TUAS Water Reclamation Plant - A Resource Recovery Factory

EXECUTIVE SUMMARY PUB, Singapore's National Water Agency, is implementing Tuas Water Reclamation Plant (Tuas WRP), which will be one of the most revolutionary facilities of its kind in the world with resource recovery in the form of NEWater, Industrial Water and energy. Two of the key objectives of Tuas WRP are: -Minimise energy usage and maximise energy recovery -Harness synergies from water-energy-waste nexus by integrating used water and solid waste treatment under a single facility: Tuas Nexus This paper will address how the design achieves the above objectives. INTRODUCTION Singapore's Deep Tunnel Sewerage System (DTSS) was conceived in the mid-1990s to serve Singapore's long-term used water needs. The overall concept of the DTSS is to use deep tunnels to intercept the flows in existing gravity sewers upstream of pumping installations and channel the flows by gravity to centralised treatment plants strategically located in coastal areas so that the treated used water can be properly discharged to the sea. DTSS Phase 1, serving the eastern part of Singapore, included the construction of the Changi WRP and this facility was commissioned more than 10 years ago in 2008. A key component of DTSS Phase 2, Tuas WRP will serve the western part of Singapore and receive used water via link sewers and used water tunnels. The overall Tuas WRP concept is shown schematically in Figure 1.1. Tuas WRP will be a revolutionary facility for many reasons, including: -Tuas WRP will be co-located with an incineration facility to form the Tuas Nexus; integrating various used water and solid waste treatment processes as well as non-process synergies to maximise resource recovery (Figure 1.2). -At Tuas WRP, NEWater and Industrial Water will be reclaimed from the used water streams. -The facility will be specifically designed to be energy self-sufficient (Figure 1.3 and Figure 1.4). -The proposed site for the Tuas Nexus is located on a peninsular of reclaimed land in the western end of Singapore, Tuas, with a total land area of only 68 ha, thereby necessitating an extremely compact footprint design. -When completed in 2026, Tuas WRP will be the largest MBR facility in the world. HIGHLIGHTS -Maximise carbon diversion and minimise electricity demand -Maximising energy recovery by employing advanced digestion and co-digestion -Minimise energy consumption by appropriate selection of solids handling technology METHODOLOGY Extensive mass and energy balance assessment has been conducted and many scenarios have been developed and tested to demonstrate the balance between carbon diversion and treated water total nitrogen achievable and to maximise energy recovery by utilising advanced digestion and co-digestion at TWRP. PUB operated a pilot plant at Ulu Pandan WRP which includes an A-stage primary followed by a secondary process and co-digestion of food waste to develop operational data that is utilised to develop a robust design. The operational information developed from operating a food waste digestion facility is utilised to assess the maximum loading rate to the digester, expected volatile solids destruction, biogas production and dewaterability of the sludge. Energy consumption and footprint requirements were the key criteria used to select key process units such as anaerobic digester shape, mixing system, THP pre-treatment, thickening, and dewatering facility etc. OUTCOMES The detailed design of the Tuas WRP includes the following features: -Primary treatment design and flexibility Flexibility provided in the process design for the A-stage primary treatment for Domestic Used Water (DUW), including a bypass of the biosorption tankage of the A-stage system to deliver an optimised C:N ratio in the bioreactor feed. -The advanced digestion process Use of WAS only thermal hydrolysis process (THP) to maximize energy recovery with the flexibility to hydrolyse part of A-Stage primary sludge to mitigate potential mixing limitation in anaerobic digester due to significantly high proportion of unhydrolyzed primary sludge as compared to hydrolysed WAS. -Co-digestion Assessed various scenarios to appreciate the maximum greasy waste and food waste that can be added to the digester while providing the flexibility of feeding each digester with indigenous sludge (Primary sludge + hydrolysed sludge), greasy waste and food waste -Food waste digestion trial Incorporation of output from the food waste digestion trial to develop a robust co-digestion solution. -Digester geometry Silo-shaped (or modified egg) digester geometry over pancake or egg shape digester. This was arrived at after key considerations such as mixing effectiveness, constructability and construction time frame, site layout, scum collection and foaming potential, operation and maintenance requirement, and local experience. -Dewatered cake as fuel source Use of dewatered cake as a feedstock to the IWMF incineration facility to generate steam and electricity. Steam produced is sent to the THP facility. -Highly efficient biogas use The biogas produced will be transferred to IWMF to use in the superheaters downstream of the Municipal Solids Waste (MSW) incineration boilers. This will increase the temperature and pressure of the steam resulting in high gas to electricity efficiencies than without the biogas or if Combined Heat and Power (CHP) units were used. - Higher Energy Recovery A-stage process, WAS only THP system, co-digestion with greasy waste and food waste, selection of appropriate equipment and integration with IWMF contributed towards overall higher energy efficiency. CONCLUSION The Tuas WRP will be one of the most progressive facilities of its kind in the world with resource recovery in the form of two grades of water NEWater and Industrial Water, and energy. The design adopted maximises carbon diversion by using technologies such as the A-stage process for domestic used water and maximise energy recovery by implementing WAS only THP and co-digestion with greasy waste and food waste.