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Balancing Carbon for Energy Recovery and Nutrient Removal for the World's Largest MBR Facility


Abstract number: 1647

Primary Author
Emma Shen, Jacobs Engineering Group

Co-Author
Tim Constantine, Jacobs
Colin Newbery, Jacobs Engineering
Wei Hin Yong, Public Utilities Board Singapore
Koh Siong Teck, Public Utilities Board Singapore
Loh Yee Wen, Public Utilities Board Singapore
Wee Siang Liow, Public Utilities Board Singapore

Speaker
Emma Shen, Jacobs Engineering Group

Purpose:
Introduction The Tuas Water Reclamation Plant (TWRP), which is a key component of the Singapore Deep Tunnel Sewerage System Phase 2 project, will be one of the most revolutionary facilities of its kind in the world. Located in western Singapore, the TWRP will receive domestic used water from the existing Ulu Pandan WRP and Jurong WRP, as well as high-strength industrial used water, conveyed through a series of link sewers and deep tunnels. Domestic used water will be treated in a 650 ML/d average flow module and then further purified to produce NEWater, while industrial used water will be treated in a separate 150 ML/d average flow module and sent back to industries for reuse. With an initial total treatment capacity of 800 ML/d, the TWRP will be the largest membrane bioreactor (MBR) facility in the world, but with a 30 percent more compact footprint compared to conventional plants. The TWRP will be co-located with an Integrated Waste Management Facility (IWMF), which is implemented by the National Environment Agency. A key feature of the co-located facility will be energy self-sufficiency, where electricity generation at IWMF will exceed the needs of TWRP treatment. The overall TWRP concept is presented in Figure 1. Maximizing energy production and minimizing energy usage from the TWRP depends largely on how carbon is managed through the treatment process. More specifically, maximizing the diversion of carbon in the primary treatment step allows for greater biogas production (energy generation) while reducing the oxygen demand, and therefore blower aeration requirements (energy usage), in the MBR process. However, such diversion needs to be balanced with the carbon needs to drive biological nutrient removal requirements in the MBR. This paper will provide details related to the primary treatment and MBR biological treatment designs to achieve the optimal balance in carbon diversion, and flexibility in these designs to achieve improved performance through the potential harnessing of anammox-based biological treatment. Methods Limiting the carbon load on the mainstream biological process may contribute to deterioration of biological nutrient removal performance, which has traditionally been dependent on using carbon resident in the wastewater to drive heterotrophic denitrification and/or enhanced biological phosphorus removal. In a best-case scenario, an optimal balance is achieved by maximizing carbon redirection to anaerobic digestion while maintaining the required effluent compliance by optimizing the carbon to nitrogen (C:N) ratio of the bioreactor feed while providing flexibility in the downstream bioreactor facility to manage heterotrophic denitrification and potentially harnessing the recently discovered anammox bacteria in a mainstream capacity (Cao et al, 2017). One of the key activities carried out in the design of the TWRP was to evaluate the degree of flexibility required in the design of the overall treatment process to achieve this balance via process modelling. A series of process modelling exercise were completed to facilitate the design of TWRP using BioWin™ and Jacobs' in-house Professional Process Design (Pro2D) model, to answer a number of key questions related to design flexibility to accommodate various potential scenarios. The overall BioWin™ model flow sheet for the TWRP is provided in Figure 2. This paper focuses on design of the domestic liquid module (DLM) and demonstrates the balance between carbon re-direction and effluent nutrient levels achievable. Results and Discussion Several features were incorporated into the design of the TWRP DLM based on process modelling results, with a goal to provide operational flexibility to optimize carbon re-direction and associated energy benefits while achieving effluent nutrient limits for NEWater production and combined outfall discharge. The full paper will provide details on the operational flexibility provided in the design of the DLM A-stage primary treatment and bioreactor facilities. Primary Treatment Design and Flexibility: The design of the A-stage facility includes the provision of controlled biosorption bypass(es) to deliver a minimum C:N ratio of the bioreactor feed, depending on the biological nutrient removal mechanisms. Specifically, if the conventional nitrification/denitrification pathway is utilized, allowing a portion of flow bypassing the biosorption tank can improve the denitrification process (by delivering more carbon into the downstream MBR biological process) and achieve effluent nutrient limits for nitrogen species, while avoiding alkalinity deficiency (as denitrification process recovers some alkalinity consumed for nitrification). Some of the key modelling results are summarized in Table 1. Process modelling suggests that, if the conventional nitrification/denitrification pathway is utilized , biosorption bypass would not be required to achieve the effluent nitrate (< 20 mg-N/L) and alkalinity (> 50 mg/L) targets when the influent C:N ratio is higher than 15:1, while complete biosorption bypass would be required when the influent C:N ratio is lower than 8:1. Biogas production from the anaerobic digestion process could be significantly impacted by diverting wastewater around biosorption; modelling results suggest that 50 percent biosorption bypass may decrease the overall biogas production by approximately 10 percent. Bioreactor Design and Flexibility: Flexibility to adjust the step-feed flow split to be proportional to the biomass in each pass (by sending more flow to the front zones and less to the downstream zones) will be provided, therefore accommodating a range of C:N ratios of the bioreactor feed from the A-stage system. More specifically, if a significant degree of anammox-based treatment can be harnessed, lower C:N ratios can be accommodated while not sacrificing effluent quality. The provision for anammox granule capture from the waste activated sludge (WAS) will also be provided to optimize the degree of anammox-based treatment. Conclusions and Recommendations The TWRP will be one of the most revolutionary facilities of its kind in the world, allowing for production of NEWater and industrial water for reuse from MBR filtrate, and striving to achieve energy self-sufficiency for the overall process. This study focused on the design considerations for the TWRP to provide operational flexibility for carbon balancing between energy recovery and biological nutrient removal. Overall, the process modelling demonstrated that providing the flexibility to partially bypass the biosorption step of A-stage treatment would allow for reliable nutrient removal in the MBR, but at the expense of reduce energy production compared to full A-stage primary treatment. The modelling also demonstrated that maximum biogas production (via full A-stage treatment) would be possible should mainstream short-cut nitrogen removal be achieved at the TWRP. Flexibility in A-stage design, among other processes, has been incorporated into the final TWRP design.

Results:
Cao, Y., van Loosdrecht, M.C.M., and Daigger, G.T. (2017). Mainstream partial nitritation–anammox in municipal wastewater treatment: status, bottlenecks, and further studies. Applied Microbiology and Biotechnology, 101(4), 1365–1383.

Keywords
Carbon (BOD) Removal
Circular Economy, Featuring One Water
Design and Technology
Nutrient Removal
Primary Treatment
Treatment: Preliminary and Primary (Does not include handling and disposal of grit, screenings, and solids- see Residuals)