The viability of a variable salinity water desalination (VSD) treatment plant to harvest water from estuarine catchments in Singapore has been explored. The plant is able to treat both low-salinity river water and high-salinity seawater; the treated water is of high grade and energy consumption is half that of seawater desalination plants.
Singapore, a small island city-state of some 700 square kilometres and a population of 4.5 million, needs to find new water sources. The mainstay of raw water supply for Singapore is the water catchments, which will be expanded to occupy approximately two-thirds of the island in 2011. The remaining part of the island consists of small catchments around the fringes which experience bouts of heavy rain that reduces to a trickle or dries up during the dry season. Rain falling in the fringe catchments is currently not harvested and flows into estuaries and sea.
Feasibility studies showed that using conventional treatment concepts to harvest water from the fringe estuarine catchments is not viable. Small reservoirs were only able to provide small reliable yields and support very small water treatment plants. The low storage capacities resulted in high loss of storm water. On the other hand, large capacity water treatment plants suffered from low plant utilisation; only about 20% of the days in the year have heavy rainfall which usually lasts for a few hours. In bother cases, unit production cost was high and exceeded that of desalination plants.
The variable salinity water desalination (VSD) concept described by Seah et al. in the Journal of Water Supply: Research and Technology allows medium-sized water treatment plants to be built and operated at high plant utilisation. Adjustable weirs can be constructed across the mouths of canals and rivers to maximise freshwater harvesting. In the absence of fresh water, the plant will switch its operation mode to treat seawater. The VSD concept provides flexibility in terms of the source of feedwater that can be treated, which greatly enhances plant utilisation.
Research began with a pilot-scale VSD plant of 240 cubic metres per day of seawater in 2004 to test the feasibility of the concept, which was the first of its kind. The pilot plant incorporated microfiltration for pre-treatment before reverse osmosis (RO) membranes for the final product, which met WHO and USEPA Drinking Water Guidelines and Standards. The pilot testing was successful with reduced chemical usage for pH correction, anti-scaling in RO and bio-fouling. This led to a medium-sized demonstration plant of projected production capacity of 4,500 cubic metres per day (seawater desalination mode) and 9,000 cubic metres per day (brackish water desalination mode) located at Pasir Ris, along Tampines River.
The VSD plant treated brackish water with total dissolved salts (TDS) of 30-250 ppm and seawater with TDS of 30,000-35,000 ppm. In both modes, the plant was able to produce potable water that met the WHO and USEPA Drinking Water Guidelines and Standards. The overall production cost of the VSD plant is about half that of a seawater desalination plant and power consumption is less than half of that required for seawater desalination. These results were achieved between January and December 2008, when the plant was treating brackish water 60% of the time and seawater the remaining 40% of the time.
The authors describe how the ability to treat both seawater and brackish water has reduced the unit energy consumption to a much lower level than that of a typical seawater desalination plant. With energy consumption being the major cost component of a desalination plant, lower energy consumption means that the production costs are reduced tremendously. The VSD plant is therefore an attractive and viable option in terms of production costs.
