day) day−1

5 0.5 0.62 0.3 2.1 0.06 0.28 0.5 0.8 0.04 0.5

1 1 1.029 1.029 1.029 1 1 1 1.029 1.029 1

day−1 mg COD/L –

3.2 5 0.125

1.029 1 1

day−1

0.1

1.05

mg COD/L

0.15

1

3. Results and discussion 3.1. Sludge solubilization The average characteristics of raw and pretreated WAS are shown in Table 2. After pretreatment, TCOD remained almost constant compared to the raw untreated WAS (Control). As expected, the thermo-oxidative pretreatment led to sludge solubilization; and after pretreatment SCOD significantly increased by 2.5 times. Compared to raw WAS, the TSS and VSS in pretreated sludge decreased by an average of 9% and 17%, respectively. It is hypothesized that an increase in SCOD after pretreatment may originate from the disruption of microbial cells of WAS, resulting in the release of various EPS (carbohydrates, proteins, lipid, etc.) and volatile fatty acids. As shown in Table 2, after pretreatment the total volatile fatty acid (TVFA) and soluble protein concentrations in pretreated sludge were significantly higher than the raw WAS. Protein

Table 2 Characteristics of raw and pretreated WAS.

2.5. BioWin® modeling To evaluate the impact of thermo-oxidative pre-treatment on the hydrolysis rate coefficient (kh ), BioWin® 3.0 (EnviroSim Associates Ltd., Flamborough, Ontario, Canada) model has been used. BioWin® is based on the “four population” framework (heterotrophs, acetogenic bacteria, acetoclastic methanogens, hydrogenotrophic bacteria). A schematic diagram of the BioWin® model for anaerobic degradation pathways is shown in Fig. 1. For the details of the structure of BioWin® model for anaerobic digestion readers are referred to the user manual for BioWin® 3.0 [19]. The model was calibrated based on the influent sludge characteristics. The influent characteristics of the WAS used in this study were simulated using the influent specifier associated with BioWin® model. The default kinetic parameters of BioWin® 3.0 are shown in Table 1. For the control digester simulation, all kinetic and stoichiometric parameters were kept at default values, while for pretreated feed digester a sensitivity analysis was conducted for different hydrolysis rate (0.1–0.25 day−1 ) to match the experimental digested sludge characteristics. The optimum hydrolysis rate coefficient has been estimated using mean average percentage error (MAPE) analysis.

Parameter

Raw WAS

TCOD (mg/L) SCOD (mg/L) TSS (mg/L) VSS (mg/L) TVFAa (mg COD/L) Ammonia (mg/L) TKN (mg/L) STKN (mg/L) Particulate protein (mg/L) Bound protein (mg/L) Soluble protein (mg/L) Particulate carbohydrate (mg/L) Soluble carbohydrate (mg/L) pH Alkalinity (mg as CaCO3 /L)

21,000 1450 18,200 13,200 350 140 1150 700 2600

± ± ± ± ± ± ± ± ±

Pretreated WAS

900 100 700 600 20 10 50 15 270

21,000 3600 16,500 11,000 420 140 1200 750 1900

± ± ± ± ± ± ± ± ±

Increment/ Decline After Pretreatment (%)

700 150 750 400 25 20 125 20 210

– +157 −9 −17 +20 – +4 +9 −27

800 ± 60 450 ± 100 1750 ± 250

500 ± 130 1200 ± 150 1400 ± 100

−38 +167 −20

100 ± 25

350 ± 50

+250

7.1 ± 0.1 3000 ± 200

6.9 ± 0.1 4500 ± 250

−3 +5

Note. Values represent average ±standard deviation of 12 samples. a Summation of acetic acid, propionic, butyric, iso-butyric, and valeric acids.

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169

Fig. 1. Schematic diagram of BioWin® model for anaerobic digestion. Adapted from the user manual for BioWin® 3.0).

content in sludge is usually divided into three different fractions: cell, bound and soluble [20]. The cell protein represents the protein fraction inside the microbial cell, and the bound protein is the loosely attached protein to the microbial cell wall, while the soluble protein represents the protein fraction available in the aqueous phase. Particulate protein represents the combination of cell protein and bound protein [20]. As shown in Table 2, thermo-oxidative pretreatment had a significant impact on protein solubilization. Compared to the raw untreated sludge, particulate (cell + bound) protein, and bound protein decreased by on average of 27% and 38%, respectively in the pretreated sludge; while the average soluble protein concentration in pretreated sludge was 2.7 times higher than the raw sludge. Therefore, the average reduction in particulate protein (cell + bound) concentration of 700 mg/L agrees well

with the average 750 mg/L increase in soluble protein concentration. The results suggest that along with protein solubilization, thermo-oxidative pretreatment can decrease the odor generation potential of sludge by reducing bound protein prior to the anaerobic digestion, as bound protein has been identified as responsible for VSCs production in anaerobic digestion [21]. After pretreatment, particulate carbohydrate in pretreated sludge decreased on average by 18%; while soluble carbohydrate increased by almost 3.5 times compared to the raw untreated sludge. These results indicate that the thermo-oxidative pretreatment has significantly released the various EPSs through the solubilization of sludge. As expected, the TKN remained almost constant after pretreatment; while STKN slightly increased after the pretreatment. However, no ammonia nitrogen (NH4 -N) solubilization was observed after

B.R. Dhar et al. / Chemical Engineering Journal 174 (2011) 166–174

Table 3 Summary of methane production results. Performance parameter

Control

Pretreated

Methane production rate (L CH4 /day) Methane production (L CH4 /L sludge treated -day) Methane yield (mL CH4 /g COD removed ) Average CH4 in biogas (Vol. %)

2.6 ± 0.2 3.5 ± 0.4 420 ± 30 63

3.1 ± 0.15 4.1 ± 0.3 410 ± 15 66

pretreatment. Therefore, the increase in STKN concentration observed after pretreatment reflects the solubilization of organic nitrogen compounds such as protein in liquid phase. This trend was previously observed in other thermo-oxidative pretreatment studies [8,9]. The statistical-paired student’s t-test was used to evaluate the observed differences in different parameters in raw and pretreated WAS, and results have shown that the differences in SCOD, TSS, VSS, TVFA, particulate protein, bound protein, soluble protein, total carbohydrate, and STKN concentrations were statistically significant at the 95% confidence level, while the differences in TCOD, ammonia nitrogen, and TKN concentrations were statistically insignificant at 95% confidence level. Thus, it is evident that WAS pretreatment at 60 ◦ C in presence of oxidants has exhibited a significant impact on sludge solubilization through cell disintegration.

Removal Efficiency (%)

a

80 70

Control Digester Pretreated Feed Digester

60 50 40 30 20 10 0

b Removal Efficency (%)

170

TCOD 70 60 50

SCOD

Control Digester Pretreated Feed Digester Overall Solids Removal with Pretreatment

40 30 20 10 0

TSS

VSS

Fig. 2. (a) COD removal efficiency for raw and pretreated feed digesters, (b) solids removal efficiency.

3.2. Impact on anaerobic digestion 3.2.1. Methane production Table 3 summarizes the steady state methane production for the control and pretreated feed digesters. As expected, compared to the control digester, a significant increase of 20% in methane production rate to 3.1 L CH4 /day was observed for the digester fed with pretreated sludge (Table 3). Methane produced per gram of COD removed provides a good measurement on COD closure in the digester. As shown in Table 3, the values obtained for both reactors are close to the theoretical value of 0.4 L CH4 /g CODremoved at 37 ◦ C. The average methane contents in biogas were 63% and 66% for control and pretreated feed digesters, respectively. The results indicate that the pretreatment did not have significant impact on the methane content in biogas. 3.2.2. COD and solids removal Fig. 2(a) shows the steady state COD removal efficiency for both digesters. The average TCOD concentrations in influent was almost the same for both digesters, while the SCOD was significantly higher in pretreated sludge compared to the raw untreated WAS. Compared to the control digester, an enhancement in TCOD removal efficiency during digestion was observed for the digester processing the pretreated sludge, and the TCOD removal efficiency for control and pretreated feed digesters were 38% and 48%, respectively. Compared to the control digester a significant increase in SCOD concentration removal was observed for the pretreated feed digester of 74% relative to the 54% for the control digester. Fig. 2(b) shows the steady-state suspended solids removal efficiencies for both digesters during digestion as well as the overall suspended solids removal efficiency achieved due to the thermo-oxidative pretreatment and digestion. Although prior to the anaerobic digestion the thermo-oxidative pretreatment has significantly reduced the TSS and VSS concentrations compared to the raw WAS, steady-state TSS and VSS removal efficiency (during anaerobic digestion) for both reactors were almost same. At the 10-days SRT, the VSS removal efficiencies during digestion for the control and pretreated feed digester were 39% and 41%, respectively. However, compared to the digestion of raw WAS the overall TSS and VSS removal for digestion with thermo-oxidative pretreatment (digestion + pretreatment) increased by 9% and 12%,

respectively. The lack of improvement observed in suspended solids reduction despite a nominal increase in VSS destruction efficiency from 39% to 41% during digestion of pretreated sludge suggests that the thermo-oxidative pretreatment did not improve the biodegradability of particulate matter. Cacho et al. [8] have also reported no improvement in VSS removal during continuous anaerobic digestion (5 days SRT) of excess municipal sludge using thermal pretreatment at 65 ◦ C at different H2 O2 dosages varying from 0.1 to 0.5 g H2 O2 /g VSS, and the enhancement in suspended solids removal occurred during the pretreatment only; for few cases the VSS removal efficiency was slightly lower than the control. Although the steady-state suspended solids removal efficiency was the same for both digesters during digestion, the increase in TCOD removal observed during digestion for pretreated feed digester is due to the higher SCOD removal. 3.2.3. Digested sludge quality The various effluent quality parameters were monitored for the both digesters to evaluate the quality of digested sludge as well as the digestion performance. Fig. 3(a) shows the protein and carbohydrate concentrations in influent and digested sludge (effluent) for both digesters at steady-state. For the control digester, the particulate protein, bound protein, and particulate carbohydrate steady-state removal efficiencies (during digestion) were 68%, 62%, and 39%, respectively; while for the pretreated feed digester, steady-state removal efficiencies (during digestion) of the particulate protein, bound protein, and particulate carbohydrate were 67%, 46%, and 77%, respectively. Although compared to the control digester, bound protein removal efficiency during digestion was lower for pretreated feed digester, the concentration of bound protein in the digested sludge of pretreated feed digester was significantly lower than the digested sludge of the control due to the reduction of bound protein during thermo-oxidative pretreatment. The results indicate that the particulate protein, bound protein, and particulate carbohydrate concentrations in the control digester effluent were significantly higher than the pretreated feed digester effluent. Due to the pretreatment, the soluble protein and soluble carbohydrate concentrations in pretreated sludge (pretreated feed digester influent) were significantly higher than the raw WAS (Table 2). During digestion, the soluble protein removal

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171

Fig. 3. (a) Proteins and carbohydrates concentrations for control and pretreated feed digester, (b) nitrogen and TVFA concentrations for control and pretreated feed digester.

3.3. Volatile sulfur compounds (VSCs) removal 3.3.1. Dissolved sulfate and sulfide The major source of inorganic sulfur in sludge is dissolved sulfate and sulfide. The dissolved sulfate and sulfide were analyzed before and after pretreatment to assess the impact of pretreatment on dissolved sulfur compounds (Fig. 4). As shown in Fig. 4, after thermo-oxidative pretreatment the dissolved sulfide concentration decreased by 19% compared to the control, while dissolved sulfate

concentration remained almost constant. During pretreatment, the H2 S concentration was also monitored in the headspace of pretreatment flask, and no stripping of sulfides was observed. Therefore, the results suggest that the dissolved sulfide might have been converted to ferrous sulfide (FeS) or colloidal sulfur during pretreatment. The dissolved sulfide and sulfate concentrations were also measured in the digested sludge. The dissolved sulfide concentrations in the digested sludge were 70 and 56 mg/L for the control and the pretreated digester, respectively. As shown in Table 4, the dissolved sulfate and sulfide removal efficiencies during digestion were the same for both digesters. The very low sulfate removal (<10% for both digesters) during anaerobic digestion reflects the 90 80

Concentration (mg/L)

efficiencies for the control and the pretreated feed digester were 60%, and 67%, respectively; soluble carbohydrate removal efficiencies were 45%, and 77%, respectively. The results indicate that the two major extracellular polymeric substances (protein and carbohydrate) removal efficiencies during digestion for pretreated feed digester were significantly higher than the control digester. Since high EPS concentrations in sludges have been correlated with poor dewatering properties [22], the findings of this study suggest that the pretreated digested sludge may be easier to dewater than the untreated digested sludge. In the digested sludge, the average TKN and STKN concentrations were slightly lower than expected for the both digesters (Fig. 3(b)). For both digesters, the effluent ammonia nitrogen concentrations significantly increased with solids reduction. The TVFA removal efficiencies for the control and the pretreated feed digester were 50% and 60%, respectively; and the effluent TVFA concentrations for the control and the pretreated feed digesters were 175 ± 25 and 170 ± 10 mg/L (Fig. 3(b)).

70 60

Control Digester Pretreated Feed Digester

50 40 30 20 10 0 SO42-influent

SO42-effluent

S2-influent

Fig. 4. Dissolved sulfate and sulfide concentrations.

S2-effluent

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Table 4 Steady-state inorganic sulfur balance and removal efficiency.

SO4 2− –Sinfluent (mmol) S2− –Sinfluent (mmol) Total-Sin (mmol) SO4 2− –Seffluent (mmol) S2− –S effluent (mmol) H2 S–sbiogas (mmol) Total-Sout (mmol) SO4 2− removal efficiency a (%) S2− removal efficiency (%) a

Control digester

Pretreated feed digester

0.27 1.88 2.15 0.25 1.64 0.005 1.9 7 13

0.28 1.64 1.92 0.26 1.41 0.001 1.7 7 14

Steady-state removal efficiency during anaerobic digestion.

low population of sulfate reducing bacteria (SRB) in the anaerobic digesters (Table 4). The statistical paired student’s t-test was used to evaluate the differences in dissolved sulfate and sulfide concentrations for both digesters. The dissolved sulfide concentration for pretreated feed digester was significantly different from the control digester, while there was no significant difference in sulfate concentrations at the 95% confidence level. 3.3.2. Volatile sulfur compounds (VSCs) in biogas During anaerobic digestion, various volatile sulfur compounds are produced in the biogas through the degradation of inorganic and organic sulfur compounds. In this study, for both digesters the concentrations of three major VSCs (H2 S, MT and DMS) were regularly monitored in the biogas, and the average H2 S, MT and DMS concentrations for both digesters are shown in Fig. 5. As expected, a significant decrease in VSCs concentrations was observed for the pretreated feed digester relative to the control, and the H2 S, DMS and MT concentrations in the biogas decreased by on average 75%, 40% and 10% compared to the control digester, respectively. The statistical-paired student’s t-test has shown that compared to the control digester, the gaseous H2 S and DMS reductions were statistically significant at the 95% confidence level, while gaseous MT reduction was insignificant at the 95% confidence level. During pretreatment, the H2 S concentration was monitored in the headspace of pretreatment flask, and no stripping of sulfides was observed. Besides, no increase in dissolved sulfate (SO4 2− ) concentration was observed after pretreatment. Therefore, the results suggest that the gaseous H2 S reduction observed in the pretreated feed digester was due to the reduction of dissolved sulfide (S2− ) through thermooxidative pretreatment before digestion. The reduction in VSS could be another reason for this observation, as different studies have shown that VSS destruction has an impact on odor removal [4,23]. Different types of organo-sulfur compounds or mercaptans are produced from sulfur-containing bound proteins and methylation of sulfide [21]. The results indicate that the reason behind the reduc-

Fig. 5. Average volatile sulfur compounds (VSCs) concentrations in biogas.

tion in mercaptans in biogas might be due to the reduction in bound protein. The biological conversion of DMS to methane and MT reported by Rasi et al. [3] may explain the insignificant differences in MT concentrations in biogas between control and pretreated feed digester. The results indicate that the addition of oxidants during thermal pretreatment has shown a significant impact on dissolved sulfide reduction as well as H2 S and DMS concentrations in biogas. Compared to the earlier study by Dhar et al. [4] using mechanical pretreatment of WAS through depressurization of sludge in the presence of oxidants (0.6 mg H2 O2 + 1.5 mg FeCl2 /g S2− ), the thermal pretreatment of WAS at 60 ◦ C at the same aforementioned oxidant dosages, the observed reduction in gaseous H2 S is significantly increased by ∼30% in the current study. 3.3.3. Inorganic sulfur balance Based on the steady state experimental results of sulfur compounds (sulfate, dissolved sulfide, and H2 S in biogas) the inorganic sulfur mass balance closed reasonably well for both digesters (Table 4). Total inorganic sulfur exiting the digesters were 88% and 89% of the influent for the control and test digester, suggesting a loss of about 10% through the formation of sulfur. However, the small differences in total sulfur for raw and pretreated sludge before anaerobic digestion are due to the formation of ferrous sulfide and colloidal sulfur, which were not considered in the sulfur balance. 3.4. BioWin® simulation results Estimation of biochemical parameters is critical for anaerobic digester design and operation. Since the premise of all sludge pretreatment is enhancement of VSS disintegration and hydrolysis, using BioWin® 3.0 (EnviroSim Associates Ltd., Flamborough, Ontario, Canada) a process model was developed and calibrated based on the influent characteristics of the sludge to evaluate the impact of thermo-oxidative pretreatment on the hydrolysis rate. It must be noted that BioWin® does not predict the VSCs produced during anaerobic digestion. The default kinetic and stoichiometric parameters included in BioWin® 3.0 are based on the typical characteristics of municipal sludge. Therefore, all kinetic and stoichiometric parameters including hydrolysis rate coefficients were set at default values of BioWin for the simulation of control digester. The simulated digested sludge characteristics for the control and pretreated feed digester are shown in Table 6. For the pretreated feed digester, all kinetic and stoichiometric parameters were set at default values of BioWin except the hydrolysis rate coefficient (kh ). A sensitivity analysis was conducted for different hydrolysis rate coefficients ranging from 0.1 day−1 to 0.25 day−1 , as suggested by IWA Anaerobic Digestion Model No. 1 (ADM 1) [24]. The sensitivity analysis results are shown in Table 5. The results suggest that the effluent SCOD concentration and inorganic suspended solids (calculated as the difference between TSS and VSS) concentrations are not sensitive to the hydrolysis rate coefficient. On the other hand, TCOD, VSS and methane production rates were sensitive to the hydrolysis rate coefficient. To estimate the hydrolysis rate coefficient, the equally-weighted mean average percentage error (MAPE) of the three aforementioned parameters (calculated as the absolute difference between experimental and model values divided by experimental) for the various kh values were calculated (Table 5). The results suggested that the optimum hydrolysis rate coefficient for anaerobic digestion operated with thermo-oxidative pretreatment is 0.13 day−1 . The simulated effluent quality parameters predicted by the BioWin® model using hydrolysis rate coefficient of 0.13 day−1 are shown in Table 6. As apparent from Table 6, the digested sludge quality was well predicted by the BioWin® model. It is interesting to observe that both experimental and simulated results showed a reduction in TKN during digestion, possibly due

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173

Table 5 Sensitivity analysis for hydrolysis rate coefficient estimation for pretreated feed digester. Effluent concentration (mg/L)

Hydrolysis rate coefficient (day−1 )

Experimental 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.18 0.20 0.22 0.25

Methane production rate (L CH4 /day)

TCOD

SCOD

TSS

VSS

11,000 ± 200 10,845 10,595 10,373 10,176 10,000 9841 9697 9447 9237 9057 8834

925 ± 100 1059 1055 1052 1049 1048 1046 1044 1041 1040 1038 1036

11,300 ± 650 13,382 13,179 12,999 12,839 12,695 12,566 12,450 12,245 12,074 11,928 11,744

6500 ± 350 7938 7734 7553 7392 7248 7118 7000 6795 6622 6475 6291

3.1 ± 0.15 2.90 3.00 3.05 3.10 3.15 3.20 3.25 3.35 3.40 3.45 3.50

Mean absolute percentage error (MAPE)

10 8.6 7.8 7.1 7.4 7.8 8.1 8.9 9.2 9.8 11.9

Table 6 Measured and simulated data using BioWin® 3.0. Parameter

Control digester

Pretreated feed digester

Experimental Influent VSS (mg/L) TSS (mg/L) SCOD (mg/L) TCOD (mg/L) STKN (mg/L) TKN (mg/L) Alkalinity (mg as CaCO3 /L) pH TVFA (mg/L) Ammonia (mg/L) VSSremoval (%) CH4 production rate (L/d)

13,200 18,200 1400 21,000 700 1150 3000 7.1 350 140

± ± ± ± ± ± ± ± ± ±

Simulated Effluent

600 700 100 900 15 50 200 0.1 20 10

8000 13,000 650 13,000 650 930 4400 7.5 175 630

± ± ± ± ± ± ± ± ± ±

620 400 30 300 20 80 450 0.15 25 30

Experimental

Influent

Effluent

Influent

15,039 20,061 1650 21,000 721 1128 3000 7.1 360 143

9226 14,287 703 12,054 638 991 4695 7.3 188 609

11,000 16,500 3600 21,000 750 1200 4500 6.9 420 140

39 2.6 ± 0.2

to struvite formation and ammonia stripping that could not be confirmed experimentally. It is thus evident that the thermo-oxidative pretreatment enhanced the anaerobic hydrolysis rate by 30% during digestion. 4. Conclusions Based on the experimental results and simulation, the following conclusions can be drawn: • Due to pretreatment, the SCOD increased by 2.5 times in pretreated sludge compared to raw WAS along with a significant solubilization of EPSs (protein and carbohydrate), accompanied by a 9% and 17% decrease in TSS and VSS, respectively. However, during digestion suspended solids removal efficiencies for both digesters were almost the same, and the overall TSS and VSS destruction efficiencies with pretreatment and digestion were 9% and 12% higher than the control. • Compared to the control, the methane production rate for pretreated feed digester increased by 20% along with a slight increase in average methane content in biogas from 63% to 66% (vol.). • For the pretreated feed digester, the H2 S, DMS and MT concentrations in biogas decreased by 75%, 40%, and 10% compared to the control digester, respectively. The observed differences in H2 S and DMS between the control and pretreated feed digesters were statistically significant at the 95% confidence level, while the difference in MT concentrations were not statistically significant. • The methane production rate and VSS removal efficiency predicted by the BioWin® model were in good agreement with the observed experimental results, and the results also suggest that

± ± ± ± ± ± ± ± ± ±

39 2.7

Simulated Effluent

400 750 150 700 20 125 250 0.1 25 20

6500 11,300 925 11,000 620 960 5850 7.6 170 670

41 3.1 ± 0.15

± ± ± ± ± ± ± ± ± ±

350 650 100 200 25 70 500 0.15 10 50

Influent

Effluent

12,132 17,534 3600 21,000 809 1200 4500 7.1 420 143

7392 12,839 1049 10,176 686 996 6300 7.4 188 655 39 3.1

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