Water. Desalination + reuse
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14 In Site March 2017 Water. desalination + reuse Integrated validation plant the first step The TeSTS • A 1,000 m3/d pilot-scale integrated validation plant to test MBR systems was completed in 2014 • Testing at the pilot plant is ongoing • A bigger, 12,500 m3/d demonstration-scale plant was awarded to Mitsubishi Corporation in 2015 • Demonstration-scale plant is due to complete in mid 2017 The OUTCOMeS • PUB is adopting lower solids retention times in its membrane bioreactor systems • Improvements to biosorption process may save costs • Co-digestion of food waste and sludge produces more energy • The results of the pilot and demo plant could influence the choice of technologies used in the future Tuas waste reclamation plant Singapore's national water agency, PUB, completed the construction of a 1,000 m3/d pilot-scale integrated validation plant (IVP) to test MBR systems for water reuse in 2014. "PUB gained good experience of operating the plant, and through this we have learned to operate the MBR system more efficiently and reliably," says Harry Seah, PUB chief technology officer. While testing in the pilot plant continues, PUB is now also maximising the operation with a 12,500 m3/d demonstration- scale plant. Specifically, the demonstration plant will be used for testing the technologies that could be implemented at the future Tuas WRP. "These technologies are aimed at enhancing cost savings and energy efficiency of the used water treatment process at Tuas WRP," Seah says. The tender for the demonstration plant was awarded to Mitsubishi Corporation, which in turn contracted Mitsubishi Heavy Industries Asia Pacific to build the plant. Work began in August 2015, and the expected completion date is mid 2017. One significant outcome of the pilot testing is that Singapore is adopting a lower solids retention time (SRT) in its MBR systems to reduce Technology testing Singapore ratchets up developments in water reuse • Ambitious, long-view plans to build resilient water supplies • Flurry of recent activity undertaken to fulfil city-state's aspirations • Culture of pilot and full-scale demonstration plants to test new water treatment technologies Singapore has an ambitious programme to establish sustainable, resilient supplies of water for its citizens and industries over the long term. As part of this plan, there has been a flurry of activity over the past quarter. Keppel has been appointed to build the 30 MGD (137,000 m3/d) Marina East desalination project. Four companies have been shortlisted to build another 137,000 m3/d desalination plant in Jurong Island. CH2M Hill Singapore was selected to work up detailed designs for a mega reuse facility, the Tuas water reclamation plant (WRP), which is expected to have a staggering 800,000 m3/d total capacity. And a new 228,000 m3/d water reuse plant has been commissioned at Changi (see page 17). It's a dizzying rate of development. These steps all form part of the bigger journey Singapore is on to secure its water supply now and for the future. The city-state has been developing a culture of testing and honing water treatment technologies as well as cultivating home-grown talent. The results can be seen in the latest big contract award, to CH2M, to work up detailed designs for a mega water reuse facility utilising MBR technologies that have moved through a pilot stage of testing and are now to be put through their paces in a full- scale demonstration plant. Once built, it is anticipated that Tuas WRP will be the largest MBR-based water reuse facility in the world. the energy required in used water treatment. "Current MBR systems are typically designed based on operations in temperate climates, with long SRTs of 10 days or more to retain a large bacterial population for stable treatment performance," says Seah. "In Singapore, however, the year-round warm tropical climate promotes higher bacterial growth rates. This allows us to operate SRTs as low as five days, lowering the bacterial population while still achieving excellent, stable treatment performance." Operating a lower bacterial population results in greater efficiency when transferring oxygen from the aerators to the bacteria, which translates into lower aeration requirements and energy savings. Another important process that will be piloted at the IVP demonstration plant is the biosorption process, where organics from fresh, raw influent are captured early in the pretreatment process. This high carbon removal is two-pronged: it serves to reduce the aeration requirement downstream and it diverts more organics to the anaerobic digesters for biogas production. The other part of the equation to achieve energy efficiency in used water treatment is to maximise energy recovery from sewage sludge. PUB is piloting and validating different technologies to enhance biogas production from sludge. One such is thermal hydrolysis, whereby sludge is subjected to heat and pressure to make it more biodegradable, improving digestion performance • Water demand in Singapore averages 400 MGD (1.8 million m3/d) • Recycled water currently meets 40 per cent of demand • PUB wants reused water to meet 55 per cent of demand by 2060 • Tuas WRP project aims to build reused water capacity of 800,000 m3/d Reuse to rise as share of demand 60 50 40 30 20 10 2016 2017 Demand share (per cent) continued on page 17