Extended thermophilic digestion enhances hydrolysis and methane production in subsequent second stage mesophilic digestion
Chua, Xin Yi
Date of Issue2017-12-18
School of Civil and Environmental Engineering
Two-stage anaerobic digestions have been established to improve overall performance of sludge treatment. Temperature phase anaerobic digestion (TPAD) was often used as the first stage as pre-treatment of sludge at thermophilic temperature for short SRT followed by mesophilic temperature at longer SRT. Temperature and solids retention time are important parameters in anaerobic treatment. Higher temperature accelerates most biological processes while longer SRT provides better treatment efficiency. The hypothesis in this study was given as a longer first stage thermophilic anaerobic digestion with subsequent mesophilic anaerobic digestion can improve overall methane production and volatile matter reduction. This is because a prolonged thermophilic anaerobic digestion enhances hydrolysis rate which is often the rate limiting step in anaerobic digestion. Two phases of experiments were conducted to test the hypothesis. Four laboratory-scale reactors were operated for 7 months. Two of the reactors were operated in thermophilic temperature at 47°C while the other two reactors were operated at mesophilic temperature of 35°C. There were no replicates due to the constraint of the number of reactors available during phase of study. In Phase 1, there were two combined system (thermophilic-mesophilic) with an overall SRT of 20 days. First set up was called T2M18 and it consisted a thermophilic reactor (2days SRT) followed by mesophilic reactors (18days SRT) while the second set up T5M15, consisted of a 5 days SRT thermophilic reactors followed by 15 days SRT mesophilic reactor. In phase 2, there is a switch of thermophilic reactors at different SRT between the two set ups. The results obtained in this study showed that T5M15 produced 43% more methane than T2M18, suggesting that a longer thermophilic anaerobic digestion enhances overall methane production. However, T2M18 achieved higher volatile matter reduction of 28% than T5M15 which disagreed to what was hypothesize. In addition, comparison between the mesophilic digesters between the two phases did not support the hypothesis. Instead, these results suggested that a shorter thermophilic anaerobic digester is more preferred as a pre-treatment mechanism.
Final Year Project (FYP)
Nanyang Technological University