Integrated Anaerobic and Aerobic Processes for Treatment of Municipal Wastewater

In this study, integrated anaerobic and aerobic treatment processes for municipal wastewater treatment were investigated for its feasibility to replace the existing conventional activated sludge process. Three types of anaerobic systems − anaerobic sludge blanket, anaerobic sequencing batch reactor and anaerobic filter, as the pretreatment step were studies. High solids and organics removal efficiency was observed during the start-up period at a hydraulic retention time (HRT) of 16 h, as well as during 6 h HRT operation. The UASB was found to require the shortest start-up time (80 days) and it produced the highest amount of methane gas (0.16 L/(gtCOD removed) during start-up and 0.13 L/(g tCOD removed) during 6 h HRT operation). However, it was a challenge to operate the UASB due to sludge floatation problems encountered during the start-up period when the HRT was 16 h. The performance characteristics of conventional activated sludge system and membrane bioreactor were also assessed to evaluate their effectiveness for treating the effluent of the UASB system. Both aerobic systems were able to produce an excellent final effluent with quality meeting that of the secondary effluent discharge standards. However, the membrane bioreactor was noted to be a better option as it offered the advantages such as smaller footprint and higher removal efficiencies.

Aerobic treatment systems such as the conventional activated sludge (CAS) process are widely adopted for treating low strength wastewater (<1000 mg COD/L) like municipal wastewater. CAS process is energy intensive due to the high aeration requirement and it also produces large quantity of sludge (about 0.4 g dry weight/g COD removed) that has to be treated and disposed of. As a result, the cost of operation and maintenance of a CAS system is considerably high.

On the other hand, anaerobic process for domestic wastewater treatment has presented an alternative that is potentially more economical and holistic conceptually (Mergaert et al., 1992), particularly in the sub-tropical and tropical regions where the climate is warm consistently throughout the year. Anaerobic process does not require aeration and it produces biogas and less sludge.

Research has shown that anaerobic systems such as the Upflow Anaerobic Sludge Blanket (UASB) (Behling and Sant’Anna, 1997; Barbosa et al., 1989), the Anaerobic Sequencing Batch Reactor (AnSBR) (Sung and Dague, 1992; Ng, 1989) and the Anaerobic Filter (AF) (Ng and Chin, 1986; Chernicharo and Machado, 1998) can successfully treat high-strength industrial wastewater as well as low-strength synthetic wastewater. However, there have been limited
reports on using anaerobic systems as a pretreatment for municipal wastewater.

The anaerobic process alone would not be able to produce an effluent of a quality that meets typical secondary effluent standards. Post-treatment will therefore be required. However, the size of the aerobic system for the integrated anaerobic and aerobic treatment processes (IAATP) will be greatly reduced because the wastewater has been pretreated by the anaerobic system. Thus, the IAATP is potentially a more cost-effective technology for treating municipal wastewater by reducing the aeration requirement and sludge production while achieving secondary effluent

In view of the above observations, this study was conducted to investigate the feasibility of using an IAATP for treating municipal wastewater. Three types of anaerobic system − UASB, AnSBR and AF were investigated. Comparisons of the anaerobic systems were based on length of startup period, effluent quality, biogas production and ease of operation. Two aerobic systems, a CAS and a membrane bioreactor (MBR), were also used to treat the effluent from the UASB. The aerobic systems will be compared based on their effluent quality.

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