Enhancement of methane production from anaerobic digestion of wastewater treatment sludge

Bernice Ng

Anaerobic digestion is a commonly used technique in treating industrial, rural effluents and sewage sludge. Methane, CH4, produced in anaerobic digestion is a valuable renewable energy source. In wastewater treatment plant (WWTP), primary and secondary sludge are produced at different stages of wastewater treatment process. They are different in terms of composition and degradability. Experiments (batch/ semi-continuous) showed that primary sludge was more degradable and produced higher methane than secondary sludge. As primary sludge is highly degradable, the possibility of increasing feeding rate was examined. This study found that the increased in feeding rate eventually leaded to massive methanogens washout and hence digester failure. In order to maintain the stability of primary sludge reactor at shorter hydraulic retention time (HRT), a biomass recycling method was investigated. It was found that the reactor remains stable at shorter HRT (16 days). However, further investigations are required for this technique. Secondary sludge is made up of microbial cells from secondary treatment in WWTP. Therefore, it is hydrolysis limited in anaerobic digestion due to the low degradability characteristics. To increase the hydrolysis rate of secondary sludge anaerobic digestion, two pre-treatments were examined and compared. Thermal pre-treatment increased the methane production of secondary sludge by 50% at the highest temperature (150°C), and 80°C was not found effective when treatment time is short (1 hour). However, the energy consumption of thermal pre-treatment was too high. Energy consumption was higher than the energy gained from anaerobic digestion. Hence, better heating technology is needed. Electrolysis is an emerging technology for secondary sludge pre-treatment. It is an adaptation from the theory of water electrolysis. During electrolysis treatment, the pH of sludge changed due to the redox activity occurring at anode and cathode. The high and low pH in the anode and cathode chambers disrupted microbial cells in the sludge, and increased methane production by 30%. Energy consumption of electrolysis was lower than the energy gained from anaerobic digestion, thus it is more energy favourable compared to thermal pre-treatment in this study.

Enhancement of methane production from anaerobic digestion of wastewater treatment sludge

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