Every raindrop counts
PUB ramps up world-first technology that will make 90% of the island a catchment area
Woo Sian Boon Today Online 4 Jun 12;
SINGAPORE - For a country that brought to the world the technology to recycle sewage into high-grade reclaimed potable water, the Republic is now making strides in another world-first: Variable salinity technology, which would turn almost the entire island into a water catchment area, as national water agency PUB strives to make use of every drop of rainwater.
TODAY has learnt that, following a successful pilot, PUB plans to ramp up the technology, having recently identified eight potential sites across the island, including one at Jurong, at which to build new plants.
PUB had revealed that it was looking at using more of the technology - without giving details - in a letter to this newspaper last month, in a response to a reader's suggestion to harness water in the streams and rivulets near the shoreline.
Currently, two-thirds of Singapore's land surface is a water catchment area. But, according to PUB, a considerable amount of rainwater still goes to waste at smaller rivulets situated at the island's outskirts. With the new technology - which has the dual function of generating potable water from brackish water and sea water - about 90 per cent of the island will become a water catchment area.
During wet spells or periods of heavy rainfall, an inflatable rubber weir built into a canal retains rainwater that is otherwise drained into the sea.
The water from the canal is transferred to a variable salinity plant. During the purification process, the water is filtered to remove particles of less than 100 micrometres in size. The water is then desalted when it is run through reverse osmosis membranes.
During dry spells, however, the variable salinity plant switches to "seawater" mode, and seawater is pumped into the plant from a pipe spanning 190m offshore.
The seawater stream is passed through seawater reverse osmosis membranes and then through the brackish water reverse osmosis membranes to obtain salt-free water.
PUB spokesperson Sarah Hiong told TODAY: "The variable salinity plant is the first of its kind in the world, and is designed to produce drinking water from seawater and brackish water at an affordable cost."
With some parts of the island seeing ony minimal rain for up to four-fifths of the year, it would not have been cost effective to build conventional systems for small catchments such as streams and rivulets near the shorelines.
The PUB launched a prototype variable salinity plant at the Bedok NEWater facility in 2004. Bouyed by its success, a S$7.4-million demonstration-scale plant was constructed in 2007 at Sungei Tampines. And for the past five years, the plant has been working around the clock, processing up to 2 million gallons (9.1 million litres) of brackish rainwater per day - or about 0.5 per cent of Singapore's daily water consumption, which stands at 380 million gallons.
Like NEWater, the water processed by this latest technology is safe for drinking, meeting or surpassing drinking water quality guidelines and standards by the World Health Organization and the United States Environmental Protection Agency.
But as the yield is small, all product water is currently used to augment the supply of NEWater used to meet industrial needs.
Ms Hiong noted that 55 per cent of Singapore's total water demand comes from the industrial sector. "NEWater is primarily supplied to industries, and (the new technology) frees up potable water from catchment areas for household consumption," she said.
Four of the potential sites identified for new variable salinity plants are in the eastern part of the island, and two each in the western and north-western parts. One of the sites -to be situated in Jurong - will ultimately be capable of processing 1.3 million gallons of seawater or 2.6 million gallons of brackish water per day.
As the new technology make inroads here, members of the public have more reasons not to litter.
"Rubbish thrown on the ground can be washed into drains when it rains, and end up in a canal, which is collecting rainwater for our water supply," Ms Hiong pointed out.
