Operational Start-Up Performance of an Innovative Anaerobic Stage Reactor (ASR) using Synthetic Wastewater

Rosnani Alkarimiah1, Siti Baizura Mahat1, Ali Yuzir1, Mohd. Fadhil Md. Din1 and Shreeshivadasan

Chelliapan2

1Department of Environmental Engineering, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, Johor Bahru, Malaysia.

2UTM Razak School of Engineering and Advanced Technology, Universiti Teknologi Malaysia (International Campus), Jalan Semarak, 54100, Kuala Lumpur, Malaysia.

Abstract. In general, the start-up of an anaerobic reactor is a relatively delicate process. Start-up procedures will depend on various factors, including wastewater composition, available inoculum, reactor operating conditions, and reactor configuration. Systematized operational procedures are very important, mainly during the start-up of an anaerobic reactor. In this paper, the start-up performance of an innovative Anaerobic Stage Reactor (ASR) using synthetic wastewater (glucose) at various organic loading rates was investigated. In Phase 1 of the experimental study, the ASR system was operated at hydraulic retention time (HRT) of 1 d with corresponding organic loading rate of 1.07 kg COD.m-3.d-1. During this period, the reactor was operated continuously for a period of 34 days. Thereafter, the reactor was operated at intermittent feeding (Phase 2), with HRT of 1.4 d and OLR of 0.82-2.45 kg COD.m-3.d-1. The treatment efficiency of the reactor was characterised in terms of its chemical oxygen demand (COD) reduction, biogas composition (methane and carbon dioxide), solid washout and pH. Results showed up to 71% COD reduction in the Phase 1 of the experimental study. Nevertheless, in the Phase 2, when the reactor was operated at intermittent feeding, the COD removal efficiency increased from 75 - 92% (at OLR of 0.82-2.45 kg COD.m-3.d-1). It can be concluded that the ASR system performed better at intermittent feeding, indicating the reactor required low loading rate and sufficient HRT for gradual acclimatization for reactor start-up. The reduction of the period necessary for the start-up and improved operational control are important factors to increase the efficiency and the competitiveness of the ASR system.

Keywords: Anaerobic Stage Reactor (ASR), anaerobic start-up, biomass, glucose wastewater

1. Introduction Anaerobic digestion has proven to be stable process for a variety of wastewaters when operated properly.

It has several advantages over the aerobic and physic-chemical process such as low sludge production, higher loading potential, low operating cost and methane production [1]. Anaerobic methanogenic digestion an effective method for treatment of many organic wastes is a topic of increasing interest throughout the world. A number of designs and their performance have already been described by several researchers [2]. However, the fact remains that anaerobic process has not been utilized as widely as aerobic process. Up to now, the technology of anaerobic digestion has not been able to meet the predicted expectation to its potential. Compared with other processes, its advantages are less energy requirement, high treatment efficiency and usable gas produced.

Start-up procedures for anaerobic reactor will depend on various factors such as wastewater composition and strength, available inoculum, organic loading rate (OLR), hydraulic retention time (HRT), and reactor configuration [3]. Shorter start-up time can be obtained by using wastewater low in particulate organics. + Corresponding author. Tel.: + 006-03-26154581; fax: +006-03-26934844 E-mail address: shreeshivadasan@ic.utm.my

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2011 International Conference on Environment and Industrial Innovation IPCBEE vol.12 (2011) © (2011) IACSIT Press, Singapore

In general, high-rate anaerobic processes can be operated with organic loads much higher than those of the conventional anaerobic reactors, but frequently these highly efficient processes require longer start-up periods, better operational control and more qualified operators. Systematized operational procedures are very important, mainly during the start-up of high-rate systems. The start-up of anaerobic reactors is determined by the initial transient period, marked by operational instabilities.

The Anaerobic Stage Reactor (ASR) is similar in design and application to the anaerobic baffled reactor (ABR) [4]. Each stage of the ASR system represents a separate compartment. A stage reactor can provide high treatment efficiency since recalcitrant substrates will be in an environment more conducive to degradation [5]. The design of the ASR results in the separation of acidogenesis and methanogenesis, which has potential benefits for reactor performance. With no moving parts or mechanical mixing, no requirement for biomass with unusual settling properties, and a high degree of stability to hydraulic and organic shock loads, the stage reactor has the potential be applied economically as a pre-treatment system for many trade effluents [6].

Start-up is often considered to be the most unstable and difficult phase in anaerobic digestion. Therefore, the main objective of this study is to observe and evaluate start-up performance of ASR system using synthetic wastewater (glucose) at various organic loading rates (OLRs). The performance of ASR was evaluated based on the COD removal efficiency, solid washout, pH stability and gas composition.

2. Materials and Methods

2.1. Anaerobic stage reactor (ASR) The Anaerobic Stage Reactor (ASR) consists of 4 units of transparent identical cylindrical Plexiglas

compartments (stages), 160mm internal diameter by 1100mm height with head plate (Fig. 1). The active volume of the ASR system was 90 L (4 stages of 22.5L). The flow diagram of reactor system design is presented in Fig. 1. Each stage of the reactor had a 3-phase separator baffle, placed below the effluent ports, to prevent floating granules from washing out with the effluent. Effluent from each stage of the reactor flowed by gravity to the next, as each stage was placed on stepped platform. Each stage of the reactor had a temperature controller to maintain the reactor temperature at 37o C. Peristaltic pumps were used to control the influent feed rate to the first stage of the ASR. Gas production was monitored separately for each stage using gas-water displacement method.

Fig. 1: ASR system and flow regime

2.2. Operation and analytical method The ASR was seeded using anaerobic digested palm oil mill effluent (POME) sludge (Felda Palm

Industries Pte. Ltd., Negeri Sembilan, Malaysia). The sludge was sieved pass 2 mm mesh giving solid contents of 53,750 mg TSS.L-1 (41,500 mg VSS.L-1). 7.5 L of sieved sludge was added to each reactor stage, the remaining volume being filled with tap water. Throughout the experiment, the reactor was supplied with

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134

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136

We would like to thank Ministry of Science, Technology and Innovation (MOSTI), Malaysia for funding this research under the e-science fund category; project number 03-01-06-SF0568.

5. Conclusions The intermittent feeding during reactor start-up shows better performance compared to continuous

feeding. At an OLR 2.45 kg COD.m-3.d-1, up to 92% COD removal efficiency was observed in the ASR, indicating optimum operational condition for reactor start-up. However, low pH values had effected the performance of the reactor during each step increases in the OLR. To improve the performance, it is always a good practice not to let the pH in the anaerobic reactor reduced to a value less than 6.5. Although COD degradation efficiency might be affected by the lower pH, long HRT in the ASR can lessen these effects.

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