Variable Salinity Desalination Plant Feasible for Treating Inconsistent Rainfall

Desalination plants are usually geared to be very efficient at processing water of uniform salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–>. In Singapore, heavy rains occur often, but usually in short, interrupted bouts. As a result, a plant geared to the low salinity of rainwater would be very inefficient during dry periods. A solution to this problem is variable salinity desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> (VSD) that is able to switch between low salinity rainwater and high salinity seawater. Seah et al. (2010) evaluated a pilot VSD plant on the Tampines River in Singapore and found that it was successful at treating both water feeds. This VSD plant utilized reverse osmosis treatment with microfiltration of the feed water. They found that due to the low salinity of rainwater, the energy consumption of the plant is greatly reduced since the energy needed to process low salinity water is about one-fourth the energy needed for high salinity water, and the overall energy usage of the VSD plant is about half that of conventional desalination plants. In addition, the authors found that fouling of the reverse osmosis membrane could be avoided by switching between high and low salinity feedwater every three days. —Erin Partlan
Seah, H., Khoo, K. L., Chua, J. Y., Toh, D., Chua, S. C., 2010. Cost effective way to harvest estuarine water: variable salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> concept. Journal of Water Supply: Research and Technology 59, 452–458.

This plant is located in an estuarine section of the Tampines River. Low salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> water is obtained from the river, largely supplied by rainwater, and high salinity water is obtained from the ocean. In particular, this plant uses reverse osmosis desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> technology prefaced by screening and microfiltration and followed by disinfection. The product of the plant is added to existing high-grade reclaimed water. Technologically, in order to process both high and low salinity, the plant reroutes water through similar infrastructure when the feedwater is switched. Though the plant was designed for 50:50 division between low and high salinity processing, in 2008, this plant operated at low salinity 60% of the time and at high salinity 40% of the time. Since low salinity desalination requires significantly less energy than high salinity (seawater) desalination, the energy requirements of the VSD plant were greatly reduced. Annual energy usage was 1.11 kWh/m3 for low salinity desalination and 4.86 kWh m–3 for high salinity desalination, averaging 1.86 kWh m–3.
Seah et al. discuss current and future improvements of the VSD plant over conventional desalination<!–[if supportFields]>XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> plants. First, they discuss a rubber weir that is used for flood control of the Tampines River. They postulate that by refining control of the weir, the flow rate of water to the desalination plant can be optimized. Secondly, they note that this VSD plant successfully uses microfiltration in place of multi-media filters and dissolved air flotation. They also found that the conventional RO filter cartridge was not needed as the microfiltration outflow was sufficiently filtered, and will continue to monitor the effect of removing the cartridge. Thirdly, this VSD plant is designed to avoid scaling of precipitating solutes on the RO membrane, thus avoiding the need for pH adjustment or anti-scalants. Fourthly, this VSD plant utilizes testing for membrane and microfiltration performance. Total organic content of the permeate, or the filtered side of the membrane, is used as a continuous measure of the membrane performance, thus notifying operators of either ageing membranes or other damage. A pressure decay test is used in addition to conventional sampling tests for microfiltration performance. Fifthly, the authors found that by switching between low and high salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> feedwater, the chance of membrane fouling, a large problem in conventional plants, was greatly reduced. They found that the optimal time to spend in each mode is three days.

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