- Email: [email protected]
Enhanced disinfection and methane production from sewage sludge by microwave irradiation
Desalination 248 (2009) 279–285
Enhanced disinfection and methane production from sewage sludge by microwave irradiation C. Eskicioglua , K.J. Kennedyb, R.L. Drostea a
Department of Civil Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5 Canada email: [email protected] b Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Received 31 January 2008; revised accepted 15 May 2008
Abstract The effect of inoculum acclimation on methane production from mesophilic digestion of microwave pretreated of thickened waste activated sludge (TWAS) was investigated. In a range of 50–175 C, microwave disintegrated waste activated sludge and enhanced methane production. Soluble to total chemical oxygen demand ratios increased from 9 + 1 (control; unpretreated) to 24 + 3, 28 + 1 and 35 + 1% at 120, 150 and 175 C, respectively. Biological Methane Potential tests indicated that despite mild acute inhibition in the first 9 d, acclimated inoculum digesting sludge, irradiated to 175 C produced 31 + 6% higher biogas compared to control after 18 d of digestion. However, initial acute inhibition was more severe for non-acclimatized inoculum requiring 2 times longer recovery with only 18 + 2% higher biogas after 20 d. Inoculum acclimation did not only accelerate production of biogas, but also increased the extent of the ultimate biodegradation of pretreated waste sludge. Keywords: Microwave; Pretreatment; TWAS; Acclimation; Biodegradability; Disintegration
1. Introduction Treatment and disposal of excess sludge generated by wastewater treatment plants (WWTP) is a bottleneck in plant operation especially in both developing and industrial countries due to more stringent quality requirements regarding landfilling, ocean disposal, agricultural use and incineration. Concomitantly, there is world-wide interest in three main sludge reduction strategies, applied: (a) in the wastewater line (energy
Corresponding author.
uncouplers, alternating stream exposure to oxic and anoxic environments), (b) in the sludge line (physical, chemical and thermal pretreatments for enhanced hydrolysis before anaerobic digestion (AD)), and (c) in the final waste line (incineration and pyrolysis). It is widely documented that in the sludge line pretreatment processes such as; mechanical, thermal, ultrasound, chemical, enzymatic and thermochemical methods could disrupt the extracellular polymeric substances (EPS) and divalent cation network and increase the extent of waste activated sludge biodegradability through enhanced hydrolysis.
Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International Workshop Edinburgh, Scotland, UK, 28–30 May 2008. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.066
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Current research has focused on a novel in the sludge line process: microwave pretreatment of thickened waste activated sludge (TWAS) before AD for enhanced sludge solubilization and methane production. Initial microwave pretreatment of waste sludge studies have been conducted at temperatures less than 100 C by household type microwave ovens [1–3]. Using a household microwave oven, fecal coliforms were not detectable in primary sludge and waste activated sludge pretreated to 65 and 85 C, respectively [1]. Microwave irradiation also solubilized the particulate (>0.45 mm) chemical oxygen demand (COD) of waste sludge. Soluble to total COD ratios (SCOD/ TCOD 100) of waste activated sludge increased from 8% (control) to 18% after microwave irradiation to 72.5 C [4] and from 6% (control) to 18% after irradiation to 96 C [3]. SCOD/TCOD ratios of 19% and 21% were also reported for waste activated sludge, irradiated to 91 and 100 C, respectively [2]. The variability in SCOD/TCOD ratios of waste activated sludge pretreated to similar temperatures is likely due to different sludge source and extracellular polymeric substance characteristics. Besides solubilization experiments, anaerobic biodegradability of microwave-irradiated samples were also studied in mesophilic batch and continuous-flow digesters. In a microwave (kitchen type) pretreatment range of 50–96 C, waste activated sludge microwaved to 96 C resulted in the highest improvement in cumulative biogas produced with 15 + 0.5 and 20 + 0.3% increases over controls after 19 d of batch digestion at low [1.4% (w/w) total solids (TS)] and high [5.4% TS (w/w)] sludge concentrations, respectively [3]. In continuous-flow digesters (mostly used in full-scale operations), waste activated sludge pretreated to 96 C by microwave and conventional heating achieved 29% and 32% higher TS and 23% and 26% volatile solids (VS) removal efficiencies compared to controls at a sludge retention time (SRT) of 5 d, respectively [5]. In another study [2], continuous-flow mesophilic digestion was studied at 8, 10, 12 and 15 d SRTs for waste activated sludge, microwaved to 91 C and 64% and 31% higher COD removal and methane productions were reported, respectively, compared to controls at an SRT of 15 d. While under boiling point thermal (both microwave and conventional heating) pretreatment studies
mentioned above resulted in SCOD/TCOD ratios around 20%, autoclave studies above boiling point indicated a potential to reach ultimate solubilization of waste activated sludge with a SCOD/TCOD ratio of 60% at 170 C [6]. Another study [7] reported 42% and 48% COD solubilization for autoclave heated waste activated sludge samples at pretreatment temperatures of 170 and 190 C, respectively. Presently no sludge pretreatment study using a programmable high temperature and pressure microwave system with closed reaction vessels, (used in this study), has been reported for enhanced methane production. This study focused on the effects of microwave pretreatment on disintegration of waste activated sludge floc structure above boiling point (120, 150 and 175 C). The potential toxicity of microwave-irradiated samples on mesophilic inoculum was further studied in biochemical methane potential (BMP) tests with both acclimatized and non-acclimatized inoculums. The data obtained for under boiling point temperatures (50, 75 and 96 C) were also reported for the completeness of presentation.
2. Materials and methods 2.1. Sample collection Both microwave solubilization and methane potential tests were done on TWAS obtained from the thickener centrifuge at the Robert O. Pickard Environmental Center (ROPEC) sewage treatment plant in Gloucester (ON, Canada). ROPEC has preliminary and primary treatment followed by a conventional aerobic activated sludge unit operated at an average SRT of 5 d. Ferric chloride is added to waste activated sludge for P removal prior to thickening. Characterization of ROPEC-TWAS is given in Table 1 along with both acclimatized (to microwave-irradiated waste activated sludge) and non-acclimatized mesophilic inoculum used for biological studies. 2.2. Ultimate solubilization Before microwave solubilization of the sludge, ultimate solubilization of ROPEC-TWAS was tested by harsh alkaline (NaOH) treatment for comparison. There is no established method to determine the
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Table 1 Characterization of raw thickened waste activated sludge (TWAS) and mesophilic inoculuma Parameters pH [–] TS [% (w/w)] VS [% (w/w)] VS/TS 100 [%] TCOD [mg/L] SCOD (<0.45 mm) [mg/L] SCOD/TCOD 100 [%] a b
Raw TWAS
Non-acclimatized inoculum
Acclimatized inoculum
7.0 (0.0; 2)b 4.6 (0.0; 4) 3.1 (0.0; 4) 67 (0.2; 4) 45,714 (0.0; 2) 4286 (0.0; 2) 9 (0.0; 2)
7.7 (0.0; 2) 2.5 (0.0; 2) 1.5 (0.0; 2) 58 (0.1; 2) 26,600 480 2
7.9 2.6 1.0 38.5 17,770 400 2.3
TS, VS: total and volatile solids; TCOD, SCOD: total and soluble chemical oxygen demand, respectively. Data represent arithmetic mean of duplicates (absolute error; number of data points).
maximum COD solubilization of waste activated sludge, however, alkaline addition (0.5 mol NaOH/L for 24 h at 20 C) was previously applied [8,9]. It is obvious that sample characteristics, such as initial solubility and solid content of TWAS, pH, the amount of NaOH added, contact time and contact temperature are expected to create differences among the ultimate solubilization results. In this study, a wide range of NaOH dosage (0.12–2 mol/L) was tested on sludge. Volumes of 350 mL of diluted [1.9 + 0.01% TS (w/w); 20,286 + 2363 mg/L TCOD] TWAS (in 500 mL glass Wheaton bottles with polypropylene caps) were incubated with 0.12, 0.25, 0.5, 1 and 2 mol/L NaOH for durations of 1, 2, 3, 6, 12, 24, 48 h at a pH range of 12–13.5 in a shaker (90 rpm) at room temperature (24 + 1 C). After taking samples, they were immediately neutralized with concentrated H2SO4 and SCOD was measured. After 48 h, sample bottles were kept on the shaker and the last SCOD measurements were made after a contact time of 14 d to validate ultimate solubilities.
2.3. Microwave pretreatments The experimental design covered a wide range of pretreatment temperatures (50, 75, 96, 120, 150, 175 C) using a Microwave Accelerated Reaction System [MARS-5, CEM Corporation, 0–1250 W, 2450 MHz frequency, maximum temperature: 260 C, maximum pressure: 500 psig (33 bars) equipped with fiber optic temperature and pressure probes within the cavity and a turning carousel with a maximum of 14 pressure
sealed vessels of 100 mL each]. A total of 500 g of TWAS was cooked in 10 vessels (50 g sample per vessel) rotating on the carousel. MARS-5 operates with a focused microwave irradiating beam and is capable of heating and holding at desired cooking (or temperature ramping) rates and holding times. Although MARS-5 is capable of reaching temperatures above the boiling point very quickly (such as ramping from 20 + 2 to 175 + 2 C in 10 min), preliminary microwave studies suggest that sludge solubilization increases with duration of microwave exposure [10]. Therefore, in this study, slow cooking profiles (1.2–1.4 C/min) were applied. After target temperatures were reached, samples were removed from the heating source (no holding time at desired temperatures) and cooled to room temperature in closed vessels to avoid evaporation of organics and then stored at 4 C in a refrigerator.
2.4. Biological studies The anaerobic degradability of control (untreated) and microwave-irradiated samples were determined by batch mesophilic BMP tests in 125 mL serum bottles sealed with butyl rubber stoppers. A total of 32 BMP tests were performed for various microwave irradiation temperatures with both acclimatized and non-acclimatized inoculums including duplicates and controls according to [11]. Non-acclimatized (to microwave-irradiated TWAS) inoculum for the BMP test was originally taken from the effluent line of the anaerobic sludge
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digesters (SRT of 15–20 d) treating a mixture of TWAS and primary sludge [58:42 (v/v)] at ROPEC. In order to analyze the toxicity effects of microwave irradiation on TWAS, 16 of 32 BMP assays were setup with non-acclimatized inoculum and 16 with acclimatized inoculum. For acclimation, one 5 L anaerobic semi-continuous reactor (35 C), fed with microwaveirradiated (to 175 C sludge by MARS-5) sludge, was run at an approximately 20 d SRT over a year. Microwave toxicity effect on secondary sludge digestion was expected to increase with temperature [3,12]; therefore, microwave irradiation at 175 C was used to pretreat feed sludge for the acclimation reactor. For BMP assays, acclimated inoculums (15 mL) were placed into serum bottles and then sludge samples (70 mL) were added. Nitrogen sparging was applied to batch reactors when sludge samples and inoculum were mixed to prevent exposure to air and serum bottles (120 mL) were sealed after addition of an equal mixture of NaHCO3 and KHCO3 to achieve an alkalinity of 4000 mg/L (as CaCO3). BMP assays were kept in a darkened temperature controlled incubator shaker at 33 + 1 C and at 90 rpm until they stopped producing biogas. Daily biogas produced was measured by inserting a needle attached to a manometer. 2.5. Analysis Total and volatile solids (TS/VS) were determined based on Standard Methods procedure 2540G [13]. For supernatant solid determination, centrifugation [for 20 min at 5856 relative centrifugal force (RCF) in a Dupont instruments Sorvall SS-3 automatic centrifuge] and then coarse filtration (1.2 mm) was used. Colorimetric COD measurements were done based on Standard Methods procedure 5250D [13] with a Coleman Perkin-Elmer spectrophotometer Model 295 at 600 nm light absorbance. Before SCOD determination, sludge samples were centrifuged (for 20 min at 5856 RCF) and filtered through membrane disc filters with 1.2 and 0.45 mm pore sizes.
3. Results and discussion The effects of microwave pretreatment temperature on SCOD/TCOD ratios were investigated. For comparison, maximum achievable COD solubilization was
SCOD/TCOD * 100 (%)
282
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
NaOH = 0.12 mol/L NaOH = 0.25 mol/L NaOH = 0.5 mol/L NaOH = 1 mol/L NaOH = 2 mol/L 0
1
2
3
4
5
6 7 8 9 10 11 12 13 14 15 Contact time (d)
Fig. 1. Estimation of the ultimate solubilization ratio of ROPEC – thickened waste activated sludge [SCOD/ TCOD: soluble to total chemical oxygen demand ratio].
also quantified by NaOH addition and results are shown in Fig. 1. SCOD/TCOD (%) ratio of control (9 + 1) increased to 51 + 0.5%, 57 + 0.6%, 60 + 0.6%, 59 + 0.6%, 65 + 0.6% at 0.12, 0.25, 0.5, 1 and 2 mol/L NaOH concentrations after 2 weeks of contact time. Chemical solubilization of samples was initially quite fast. Within 3 h, half of the samples achieved about 30% solubilization (50% of ultimate solubilization) and within 2 d, waste activated sludge contacted with 2 mol NaOH/L achieved 92% of ultimate solubilization which was 60 + 0.6% SCOD/TCOD. These results lead to a conclusion that chemical solubilization contact times less than 2 d will underestimate the ultimate solubility of ROPEC-TWAS. Particulate COD solubilizations achieved at different microwave pretreatment temperatures are shown in Fig. 2. In a pretreatment range of 50–175 C and temperature ramp of 1.2–1.4 C/min, solubilization profiles
Fig. 2. Effect of microwave irradiation on ROPEC-TWAS solubilization [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures; SCOD/TCOD: soluble to total chemical oxygen demand ratio].
C. Eskicioglu et al. / Desalination 248 (2009) 279–285 a)
indicated a linear relation with SCOD/TCOD ratios of 9 + 0%, 12 + 1%, 21 + 0%, 24 + 1%, 24 + 3%, 28 + 1% and 35 + 1% at microwave pretreatment temperatures of control, 50, 75, 96, 120, 150 and 175 C, respectively. At 175 C, microwave achieved 54% of the ultimate chemical solubilization ratio of ROPECTWAS (65 + 0.6%; Fig. 1) obtained with 2 mol NaOH/L. Disintegration of TWAS floc structure was also evaluated by analyzing the supernatant solids (<1.2 mm) after pretreatment. TS, VS and fixed solids (FS ¼ TS VS) were measured in the whole sludge and in the supernatants after centrifugation for 20 min at 5856 RCF and then coarse filtration (<1.2 mm) of control and pretreated samples. Fig. 3 indicates significant disintegration of TS, mainly composed of VS, as microwave temperature increases. At 175 C, 32 + 0.4% of TS, composed of 91% of VS (organic portion), were in the supernatant phase while for the untreated control this value was only 8 + 0.4%. Biodegradability and potential toxicity of microwave-irradiated TWAS relative to control digesters studied in batch mesophilic (35 + 2 C) BMP assays. The first week of incubation was critical since maximum substrate utilization generally occurs in the first 5–7 d. Initial toxicity of pretreated samples on methanogenesis (converting acetic and propionic acids to methane) could be reduced by acclimation of inoculum to the harshest irradiated sludge (at 175 C). Non-acclimatized inoculum was also used in this study to evaluate the toxicity effects of pretreated sludge; however this would not represent full-scale digestion
Cumulative biogas production (mL)
800 700 600 500 400
Control MW−50 MW−75 MW−96 MW−120 MW−150 MW−175
300 200 100 0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Digestion time (d) b)
Cumulative biogas production (mL)
Fig. 3. Organic and fixed solid material solubilization at different microwave temperatures [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures; TS: total solids].
283
800 700 600 500 400
Control MW−50 MW−75 MW−96 MW−120 MW−150 MW−175
300 200 100 0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Digestion time (d)
Fig. 4. Cumulative biogas productions from control and pretreated thickened waste activated sludge samples with (a) acclimatized inoculum, (b) non-acclimatized inoculum [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures].
(continuous-flow) with an initial acclimation period. Cumulative biogas productions with acclimatized and non-acclimatized inoculum are presented in Fig. 4a and b, respectively. Biogas productions from the inoculum controls were subtracted from the biogas production from the mixtures to obtain biogas from sludge samples only. From comparison of Fig. 4a and b, starting with acclimatized inoculum did not only affect the rate of biogas production but also the extent of waste activated sludge AD. In a temperature range of 50–175 C with acclimatized inoculum, all pretreated digesters improved cumulative biogas production, pretreated sludge irradiated at 1.2 C/min to 175 C produced the highest cumulative biogas production which was
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31 + 6% higher than the control after 18 d of digestion. This result is in agreement with the level of improvement achieved in sludge solubilization microwave heated at 1.2 C/min to 175 C (Fig. 2). As digestion continued further, the difference in cumulative biogas productions became smaller (28 + 6% around the 27th day), since given sufficient time, more difficult to degrade organics in control samples would continue to be degraded. The fact that the exponential phase was longer for pretreated samples than the control suggests that higher amounts of readily degradable substances were present in the pretreated digesters. On the other hand, non-acclimatized inoculum digesting the identical pretreated sample (at 175 C) achieved only 18 + 2% higher biogas compared to controls after 20 d (Fig. 4b). Results indicate that within a practical time of 18–28 d, microwave pretreatment enhanced the ultimate degradability of ROPEC-waste sludge. Although the inoculum used in half of the BMP assays was acclimatized to the highest temperature (175 C) in a semi-continuous reactor for one year, control digesters initially had the fastest biogas production (Fig. 4a) followed by the digesters with sludge pretreated to 50 C. Reactors digesting pretreated TWAS at 50 C recovered after 4 d and at pretreatment temperatures of 96, 120, 150 C after 7 d and digesters pretreated at 175 C recovered after 10 d of mesophilic incubation. Obviously, the severity of the pretreatment temperature influenced the biogas production in the first 10 d of batch digestion. All pretreated digesters in a temperature range of 50–175 C were consistent in their reduced relative biogas responses indicating that some product or products were being formed that resulted in a mild short term microbial inhibition compared to untreated TWAS. This mild inhibition may be of some concern for continuous digesters operated at short SRTs but may disappear with acclimation [5]. Initial acute toxicity for the non-acclimatized inoculum digesting microwave pretreated sludge was much more severe. In Fig. 4b, the recovery took almost two times longer (compared to acclimatized BMP assays) and were 5 d at a microwave temperature of 50 C, 14 d at temperatures of 75, 96, 120 C and 15 and 16 d at temperatures of 150 and 175 C, respectively. It is possible that some portion of the inoculum lost its activity under this toxic environment which created the
difference in the rate and the ultimate biogas produced from BMP assays with acclimatized and nonacclimatized inoculums. Except for the assay with sludge irradiated to 175 C, biogas data indicate that all other BMP assays with unacclimatized inoculum did not result in an increase in the ultimate digestibility of sludge over controls (Fig. 4b). Since digester pH was controlled by addition of buffer solutions before the reactors were sealed, pH values of non-acclimatized and acclimatized digesters fluctuated only in the ranges of 6.9–8.6 and 6.8–8.1 during the BMP test, respectively. The digesters with acclimatized inoculum were able to tolerate initial acute toxicity with no longterm decrease in methane production or volatile fatty acids accumulation. However, the majority of digesters with non-acclimatized inoculum stayed below the control biogas production line. Methane content of biogas produced in non-acclimatized and acclimatized digesters fluctuated in ranges of 46–76% and 61–77% depending on the pH of digesters, respectively. Average methane percentages of control, MW – 50, 75, 96, 120, 150, 175 digesters were 65 + 2, 66 + 4, 67 + 2, 67 + 4, 68 + 6, 68 + 4 and 67 + 3 for acclimatized and 64 + 4, 66 + 3, 66 + 5, 66 + 5, 65 + 7, 68 + 6 and 63 + 13 for non-acclimatized digesters, respectively. Almost all (98%) of the volatile fatty acids were converted to biogas in two weeks. Ultimate biogas production results may explain the different digestion performances reported for the similar type of sludge pretreatment studies in the literature. It is possible that these differences arise not only from different initial sludge characteristics but also from using acclimatized or non-acclimatized inoculums (not specified in majority of pretreatment studies). The cumulative biogas and methane productions and VS removals from all 32 batch BMP digesters were used and results indicated 1.04 + 0.17 L biogas per g VS removed for acclimatized and 0.99 + 0.32 L biogas per g VS removed for non-acclimatized digesters, respectively. These results correspond to methane yields of 0.70 + 0.12 and 0.64 + 0.21 L methane per g VS for acclimatized and non-acclimatized mesophilic digesters, respectively at 1 atm and 25 + 1 C. Biogas and methane yields from batch mesophilic digesters in this study were within the range of other treatment studies.
C. Eskicioglu et al. / Desalination 248 (2009) 279–285 4. Conclusions Based on the experimental data and analysis, the following conclusions are drawn. In a temperature range of 50–175 C, there was a linear relation between microwave temperature and level of hydrolysis. SCOD/TCOD ratios (%) increased from 9 + 1 (control; unpretreated) to 24 + 3, 28 + 1 and 35 + 1 at microwave temperatures of 120, 150 and 175 C, respectively. Similarly, supernatant solid (<1.2 mm) to TSs ratios of waste activated sludge increased from 8 + 0% (control) to 19 + 1%, 21 + 1% and 32 + 0% after microwave irradiation to 120, 150 and 175 C, respectively. In BMP tests, despite the first 9 d of mild acute inhibition, acclimatized inoculum digesting waste activated sludge, microwaved to 175 C, produced 31 + 6% higher biogas compared to control digester around 18 d of mesophilic batch digestion. However, initial acute toxicity was more severe for non-acclimatized inoculum requiring two times longer recovery with only 18 + 2% higher biogas around 20 d, indicating that inoculum acclimatization did not only accelerate, but also extend the ultimate mesophilic biodegradation.
Acknowledgments The authors would like to thank Mr. Nicolas Terzian for his work in acquiring the data.
References [1] S.M. Hong, J.K. Park and Y.O. Lee, Mechanism of microwave irradiation involved in the destruction of fecal coliforms from biosolids, Water Res., 38 (2004) 1615–1625. [2] B. Park, J.H. Ahn, J. Kim and S. Hwang, Use of microwave pretreatment for enhanced anaerobiosis of secondary sludge, Water Sci. Technol., 50 (2004) 17–23.
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[3] C. Eskicioglu, K.J. Kennedy and R.L. Droste, Enhancement of batch waste anaerobic sludge digestion by microwave pretreatment, Water Environ. Res., 79 (2007) 2304–2317. [4] S.M. Hong, J.K. Park, N. Teeradej, Y.O. Lee, Y.K. Cho and C.H. Park, Pretreatment of sludge with microwaves for pathogen destruction and improved anaerobic digestion performance, Water Environ. Res., 78 (2006) 76–83. [5] C. Eskicioglu, R.L. Droste and K.J. Kennedy, Performance of anaerobic waste activated sludge digesters after microwave pretreatment, Water Environ. Res., 79 (2007) 2265–2273. [6] A. Valo, H. Carrere, J.P. Delgenes, Thermal, chemical and thermo-chemical pre-treatment of waste activated sludge for anaerobic digestion, J. Chem. Technol. Biotechnol., 79 (2004) 1197–1203. [7] C. Bougrier, C. Albasi, J.P. Delgenes and H. Carrere, Effect of ultrasonic, thermal and ozone pretreatments on waste activated sludge solubilization and anaerobic biodegradability, Chem. Eng. Proc., 45 (2006) 711–718. [8] Y.C. Chiu, C.N. Chang, J.G. Lin and S.J. Huang, Alkaline and ultrasonic pretreatment of sludge before anaerobic digestion, Water Sci. Technol., 36 (1997) 155–162. [9] A. Tiehm, K. Nickel, M. Zellhorn and U. Neis, Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization, Water Res., 35 (2001) 2003–2009. [10] C. Eskicioglu, N. Terzian, K.J. Kennedy, R.L. Droste and M. Hamoda, Athermal microwave effects for enhancing digestibility of waste activated sludge, Water Res., 41 (2007) 2457–2466. [11] W.F. Owen, D.C. Stuckey, J.B. Healy, L.Y. Young, P.L. McCarty, Bioassay for monitoring biochemical methane potential and anaerobic toxicity, Water Res., 13 (1979) 485–492. [12] S.M. Hong, Enhancement of Pathogen Destruction and Anaerobic Digestibility Using Microwaves, Ph.D. Thesis, University of Wisconsin–Madison, USA, 1992. [13] APHA, Standard Methods for the Examination of Water and Wastewater, 19th ed., American Public Health Association, Washington, DC, USA, 1995.
Enhanced disinfection and methane production from sewage sludge by microwave irradiation C. Eskicioglua , K.J. Kennedyb, R.L. Drostea a
Department of Civil Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5 Canada email: [email protected] b Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Received 31 January 2008; revised accepted 15 May 2008
Abstract The effect of inoculum acclimation on methane production from mesophilic digestion of microwave pretreated of thickened waste activated sludge (TWAS) was investigated. In a range of 50–175 C, microwave disintegrated waste activated sludge and enhanced methane production. Soluble to total chemical oxygen demand ratios increased from 9 + 1 (control; unpretreated) to 24 + 3, 28 + 1 and 35 + 1% at 120, 150 and 175 C, respectively. Biological Methane Potential tests indicated that despite mild acute inhibition in the first 9 d, acclimated inoculum digesting sludge, irradiated to 175 C produced 31 + 6% higher biogas compared to control after 18 d of digestion. However, initial acute inhibition was more severe for non-acclimatized inoculum requiring 2 times longer recovery with only 18 + 2% higher biogas after 20 d. Inoculum acclimation did not only accelerate production of biogas, but also increased the extent of the ultimate biodegradation of pretreated waste sludge. Keywords: Microwave; Pretreatment; TWAS; Acclimation; Biodegradability; Disintegration
1. Introduction Treatment and disposal of excess sludge generated by wastewater treatment plants (WWTP) is a bottleneck in plant operation especially in both developing and industrial countries due to more stringent quality requirements regarding landfilling, ocean disposal, agricultural use and incineration. Concomitantly, there is world-wide interest in three main sludge reduction strategies, applied: (a) in the wastewater line (energy
Corresponding author.
uncouplers, alternating stream exposure to oxic and anoxic environments), (b) in the sludge line (physical, chemical and thermal pretreatments for enhanced hydrolysis before anaerobic digestion (AD)), and (c) in the final waste line (incineration and pyrolysis). It is widely documented that in the sludge line pretreatment processes such as; mechanical, thermal, ultrasound, chemical, enzymatic and thermochemical methods could disrupt the extracellular polymeric substances (EPS) and divalent cation network and increase the extent of waste activated sludge biodegradability through enhanced hydrolysis.
Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International Workshop Edinburgh, Scotland, UK, 28–30 May 2008. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.066
280
C. Eskicioglu et al. / Desalination 248 (2009) 279–285
Current research has focused on a novel in the sludge line process: microwave pretreatment of thickened waste activated sludge (TWAS) before AD for enhanced sludge solubilization and methane production. Initial microwave pretreatment of waste sludge studies have been conducted at temperatures less than 100 C by household type microwave ovens [1–3]. Using a household microwave oven, fecal coliforms were not detectable in primary sludge and waste activated sludge pretreated to 65 and 85 C, respectively [1]. Microwave irradiation also solubilized the particulate (>0.45 mm) chemical oxygen demand (COD) of waste sludge. Soluble to total COD ratios (SCOD/ TCOD 100) of waste activated sludge increased from 8% (control) to 18% after microwave irradiation to 72.5 C [4] and from 6% (control) to 18% after irradiation to 96 C [3]. SCOD/TCOD ratios of 19% and 21% were also reported for waste activated sludge, irradiated to 91 and 100 C, respectively [2]. The variability in SCOD/TCOD ratios of waste activated sludge pretreated to similar temperatures is likely due to different sludge source and extracellular polymeric substance characteristics. Besides solubilization experiments, anaerobic biodegradability of microwave-irradiated samples were also studied in mesophilic batch and continuous-flow digesters. In a microwave (kitchen type) pretreatment range of 50–96 C, waste activated sludge microwaved to 96 C resulted in the highest improvement in cumulative biogas produced with 15 + 0.5 and 20 + 0.3% increases over controls after 19 d of batch digestion at low [1.4% (w/w) total solids (TS)] and high [5.4% TS (w/w)] sludge concentrations, respectively [3]. In continuous-flow digesters (mostly used in full-scale operations), waste activated sludge pretreated to 96 C by microwave and conventional heating achieved 29% and 32% higher TS and 23% and 26% volatile solids (VS) removal efficiencies compared to controls at a sludge retention time (SRT) of 5 d, respectively [5]. In another study [2], continuous-flow mesophilic digestion was studied at 8, 10, 12 and 15 d SRTs for waste activated sludge, microwaved to 91 C and 64% and 31% higher COD removal and methane productions were reported, respectively, compared to controls at an SRT of 15 d. While under boiling point thermal (both microwave and conventional heating) pretreatment studies
mentioned above resulted in SCOD/TCOD ratios around 20%, autoclave studies above boiling point indicated a potential to reach ultimate solubilization of waste activated sludge with a SCOD/TCOD ratio of 60% at 170 C [6]. Another study [7] reported 42% and 48% COD solubilization for autoclave heated waste activated sludge samples at pretreatment temperatures of 170 and 190 C, respectively. Presently no sludge pretreatment study using a programmable high temperature and pressure microwave system with closed reaction vessels, (used in this study), has been reported for enhanced methane production. This study focused on the effects of microwave pretreatment on disintegration of waste activated sludge floc structure above boiling point (120, 150 and 175 C). The potential toxicity of microwave-irradiated samples on mesophilic inoculum was further studied in biochemical methane potential (BMP) tests with both acclimatized and non-acclimatized inoculums. The data obtained for under boiling point temperatures (50, 75 and 96 C) were also reported for the completeness of presentation.
2. Materials and methods 2.1. Sample collection Both microwave solubilization and methane potential tests were done on TWAS obtained from the thickener centrifuge at the Robert O. Pickard Environmental Center (ROPEC) sewage treatment plant in Gloucester (ON, Canada). ROPEC has preliminary and primary treatment followed by a conventional aerobic activated sludge unit operated at an average SRT of 5 d. Ferric chloride is added to waste activated sludge for P removal prior to thickening. Characterization of ROPEC-TWAS is given in Table 1 along with both acclimatized (to microwave-irradiated waste activated sludge) and non-acclimatized mesophilic inoculum used for biological studies. 2.2. Ultimate solubilization Before microwave solubilization of the sludge, ultimate solubilization of ROPEC-TWAS was tested by harsh alkaline (NaOH) treatment for comparison. There is no established method to determine the
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Table 1 Characterization of raw thickened waste activated sludge (TWAS) and mesophilic inoculuma Parameters pH [–] TS [% (w/w)] VS [% (w/w)] VS/TS 100 [%] TCOD [mg/L] SCOD (<0.45 mm) [mg/L] SCOD/TCOD 100 [%] a b
Raw TWAS
Non-acclimatized inoculum
Acclimatized inoculum
7.0 (0.0; 2)b 4.6 (0.0; 4) 3.1 (0.0; 4) 67 (0.2; 4) 45,714 (0.0; 2) 4286 (0.0; 2) 9 (0.0; 2)
7.7 (0.0; 2) 2.5 (0.0; 2) 1.5 (0.0; 2) 58 (0.1; 2) 26,600 480 2
7.9 2.6 1.0 38.5 17,770 400 2.3
TS, VS: total and volatile solids; TCOD, SCOD: total and soluble chemical oxygen demand, respectively. Data represent arithmetic mean of duplicates (absolute error; number of data points).
maximum COD solubilization of waste activated sludge, however, alkaline addition (0.5 mol NaOH/L for 24 h at 20 C) was previously applied [8,9]. It is obvious that sample characteristics, such as initial solubility and solid content of TWAS, pH, the amount of NaOH added, contact time and contact temperature are expected to create differences among the ultimate solubilization results. In this study, a wide range of NaOH dosage (0.12–2 mol/L) was tested on sludge. Volumes of 350 mL of diluted [1.9 + 0.01% TS (w/w); 20,286 + 2363 mg/L TCOD] TWAS (in 500 mL glass Wheaton bottles with polypropylene caps) were incubated with 0.12, 0.25, 0.5, 1 and 2 mol/L NaOH for durations of 1, 2, 3, 6, 12, 24, 48 h at a pH range of 12–13.5 in a shaker (90 rpm) at room temperature (24 + 1 C). After taking samples, they were immediately neutralized with concentrated H2SO4 and SCOD was measured. After 48 h, sample bottles were kept on the shaker and the last SCOD measurements were made after a contact time of 14 d to validate ultimate solubilities.
2.3. Microwave pretreatments The experimental design covered a wide range of pretreatment temperatures (50, 75, 96, 120, 150, 175 C) using a Microwave Accelerated Reaction System [MARS-5, CEM Corporation, 0–1250 W, 2450 MHz frequency, maximum temperature: 260 C, maximum pressure: 500 psig (33 bars) equipped with fiber optic temperature and pressure probes within the cavity and a turning carousel with a maximum of 14 pressure
sealed vessels of 100 mL each]. A total of 500 g of TWAS was cooked in 10 vessels (50 g sample per vessel) rotating on the carousel. MARS-5 operates with a focused microwave irradiating beam and is capable of heating and holding at desired cooking (or temperature ramping) rates and holding times. Although MARS-5 is capable of reaching temperatures above the boiling point very quickly (such as ramping from 20 + 2 to 175 + 2 C in 10 min), preliminary microwave studies suggest that sludge solubilization increases with duration of microwave exposure [10]. Therefore, in this study, slow cooking profiles (1.2–1.4 C/min) were applied. After target temperatures were reached, samples were removed from the heating source (no holding time at desired temperatures) and cooled to room temperature in closed vessels to avoid evaporation of organics and then stored at 4 C in a refrigerator.
2.4. Biological studies The anaerobic degradability of control (untreated) and microwave-irradiated samples were determined by batch mesophilic BMP tests in 125 mL serum bottles sealed with butyl rubber stoppers. A total of 32 BMP tests were performed for various microwave irradiation temperatures with both acclimatized and non-acclimatized inoculums including duplicates and controls according to [11]. Non-acclimatized (to microwave-irradiated TWAS) inoculum for the BMP test was originally taken from the effluent line of the anaerobic sludge
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digesters (SRT of 15–20 d) treating a mixture of TWAS and primary sludge [58:42 (v/v)] at ROPEC. In order to analyze the toxicity effects of microwave irradiation on TWAS, 16 of 32 BMP assays were setup with non-acclimatized inoculum and 16 with acclimatized inoculum. For acclimation, one 5 L anaerobic semi-continuous reactor (35 C), fed with microwaveirradiated (to 175 C sludge by MARS-5) sludge, was run at an approximately 20 d SRT over a year. Microwave toxicity effect on secondary sludge digestion was expected to increase with temperature [3,12]; therefore, microwave irradiation at 175 C was used to pretreat feed sludge for the acclimation reactor. For BMP assays, acclimated inoculums (15 mL) were placed into serum bottles and then sludge samples (70 mL) were added. Nitrogen sparging was applied to batch reactors when sludge samples and inoculum were mixed to prevent exposure to air and serum bottles (120 mL) were sealed after addition of an equal mixture of NaHCO3 and KHCO3 to achieve an alkalinity of 4000 mg/L (as CaCO3). BMP assays were kept in a darkened temperature controlled incubator shaker at 33 + 1 C and at 90 rpm until they stopped producing biogas. Daily biogas produced was measured by inserting a needle attached to a manometer. 2.5. Analysis Total and volatile solids (TS/VS) were determined based on Standard Methods procedure 2540G [13]. For supernatant solid determination, centrifugation [for 20 min at 5856 relative centrifugal force (RCF) in a Dupont instruments Sorvall SS-3 automatic centrifuge] and then coarse filtration (1.2 mm) was used. Colorimetric COD measurements were done based on Standard Methods procedure 5250D [13] with a Coleman Perkin-Elmer spectrophotometer Model 295 at 600 nm light absorbance. Before SCOD determination, sludge samples were centrifuged (for 20 min at 5856 RCF) and filtered through membrane disc filters with 1.2 and 0.45 mm pore sizes.
3. Results and discussion The effects of microwave pretreatment temperature on SCOD/TCOD ratios were investigated. For comparison, maximum achievable COD solubilization was
SCOD/TCOD * 100 (%)
282
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
NaOH = 0.12 mol/L NaOH = 0.25 mol/L NaOH = 0.5 mol/L NaOH = 1 mol/L NaOH = 2 mol/L 0
1
2
3
4
5
6 7 8 9 10 11 12 13 14 15 Contact time (d)
Fig. 1. Estimation of the ultimate solubilization ratio of ROPEC – thickened waste activated sludge [SCOD/ TCOD: soluble to total chemical oxygen demand ratio].
also quantified by NaOH addition and results are shown in Fig. 1. SCOD/TCOD (%) ratio of control (9 + 1) increased to 51 + 0.5%, 57 + 0.6%, 60 + 0.6%, 59 + 0.6%, 65 + 0.6% at 0.12, 0.25, 0.5, 1 and 2 mol/L NaOH concentrations after 2 weeks of contact time. Chemical solubilization of samples was initially quite fast. Within 3 h, half of the samples achieved about 30% solubilization (50% of ultimate solubilization) and within 2 d, waste activated sludge contacted with 2 mol NaOH/L achieved 92% of ultimate solubilization which was 60 + 0.6% SCOD/TCOD. These results lead to a conclusion that chemical solubilization contact times less than 2 d will underestimate the ultimate solubility of ROPEC-TWAS. Particulate COD solubilizations achieved at different microwave pretreatment temperatures are shown in Fig. 2. In a pretreatment range of 50–175 C and temperature ramp of 1.2–1.4 C/min, solubilization profiles
Fig. 2. Effect of microwave irradiation on ROPEC-TWAS solubilization [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures; SCOD/TCOD: soluble to total chemical oxygen demand ratio].
C. Eskicioglu et al. / Desalination 248 (2009) 279–285 a)
indicated a linear relation with SCOD/TCOD ratios of 9 + 0%, 12 + 1%, 21 + 0%, 24 + 1%, 24 + 3%, 28 + 1% and 35 + 1% at microwave pretreatment temperatures of control, 50, 75, 96, 120, 150 and 175 C, respectively. At 175 C, microwave achieved 54% of the ultimate chemical solubilization ratio of ROPECTWAS (65 + 0.6%; Fig. 1) obtained with 2 mol NaOH/L. Disintegration of TWAS floc structure was also evaluated by analyzing the supernatant solids (<1.2 mm) after pretreatment. TS, VS and fixed solids (FS ¼ TS VS) were measured in the whole sludge and in the supernatants after centrifugation for 20 min at 5856 RCF and then coarse filtration (<1.2 mm) of control and pretreated samples. Fig. 3 indicates significant disintegration of TS, mainly composed of VS, as microwave temperature increases. At 175 C, 32 + 0.4% of TS, composed of 91% of VS (organic portion), were in the supernatant phase while for the untreated control this value was only 8 + 0.4%. Biodegradability and potential toxicity of microwave-irradiated TWAS relative to control digesters studied in batch mesophilic (35 + 2 C) BMP assays. The first week of incubation was critical since maximum substrate utilization generally occurs in the first 5–7 d. Initial toxicity of pretreated samples on methanogenesis (converting acetic and propionic acids to methane) could be reduced by acclimation of inoculum to the harshest irradiated sludge (at 175 C). Non-acclimatized inoculum was also used in this study to evaluate the toxicity effects of pretreated sludge; however this would not represent full-scale digestion
Cumulative biogas production (mL)
800 700 600 500 400
Control MW−50 MW−75 MW−96 MW−120 MW−150 MW−175
300 200 100 0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Digestion time (d) b)
Cumulative biogas production (mL)
Fig. 3. Organic and fixed solid material solubilization at different microwave temperatures [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures; TS: total solids].
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800 700 600 500 400
Control MW−50 MW−75 MW−96 MW−120 MW−150 MW−175
300 200 100 0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Digestion time (d)
Fig. 4. Cumulative biogas productions from control and pretreated thickened waste activated sludge samples with (a) acclimatized inoculum, (b) non-acclimatized inoculum [MW – 50, 75,. . ., 175 indicate microwave pretreatment temperatures].
(continuous-flow) with an initial acclimation period. Cumulative biogas productions with acclimatized and non-acclimatized inoculum are presented in Fig. 4a and b, respectively. Biogas productions from the inoculum controls were subtracted from the biogas production from the mixtures to obtain biogas from sludge samples only. From comparison of Fig. 4a and b, starting with acclimatized inoculum did not only affect the rate of biogas production but also the extent of waste activated sludge AD. In a temperature range of 50–175 C with acclimatized inoculum, all pretreated digesters improved cumulative biogas production, pretreated sludge irradiated at 1.2 C/min to 175 C produced the highest cumulative biogas production which was
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31 + 6% higher than the control after 18 d of digestion. This result is in agreement with the level of improvement achieved in sludge solubilization microwave heated at 1.2 C/min to 175 C (Fig. 2). As digestion continued further, the difference in cumulative biogas productions became smaller (28 + 6% around the 27th day), since given sufficient time, more difficult to degrade organics in control samples would continue to be degraded. The fact that the exponential phase was longer for pretreated samples than the control suggests that higher amounts of readily degradable substances were present in the pretreated digesters. On the other hand, non-acclimatized inoculum digesting the identical pretreated sample (at 175 C) achieved only 18 + 2% higher biogas compared to controls after 20 d (Fig. 4b). Results indicate that within a practical time of 18–28 d, microwave pretreatment enhanced the ultimate degradability of ROPEC-waste sludge. Although the inoculum used in half of the BMP assays was acclimatized to the highest temperature (175 C) in a semi-continuous reactor for one year, control digesters initially had the fastest biogas production (Fig. 4a) followed by the digesters with sludge pretreated to 50 C. Reactors digesting pretreated TWAS at 50 C recovered after 4 d and at pretreatment temperatures of 96, 120, 150 C after 7 d and digesters pretreated at 175 C recovered after 10 d of mesophilic incubation. Obviously, the severity of the pretreatment temperature influenced the biogas production in the first 10 d of batch digestion. All pretreated digesters in a temperature range of 50–175 C were consistent in their reduced relative biogas responses indicating that some product or products were being formed that resulted in a mild short term microbial inhibition compared to untreated TWAS. This mild inhibition may be of some concern for continuous digesters operated at short SRTs but may disappear with acclimation [5]. Initial acute toxicity for the non-acclimatized inoculum digesting microwave pretreated sludge was much more severe. In Fig. 4b, the recovery took almost two times longer (compared to acclimatized BMP assays) and were 5 d at a microwave temperature of 50 C, 14 d at temperatures of 75, 96, 120 C and 15 and 16 d at temperatures of 150 and 175 C, respectively. It is possible that some portion of the inoculum lost its activity under this toxic environment which created the
difference in the rate and the ultimate biogas produced from BMP assays with acclimatized and nonacclimatized inoculums. Except for the assay with sludge irradiated to 175 C, biogas data indicate that all other BMP assays with unacclimatized inoculum did not result in an increase in the ultimate digestibility of sludge over controls (Fig. 4b). Since digester pH was controlled by addition of buffer solutions before the reactors were sealed, pH values of non-acclimatized and acclimatized digesters fluctuated only in the ranges of 6.9–8.6 and 6.8–8.1 during the BMP test, respectively. The digesters with acclimatized inoculum were able to tolerate initial acute toxicity with no longterm decrease in methane production or volatile fatty acids accumulation. However, the majority of digesters with non-acclimatized inoculum stayed below the control biogas production line. Methane content of biogas produced in non-acclimatized and acclimatized digesters fluctuated in ranges of 46–76% and 61–77% depending on the pH of digesters, respectively. Average methane percentages of control, MW – 50, 75, 96, 120, 150, 175 digesters were 65 + 2, 66 + 4, 67 + 2, 67 + 4, 68 + 6, 68 + 4 and 67 + 3 for acclimatized and 64 + 4, 66 + 3, 66 + 5, 66 + 5, 65 + 7, 68 + 6 and 63 + 13 for non-acclimatized digesters, respectively. Almost all (98%) of the volatile fatty acids were converted to biogas in two weeks. Ultimate biogas production results may explain the different digestion performances reported for the similar type of sludge pretreatment studies in the literature. It is possible that these differences arise not only from different initial sludge characteristics but also from using acclimatized or non-acclimatized inoculums (not specified in majority of pretreatment studies). The cumulative biogas and methane productions and VS removals from all 32 batch BMP digesters were used and results indicated 1.04 + 0.17 L biogas per g VS removed for acclimatized and 0.99 + 0.32 L biogas per g VS removed for non-acclimatized digesters, respectively. These results correspond to methane yields of 0.70 + 0.12 and 0.64 + 0.21 L methane per g VS for acclimatized and non-acclimatized mesophilic digesters, respectively at 1 atm and 25 + 1 C. Biogas and methane yields from batch mesophilic digesters in this study were within the range of other treatment studies.
C. Eskicioglu et al. / Desalination 248 (2009) 279–285 4. Conclusions Based on the experimental data and analysis, the following conclusions are drawn. In a temperature range of 50–175 C, there was a linear relation between microwave temperature and level of hydrolysis. SCOD/TCOD ratios (%) increased from 9 + 1 (control; unpretreated) to 24 + 3, 28 + 1 and 35 + 1 at microwave temperatures of 120, 150 and 175 C, respectively. Similarly, supernatant solid (<1.2 mm) to TSs ratios of waste activated sludge increased from 8 + 0% (control) to 19 + 1%, 21 + 1% and 32 + 0% after microwave irradiation to 120, 150 and 175 C, respectively. In BMP tests, despite the first 9 d of mild acute inhibition, acclimatized inoculum digesting waste activated sludge, microwaved to 175 C, produced 31 + 6% higher biogas compared to control digester around 18 d of mesophilic batch digestion. However, initial acute toxicity was more severe for non-acclimatized inoculum requiring two times longer recovery with only 18 + 2% higher biogas around 20 d, indicating that inoculum acclimatization did not only accelerate, but also extend the ultimate mesophilic biodegradation.
Acknowledgments The authors would like to thank Mr. Nicolas Terzian for his work in acquiring the data.
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