Novel A-B processes for energy-efficient municipal wastewater reclamation with minimized sludge production

• Main Author: Gu, Jun
• Other Authors: Liu Yu
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Environmental engineering::Water treatment
• Online Access: http://hdl.handle.net/10356/74147
• Institution: Nanyang Technological University

The conventional activated sludge (CAS) process has been widely employed for wastewater treatment for more than one hundred years. Recently, more and more concerns have been raised on the CAS process due to its high energy consumption and production of huge amount of waste activated sludge, which are inevitably linked to the issue of environmental sustainability and global climate change. Therefore, this study aimed to develop novel A-B processes towards energy-efficient municipal wastewater treatment with minimized sludge production. Anaerobic treatment could directly convert the COD into biogas with much less sludge production compared to aerobic processes. However, the dissolved methane in the anaerobic effluent is a hurdle for its application. Therefore, in the first phase of study, an A-B process in which an anaerobic moving bed biofilm reactor (AMBBR) served a lead A-stage for COD capture towards biogas production and an integrated fixed-biofilm and activated sludge sequencing batch reactor (IFAS-SBR) was employed as B-stage for biological nitrogen removal. Results showed that about 85% of wastewater COD was removed in the AMBBR with a total energy production rate of 0.28 kWh/m3 wastewater treated, while 85% of N-removal was achieved when the stable nitrite shunt was established in the IFAS-SBR. Moreover, 90% of dissolved methane in the AMBBR effluent could be removed by the proposed flash chamber at the lower energy demand of 0.12 kWh/m3 which could be offset by the potential energy harvested from produced methane. Compared to the CAS process, the production of waste sludge was reduced by about 75% due to the efficient COD capture at the A-stage, leading to significant energy savings from aeration for COD oxidation and post-treatment of waste sludge at the B-stage. Consequently, this phase of study offers in-depth insights into A-B process which should be considered as an ideal candidate for achieving the energy-efficient operation of a municipal wastewater treatment plant. With the improvement of A-stage performance, the nitritation-denitritation at B-stage in the first study could not be sustained due to the insufficiency of COD. To cope with this issue, in the second phase of study, a novel A-B process configuration in which a portion of influent was directed to B-stage was developed for sustainable and stable nitrogen removal via nitrite shunt with minimal impacts on energy recovery. The bypass flow was found to significantly shape microbial community structure by enriching ammonia oxidizer and denitrifiers against nitrite oxidizer. As the result, about 78% of total nitrogen was removed via nitritation-denitritation. In addition, the potential energy recovery and sludge reduction were not compromised in the proposed A-B process with the bypass of influent to B-stage. It is expected that this phase of study may offer a feasible engineering solution for concurrently achieving direct energy recovery from wastewater at A-stage, and sustainable nitritation-denitritation for nitrogen removal at B-stage. Anammox has unique metabolic ability to oxidize ammonium to nitrogen gas using nitrite as electron acceptor without the presence of oxygen or organic carbon. The significant energy-saving potential of anammox prompted us to integrate mainstream anammox into A-B process. Therefore, the third phase of study demonstrated the feasibility to integrate mainstream anammox into an A-2B process for municipal wastewater treatment towards energy-efficient operation with reduced sludge production. In the proposed A-2B process, an AFBR served as A-stage for COD capture, an anammox moving bed biofilm reactor (MBBR) was employed as B2-stage, which received effluent containing nitrite from a SBR at B1-stage. The results showed that under the operation conditions studied, 58% of influent COD was converted methane gas at A-stage, and 87% total inorganic nitrogen (TIN) removal was achieved with the effluent TIN concentration of 6.5 mg /L. Moreover, it was shown that at least 75% of sludge reduction was obtained due to the COD capture at A-stage. The high-throughput sequencing analysis further revealed that Candidatus Kuenenia was the dominant genus responsible for the observed anammox at B2-stage MBBR. This study clearly demonstrated a novel process configuration for sustaining mainstream anammox for municipal wastewater reclamation towards energy-efficient operation with minimized sludge production. The feasibility of single-stage mainstream deammonification in an integrated A-B process was investigated in this phase of study. NOB repression strategies developed in preceding phases based on the temporal operating SBR was successfully transferred to the continuous operating step-feed reactor. Results showed that 79% of the influent COD was directly converted to methane in the A-stage AFBR, while nitrogen removal efficiency reached up to 80% via deammonification in the B-stage step-feed mainstream deammonification reactor. Sustainable and stable mainstream deammonification was achieved without bioaugmentation for more than 100 days, which significantly reduced the operation complexity compared with other mainstream processes. In summary, this study innovatively developed the novel integrated A-B processes towards maximized energy recovery and minimized excess sludge production. The A-B processes described in this study may lead to the paradigm shift of the WWTP operation from energy-negative to energy self-sufficient.