The Integrated Wastewater Treatment (IWWT) plant is SUT Sakra Pte Ltd's second industrial biological wastewater treatment plant on Jurong Island in Singapore. The facility was designed to treat an influent at a flow rate of 110 m h -1 with chemical oxygen demand (COD) and 5-day biochemical oxygen demand (BOD 5 ) concentrations at 2878 mg /L and 1870 mg/L respectively. The influent is made up of 7 chemical wastewater streams from several chemical manufacturing plants. The treatment system uses a

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... two-stage biological process, which is a three-phase fluidized bed aerobic reactor (TPFBR), and the second is a conventional activated sludge process. -3 During commissioning, the plant was found to be able to perform up to only 80% of the design capacity of 317kg COD h -1 , provided that the final effluent COD and BOD 5 were less than 100 mg/L and 5Omg/L respectively. The cause for the underperformance was identified as inadequate biofilm attachment to the carriers in the fluidized bed reactors. A pilot study that simulated the two-stage process was conducted on site in TWWT to select a carrier with the best performance of COD and BOD 5 removal efficiencies. Altogether, there were three types of carrier studied; i.e. type Ithe existing carrier made of Polyvinyl Alcohol (PVA) coated with layers of powder activated carbon (PAC); type II -a Bio-carrier Plastic (BCP) made of plastic and type III -Kaldnes carrier made of polyethylene. In order to compare the performances of the three types of carriers, actual industrial wastewater, equivalent to 100% of the IWWT design hydraulic load and 100~110% of organic load, was fed to each pilot plant throughout the study. The total biofilm on each type of carrier and the relevant surface area loadings applied in each three-phase fluidized bed reactors (TPFBRs), in addition to the functions of activated carbon powder (ACP), were investigated in this study. The results clearly indicated that the Kaldnes carriers could maintain the most biofilm i ATTENTION: The Singapore