Water. Desalination + reuse
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RESEARCH November-December 2015 | Desalination & Water Reuse | 25 | circuit batch desalination at recovery ratio of 0.8. The system will, according to Davies, outstrip all options especially at high recovery. Davies' batch system operates in three stages: • pressurization of the feed water for reverse osmosis; • purging the system of reject brine; and •refi lling the pressure vessel with raw feed water. During the fi rst stage the pressure is gradually increased to overcome the increase in osmotic pressure as salinity rises. The pressure is applied by a piston which also presents a barrier against the unwanted mixing described above. Once a predetermined amount of permeate has been produced, the reject brine in the piston cylinder is replaced by new feed water. Rather than driving the piston with a shaft it could be more convenient to use hydraulic pressure exerted by a conventional pump. Davies' prototype deploys the pump method using feed water to push against the underside of the piston. On the return stroke of the piston, the displaced hydraulic feed water is then used to refi ll the RO side of the cylinder for the start of the cycle. To achieve maximum effi ciency, no feed water is added to the pressure circuit during the pressurization phase. A positive displacement pump powers the system and it could be driven by photovoltaics or by a conventional energy source. "Due to possible uneven energy usage, an energy storage device like a small battery or super-capacitor may be used in future designs to buffer the power supply from the PV panel," says Davies. PRESSING AHEAD The batch RO system was built largely by adapting readily available components to create a system in which energy recovery was intrinsic. It was achieved primarily by eliminating the continuous replenishment of the feed side with new source water deployed in closed loop systems. With the current set up he anticipates recovery ratios of up to 80%. Following testing with a solar array simulator Davies says the system is suitable for use with PV panels. Further research and development will include operating the system with brackish water solutions with a wider range of compositions. And the valves will be operated automatically rather than manually. "We will monitor water output and energy usage. Following monitoring of performance, the system will be improved and optimized. "We hope to deploy the system in places of need where local artisans can carry out the construction to open-access plans published online, using locally sourced materials where possible." As the United Nations' Millennium Development Goals complete their term this year, Sustainable Development Goals will come into play. Davies supports the view that the objectives of sustainable development should include economic prosperity and social inclusion for disadvantaged people. "By boosting water supplies for drinking, cultivation of crops, and making water accessible to more people, desalination can contribute to the accomplishment of Sustainable Development Goals," he says. General assembly The team at Aston aimed to create a desalination system that could be assembled in the villages and towns in areas – often in developing countries – struggling with water scarcity. "We hope to deploy the system in places of need where local artisans can carry out the construction to open-access plans published on-line, using locally sourced materials," said the team's report. Davies' system consists of a pressurized batch of brackish water passed through a semi-permeable membrane. A piston increases gradually the pressure in the fl uid to overcome the rise in osmotic pressure in the feed water as its salinity increases with permeate production. Once a predetermined amount of permeate has been produced, the reject brine remaining in the cylinder is discarded and replaced by new feed water. Nuts and bolts Davies' co-workers aimed to construct their desalinator largely from readily available materials. They formed the pressure exchange vessel from a four-inch diameter reverse osmosis pressure cylinder that housed a high-density polyethylene piston that isolated the pre-RO feed water from the feed water being used to drive the process. The researchers designed and made three pistons of lengths 0.26 m, 0.62 m and 0.69 m to test the system at different recovery ratios. The test rig was primed and operated manually using a simple electronics system and a collection of manually actuated ball valves. Powered valves may replace these. A small, central heating pump served two purposes: • to recirculate the concentrate leaving the reverse osmosis module during pressurization, and return it to the pressure exchange vessel and • to refi ll the pressure exchange vessel at the end of the pressurization cycle. The system has operated at a SEC of less than double the ideal minimum – outperforming a single stage conventional RO system with an energy recovery device (ERD) at 70% recovery. Basic Layout Water Filter Bore Pump Flow meter Pressure Gauge Concentrate Drain Pressure Gauge / Flow Meter Recirculation Pump Air Release Valve RO Module Non- return valve Non-return valve Pressure vessel Ball valve Ball valve Ball valve Piston Solar simulator Basic layout Batch system outperforms conventional RO at high recovery. Results summary (5000 ppm feed) Max pressure 1400 kPa 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Ideal min Our system Ideal 1 stage+ERD SEC (kWh/m3) 70% recovery 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Ideal min Our system Ideal 1 stage+ERD SEC (kWh/m3) 51% recovery Max pressure 1440 kPa (measured) (measured) The Aston team's system cycles through pressurize, purge, refi ll and back to pressurize.