Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge

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<ul><li><p>JOURNAL OF BIOSCIENCE AND BIOENGINEERING Vol. 95, No. 3,271 275. 2003 </p><p>Effects of Various Pretreatments for Enhanced Anaerobic Digestion with Waste Activated Sludge </p><p>JEONGSIK KIM, 1'2 C H U L H W A N PARK, l'2 T A K - H Y U N KIM, 1 M Y U N G G U LEE, 1'2 S A N G Y O N G KIM, 1. S E U N G - W O O K KIM, 2 AND J INWON LEE 3 </p><p>Advanced Energy &amp; Environment Research Team, Korea Institute of Industrial Technology, Chonan 330-825, Korea, Department of Chemical and Biological Engineering, Korea University, Seou1136- 701, Korea,; </p><p>and Department of Chemical Engineering, Kwangwoon University, Seou113 9- 710, Korea 3 </p><p>Received 5 September 2002/Accepted 14 November 2002 </p><p>The purpose of this study was to enhance the efficiency of anaerobic digestion with waste acti- vated sludge (WAS) by batch experiments. We studied the effects of various pretreatment meth- ods (thermal, chemical, ultrasonic and thermochemical pretreatments) on the biogas production and pollutants reduction owing to solubilization enhancement, particle size reduction, increased soluble protein, and increased soluble COD. The thermochemical pretreatment gave the best re- sults, i.e., the production of methane increased by more than 34.3% and soluble COD (SCOD) re- moval also increased by more than 67.8% over the control. In this case, the biogas production, methane production and the SCOD removal efficiency were about 50371 biogas/m 3 WAS, 3367 ! methane/m 3 WAS and 61.4%, respectively. Therefore, it is recognized that higher digestion effi- ciencies of the WAS were obtained through thermochemical pretreatment of the sludge. </p><p>[Key words: waste activated sludge, anaerobic digestion, pretreatment, methane, biogas] </p><p>One of the major problems facing the industrialized world today is to resolve environmental contamination and iden- tify efficient treatments. Over the last few decades, the pos- sibility of utilizing activated sludge including specific mi- croorganisms to treat various wastewaters has been widely discussed (1, 2). Activated sludge has been the most widely used biological material for industrial wastewater treatment. It facilitates the transformation of dissolved organic pollut- ants from wastewater into biomass, and these are finally converted into carbon dioxide and water by microorganisms (3, 4). The main by-product of biological wastewater treat- ment is waste activated sludge (WAS) and the amount of WAS has also increased as the result of the expansion of wastewater (5). </p><p>In addition, fossil fuels will soon be depleted and we must reduce energy consumption and very rapidly move to alternative energy supplies with the emphasis on renewable sources such as solar and biomass energy (6). Recently international support for developing these relatively new sources of energy has increased due to their benefits as assessed by reduced environmental impact, particularly re- duced greenhouse gas emissions (7, 8). For this reason, anaerobic digestion is also applied for sewage sludge stabi- lization resulting in the reduction of sludge volatile solids and the production of biogas. The anaerobic digestion proc- ess generally consists of four stages, hydrolysis, acidogene- sis, acetogenesis and methanogenesis. In anaerobic diges- tion, the biological hydrolysis is identified as the rate-limit- ing step (9-11). To reduce the impact of the rate-limiting </p><p>* Corresponding author, e-mail: sykim@kitech.re.kr phone: +82-41-589-8356 fax: +82-41-589-8330 </p><p>step, pretreatment of WAS is required such as thermal, alka- line, ultrasonic or mechanical disintegration (9-19). These treatments can accelerate the solubilization (hydrolysis) of WAS and reduce the particle size, which subsequently im- proves the anaerobic digestion (9, 12). </p><p>Thermal pretreatment has been studied to improve the anaerobic digestibility and dewatering properties (13). Al- kaline pretreatment has been also used to solubilize various substrates such as lignocellulosic materials or WAS (14). Lin et al. (17) showed that COD and volatile solids (VS) reduction, gas production and dewaterability were enhanced when WAS was pretreated with NaOH. Ultrasonic disinte- gration is a well-known method for the break-up of micro- bial cells to extract intracellular material (9), and mechani- cal pretreatment has also been shown to have the same effect on WAS as other pretreatment methods mentioned above. Nah et al. (18) examined the mechanical pretreat- ment of WAS and determined that jetting to and colliding with a collision plate at 30 bar to solubilize the sludge. </p><p>As mentioned above, many studies have investigated the pretreatment of WAS for anaerobic digestion and mostly dealt with a single pretreatment method in comparison with non-pretreatment (9-11, 17-19). However, few reports have been published on various pretreatment methods for anaero- bic digestion (10). Accordingly, the objective of this study was to investigate the effect of various pretreatments (ther- mal, chemical, ultrasonic and thermochemical pretreatments) of WAS on solubilization, particle size reduction and meth- ane production enhancement. In addition, research is ongo- ing to develop a practical way to improve the treatment effi- ciency of WAS and increase the methane production. </p><p>271 </p></li><li><p>272 KIM ET AL. J. BlOSCi. BIOENG. , </p><p>MATERIALS AND METHODS </p><p>Materials WAS was obtained from a sewage sludge treat- ment facility in Chonan, Korea and its characteristics are shown in Table 1. Rumen microorganisms were obtained from cattle dung (20). Materials were purchased from Sigma (St. Louis, MO, USA) and Aldrich (Milwaukee, WI, USA). </p><p>Pretreatment of WAS The thermal pretreatment was carried out as follows: Samples were thermally treated in an autoclave (JeioTech, Korea) at 121C and 1.5 atm pressure for 30 min. WAS was cooled to ambient temperature and the cooling time was about 2 h. For the chemical pretreatment, various alkaline agents were used such as NaOH, KOH, Mg(OH)2 and Ca(OH)2 at pH 12 (14). NaOH was added to 300 ml of WAS at final concentrations rang- ing from 0 to 21 g/l. Ultrasonic pretreatment was performed using an ultrasonicator (Branson, Danbury, CT, USA) operating at 42 kHz for various times (from 10 to 120 min). During sonication, WAS was stirred and the temperature was automatically main- mined at 25C. </p><p>Anaerobic digestion Microorganisms (seed sludge) for an- aerobic digestion consisted of those present in anaerobic sludge from the sewage sludge treatment facility and rumen microorga- nisms of cattle dung (50: 50, v/v). The reactor for anaerobic diges- tion had a volume of 1 l and its working volume was 600 ml. It was also equipped with gas and sludge sampling ports. Three hundred ml of the samples (pretreated and non-pretreated WAS) were added separately to five digesters containing 300 ml of seed sludge and the reactor was purged with helium gas to eliminate air from the reactor. By adding HC1 before feeding, the pH of the feed sludge was adjusted to pH 6.7. The mixed sludge was stirred in the digester without oxygen contact. These reactors were incubated at 37C and the gas volume generated was measured using a cali- brated sampling syringe. </p><p>Analysis Standard methods were used for the estimation of COD~ SCODcr (soluble COD), total solid (TS), VS and pH (21). TS was measured gravimetrically using a glass microfiber filter (4.7 cm diameter; Whatman, Maldstone, UK) and COD and SCOD using a colorimetric method after digestion of the samples in the COD reactor, model 45600 (HACH, Loveland, CO, USA). Addi- tionally, SCOD was measured after eentrifugation at 10,000 rpm for 10min. The degree of COD solubilization was calculated by the following equation: </p><p>COD solubilization (%)= soluble COD measured after pretreatment </p><p>total COD measured after pretreatment x 100 (%) </p><p>The amount of soluble protein was determined by the Bradford method using bovine serum albumin (BSA) as the standard (22). The particle size of sludge samples was measured using a laser particle size analyzer (MAF5001; Malvern, Worcestershire, UK). The biogas composition was analyzed by a gas chromatograph (6890N; Agilent, Palo Alto, CA, USA) equipped with a thermal conductivity detector. </p><p>RESULTS AND DISCUSSION </p><p>In this work, various pretreatment methods (thermal, chemical, ultrasonic and thermochemical pretreatments) of WAS were performed to improve the treatment efficiency. Firstly, the influence o f thermal pretreatment on SCOD and COD solubilization was evaluated (Fig. 1). The control experiment was performed using non-pretreated WAS. At ambient temperature, a value of 8.1% COD solubilization (SCOD=2250mg/ / ) was obtained. WAS was successfully </p><p>TABLE 1. Characteristics of WAS used in experiments </p><p>Item Value COD 27700 mg/l SCOD 2250 mg/l TS 38.0 g/l VS 26.0 g/1 pH 6.7 Soluble protein 30.2 mg/l </p><p>liquidized by thermal pretreatment at 121 C with an operat- ing time of 30rain. On the other hand, a value o f 17.6% COD solubilization (SCOD=4900 mg//) was achieved after thermal pretreatment. This result indicates that the organic particulates in WAS were liquidized to soluble carbohy- drates, lipids and proteins or converted into lower molecular weight compounds by thermal pretreatment. Li and Noike (11) reported that the optimum pretreatment temperature and contact time for improving the anaerobic digestion of WAS were 170C and 60 min, respectively' Through ther- mal pretreatment, the gas production from WAS was greatly increased. </p><p>Secondly, alkaline pretreatments were performed at pH 12 with various alkaline agents NaOH, KOH, Mg(OH)a and Ca(OH)2. At ambient temperature, the COD solubilization values after NaOH, KOH, Mg(OH)2 and Ca(OH)2 addition were 39.8%, 36.6%, 10.8% and 15.3%, respectively. Simi- larly, following treatment at 121C for 30 rain, NaOH addi- tion resulted in 51.8% COD solubilization with the other values being 47.8, 18.3 and 17.1, respectively. COD solubi- lization levels are shown in Fig. 2. Monobasic agents re- sulted in higher solubilization percentages than dibasic ones probably because dibasic alkali agents were only partially dissolved. Penaud et al. (14) also reported similar results. </p><p>The effect o f the addition o f various concentrations of NaOH on COD solubilization was evaluated (Fig. 3). At ambient temperature, COD solubilization increased as the dose of NaOH increased, reaching 43.5% when 7 g/l NaOH were added. From 7 to 21 g/I NaOH, a lower rate o f COD solubilization increase was observed. When alkali agents were added, COD solubilization increased through various reactions such as saponification o f uronic acids and acetyl esters, reactions occurring with free carboxylic groups and neutralization o f various acids formed from the degradation o f particular materials. For thermochemical pretreatment, </p><p>6000 -20 </p><p>4000 </p><p>8 2000 </p><p>FIG. l. </p><p>$COD COD Solubilization </p><p>- 16 </p><p>g -12 </p><p>I[ 8 </p><p>- 4 </p><p>, 0 </p><p>Thermal </p><p>Effects of thermal pretreatment on COD solubilization. </p></li><li><p>VOL. 95, 2003 ANAEROBIC DIGESTION OF BIOMASS 273 </p><p>60- </p><p>=50- </p><p>o40- 8 ~ao </p><p>o8~o </p><p>10- </p><p>FIG. 2. </p><p>~ l l Chemical pretreatrn~ "n'~=wno-ch~cal preVeatment </p><p>I1 NaOH KOH Mg(OH) 2 Ca(OH) 2 </p><p>Effects of various alkali agents on COD solubilization. </p><p>g g </p><p>e~ o (3 </p><p>FIG. 3. </p><p>100 T h e m x &gt; c h e m i c a l p ~ t m e n t </p><p>o Chemica l i ~ ' e ~ l m e n t </p><p>o - c~ o . . . . . . o . . . . . o . . . . . o o </p><p>J 6 </p><p>0 1 3 5 7 9 12 15 18 21 </p><p>N a O H (g//) </p><p>Effects of NaOH addition on COD solubilization. </p><p>the same concentrations of NaOH were added following treatment at 121C for 30 min. COD solubilization was also used to evaluate solubilization performance. When NaOH at 0 to 21 g/l was added, the COD solubilization increased from 17.6% to 86.5%. From these results, although at the maximal concentration of NaOH of 9 g/l the COD solubili- zation was 86.5%, we used the optimal concentration of NaOH (7 g//') which gave a COD solubilization of 85.4%. </p><p>The results of this type of pretreatment obtained in this study were compared with other published results and were found to be similar. Rajan et al. (15) chemically pretreated WAS with NaOH and revealed an increase of more than 46% in solubilization. Ray et al. (16) reported that a solu- bilization rate of more than 45% of particulate COD was achieved at 30 meq/l NaOH. Penaud et al. (14) also ob- tained, when more than 5 g NaOH/l was added, 75-80% COD solubilization when pretreatment was carried out at 140C for 30 min instead of 65% at ambient temperature. </p><p>The effect of ultrasonic pretreatment is shown in Fig. 4. The COD solubilization was 18.4% when the WAS was sonicated for 120 min without thermal pretreatment and was 19.1% when the WAS was sonicated for 120 min after ther- mal pretreatment. The difference was not statistically signifi- cant and sonication for 120 min without thermal pretreat- ment was selected. </p><p>As a result of various pretreatments, optimal conditions for thermal (121C for 30 min), chemical (7 g/l NaOH addi- </p><p>24- </p><p>FIG. 4. </p><p>g 16- </p><p>12- </p><p>8 0 </p><p>8 - </p><p>--e-- Themlo-ultmsol~ prelreetment </p><p> . o </p><p>o " </p><p>o </p><p>4. , , , , , , </p><p>0 10 30 60 90 120 </p><p>lime (rain) </p><p>Effects of ultrasonic pretreatment on COD solubilization. </p><p>3000 </p><p>= 2000 1983.0 1 1665.4 </p><p>,coo </p><p>0 30.2 85.7 61,7 - - - - </p><p>500- </p><p>g 4O0 ~ </p><p>.~_ </p><p>.ca 300- </p><p>~5 ~N 200- </p><p>~= 100- </p><p>0 </p><p>450 </p><p>21 c </p><p>Control </p><p>= = ~ = 10% a c c u m u l a t e d </p><p>5 0 % accumulated 9 0 % a c c u m u l a t e d </p><p>240 </p><p>,.0d ,., ,_4 T h e r m a l C h e m i c a l U l t r a s o n i c T h e r m o - </p><p>c h e m i c a l </p><p>FIG. 5. Effects of various pretreatments on particle size distribu- tion and soluble protein. </p><p>tion), ultrasonic (42 kHz for 120 min) and thermochemical (121 C for 30 min, 7 g/l NaOH addition) pretreatments were selected. Particle size distribution and soluble protein under various conditions were compared and the results are shown in Fig. 5. The "10%, 50% and 90% accumulated values" indicates that 10%, 50% and 90% of particles are size be- low 24 pm, 219 gm and 450 txm in the case of the. control, respectively. In the case of thermochemical pretreatment, 10%, 50% and 90% of the particles are below 2 gm, 29 pm and 144 I.tm, respectively. The four pretreatment methods not only reduced the particle size but also increased the level of soluble protein. In particular, thermal and thermo- chemical pretreatments resulted in significant particle size reduction. The level of soluble protein was increased by chemical and thermochemical pretreatment methods in par- ticular. Proteins are principle constituents of ,organisms and they contain carbon, which is a common organic substance, as well as hydrogen, oxygen and nitrogen (18). For this rea- son, it was considered that as the level of soluble protein in- creased, the efficiency of anaerobic digestion would be im- proved. </p><p>To investigate the SCOD reduction, VS reducldon, and gas production of pretreated WAS, an experiment was per- </p></li><li><p>274 KIM ET AL. J. BIOSCL BIOENG., </p><p>251~0 50 </p><p>I SCOD before digestion SCOD after digestion </p><p>20000 ---</p></li><li><p>VOL. 95, 2003 ANAEROBIC DIGEST...</p></li></ul>


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