effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge
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JOURNAL OF BIOSCIENCE AND BIOENGINEERING Vol. 95, No. 3,271 275. 2003
Effects of Various Pretreatments for Enhanced Anaerobic Digestion with Waste Activated Sludge
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
Advanced Energy & Environment Research Team, Korea Institute of Industrial Technology, Chonan 330-825, Korea, Department of Chemical and Biological Engineering, Korea University, Seou1136- 701, Korea,;
and Department of Chemical Engineering, Kwangwoon University, Seou113 9- 710, Korea 3
Received 5 September 2002/Accepted 14 November 2002
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.
[Key words: waste activated sludge, anaerobic digestion, pretreatment, methane, biogas]
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).
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
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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).
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.
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.
272 KIM ET AL. J. BlOSCi. BIOENG. ,
MATERIALS AND METHODS
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).
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.
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.
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:
COD solubilization (%)= soluble COD measured after pretreatment
total COD measured after pretreatment x 100 (%)
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.
RESULTS AND DISCUSSION
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
TABLE 1. Characteristics of WAS used in experiments
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
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.
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 resu