Integrated anaerobic and aerobic treatment of sewage

This thesis describes results of investigations dealing with sequential concept of anaerobic-aerobic treatment of municipal wastewater. The main purposes of the study were 1) to develop a proper anaerobic hydrolytic pretreatment unit, consisting of a Hydrolysis Upflow Sludge Bed (HUSB-) reactor and 2) to combine this system with proper aerobic post treatment processes, such as the activated sludge process or a stabilization pond system and with a combined anaerobic-aerobic process consisting of the Expanded Granular Sludge Bed (EGSB-) reactor and an upflow micro- aerophilic post-treatment process for complete sewage treatment. The newly develop HUSB reactor serves for removing SS and accomplishing a certain sludge stabilization and raising the biodegradability of the remaining COD. The HUSB-system is operated at the similar retention time as the primary sedimentation tank, viz. HRT=2.5-3.0 hours. These features of the new system result in 1) release of the troublesome high SSaccumulation problems in the post treatment, such as stabilization ponds and UASB or EGSB systems, 2) a shorter overall retention time and lower energy requirements in the different types of aerobic post treatment processes, 3) an improved applicability for some refractory industrial wastewater treatment and 4) and certain extent of sludge stabilization in the HUSB reactor itself at higher temperature conditions or in a complementary sludge recuperation tank operated in parallel with the HUSB-reactor at low temperature conditions. A new process concept, consisting of a sequential HUSB + the EGSB reactor, combined with sludge recuperation reactor, is presented in this study. The total process provides 71 % COD and 83 % SS removal efficiencies at T>15°C and 51 % COD and 77 % SS-removal at T=12°C conditions. A reasonable extent of sludge stabilization, i.e. over 50% hydrolysis of the removed SS can be obtained in the HUSB reactor at higher ambient temperatures, i.e. exceeding 19°C. The applicable hydraulic retention times are 3 hours and 2 hour for the HUSB reactor and the EGSB reactor respectively and two days for sludge recuperation tank. In the EGSB reactor up to 32 - 60% soluble COD removal efficiency can be achieved and the biogas production amounts to 23-70 NL/m 3 (sewage) at ambient temperature (9-21°C), respectively. By applying a complementary treatment using an micro- aerophilic upflow reactor operated at HRT = 1 hr., an almost complete treatment can be achieved at 13°C conditions. Regarding the shorter hydraulic retention times required in this new concept compared to conventional systems, both for the wastewater treatment and sludge stabilization and its reasonable energy recovery, the new system looks very attractive as an alternative for treatment complex wastewaters like sewage. The conventional aerobic activated sludge process and stabilization ponds both were investigated for post treatment at laboratory scale and pilot scale. The operational problems of these systems, such as occurrence of bulking sludge in the activated sludge process and the rather poor performance of stabilization ponds under cold weather condition were discussed and solutions for these problems are proposed. The experimental results obtained demonstrate the practical feasibility of the hydrolysis - aerobic treatment concept for municipal wastewater at ambient temperature. The final effluent quality is equal or better than that of the conventional activated sludge process, and the Chinese discharge standards can be satisfied satisfactorily.

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