A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Environmental Engineering and Management

Thesis (M.Eng.) - Asian Institute of Technology, 2008

Proper management of `excess sludge', more precisely, `waste activated sludge (WAS)', is a big challenge to wastewater treatment operators due to implementation of the stringent disposal regulations. To reduce time period of the rate limiting cell lysis step at the first phase of anaerobic digestion, pretreatment processes were commonly adopted prior to anaerobic digestion. Ultrasonic, chemical and chemical-ultrasonic techniques were investigated in this study for disintegrating the sludge. The results revealed that chemical-ultrasonic gave a better efficiency on sludge disintegration compared to individual chemical and ultrasonic techniques. The optimum condition of chemical-ultrasonic was found at 10 mg/g TS chemical dose and 3.8 kJ/g TS specific energy input, whereas chemical dose of 50 mg/g TS and specific energy of 3.8 kJ/g TS were the optimum operating condition of individual chemical and ultrasonic, respectively. The results from anaerobic digester indicated that pretreated sludge could enhance the performance of anaerobic digester by increasing the methane production, and also by TS and VS removal. With three (25, 15 and 10 days) operated Sludge Retention Time (SRT), 15 days SRT was found to be a suitable digestion time for both ultrasonicated and chemical-ultrasonicated sludge. Compared to control digester, the methane production of ultrasonic and chemical-ultrasonic digester at 15 days SRT increased by 23 and 31%, respectively. Beside methane production improvement, degradation rate of chemical ultrasonicated sludge was also faster than ultrasonicated and non-pretreated sludge at all SRTs. At 15 days SRT, the degradation rate of the chemical ultrasonicated sludge improved by 52%, while ultrasonic digester improved by around 26% when compared to control digester. Similarly, dewaterability of digested sludge was also found to be improved by 62 and 66% from ultrasonic and chemical-ultrasonic digester respectively compared to feed sludge. Based on energy balance and economic analysis, control digester operated at 25 days SRT was economically viable. It was found that the income from bioenergy recovery and landfill cost reduction were almost the same as expenses incurred for chemical and energy consumption. However, the 15 days SRT with ultrasonic pretreatment would be more efficient and beneficial as it requires low capital and maintenance cost, smaller reactor volume and reduce the waste volume; consequently reducing the cost for landfill, equipments, energy consumptions, land for treatment plant, etc.