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Wednesday, September 3, 2014

Batch-Like Reverse Osmosis Promises Massive Cost Savings for Water Purification

A reverse osmosis (RO) water recycling plant that started up in Singapore in mid-January employs closed circuit desalination (CCD) technology from Tel Aviv, Israel-based Desalitech Ltd. that reportedly lowers water treatment costs by up to a compelling 60 percent compared with the most advanced RO systems on the market and can shorten return on investment to less than one year. The technology is now available in North America, with Desalitech having established its U.S. headquarters in Massachusetts.

Desalitech’s CCD solutions are said to save on treatment cost by increasing recovery, reducing brine waste and reducing energy and chemical consumption compared to conventional RO processes. These advantages are achieved at competitive capital costs; Desalitech’s products use off-the-shelf components.

Israeli ROCCD plant

A brackish water treatment plant in Israel uses a semi-batch reverse osmosis process in closed circuit setup that reduces energy consumption as well as capital costs. Credit: Desalitech

The batch-like operation mode exposes membranes to varying feed compositions as the system concentrates and dilutes, and provides high cross-flow across short membrane arrays. These factors are said to have the potential to prevent fouling and scaling and improve process reliability by disrupting bio-film formation and scale precipitation, which, in turn, may reduce chemical use and membrane clean-in-place frequencies.

A system’s recovery rate can reportedly be adjusted automatically or altered manually at the control panel without modification of system hardware, providing the means to cope with source water or membrane changes. Over 97 percent recovery has been demonstrated in a single-stage CCD unit operating on a low salinity source. Energy consumption can reportedly be reduced by 35 percent with process units that cost the same or less to build than comparable conventional RO processes.

In the CCD process, a high-pressure pump feeds a closed loop comprised of a single stage of membrane elements and a circulation pump. Permeate is produced at a rate equal to the flow rate of the high-pressure pump. Brine is recirculated in batch-like operation. When a desired recovery percentage is reached, brine is throttled out of the system, displaced by feedwater from the high-pressure pump.

The exchange of brine and feedwater is executed without stopping the high-pressure pump or the production of permeate. The process then returns to closed-circuit (CC) operation with no brine reject. The initial pressure requirement of each CC sequence is just above the osmotic pressure of the feedwater, and the maximum is just above the osmotic pressure of the final brine — the same pressure as the running pressure of a conventional RO stage. The resulting average membrane feed pressure in the CC RO process is much lower than the feed pressure of typical RO systems.

The overall recovery rate in a CC process is a function of how long the brine is recirculated. Therefore, it is not necessary to use multiple stages of six- to eight-element-long membrane arrays to achieve high recovery like it is in conventional RO processes. A high-recovery CC process can be constructed with just one membrane element, for example.

More typically, CC membrane arrays consist of three or four elements to optimally balance performance and cost. Two arrays can be housed in a single center-port pressure vessel fed from both ends, with brine take-off from a center port. This center-port configuration can result in fewer pressure vessels than are required for a comparable conventional RO process, reducing both footprint and capital cost.

ROCCD schematic

CCD performance and reliability have been validated by continuous operation since 2009 in challenging waters, ranging from sea and brackish water desalination to wastewater reuse and ultrapure applications. A seawater RO-CCD process has been applied at a commercial installation at the Kibbutz Reim site in Israel to desalinate brackish ground water for agricultural use. It produces up to 835 cubic meters of permeate per day and has operated continuously with greater than 98 percent availability since February 2009, with the rare downtime being mostly associated with power outages. The feed source has variable salinity, ranging from 5,600 to 9,000 µS/cm. It contains domestic effluents and other challenging constituents, but was operated without media filtration for about two years with minimal clean-in-place operations.

A conventional RO system operating on the same feedwater to the same recovery percentage would require three stages. To balance the flux of the three stages and thereby prevent excessive fouling of the head/lead membrane elements, permeate throttling and/or interstage pressure boosting would be required.

Applications of RO-CCD systems include industrial water treatment (including supply water purification), water reuse and wastewater treatment. CCD systems are also making an impact on municipal water treatment worldwide. Desalitech offers packaged/containerized RO units, and it can also collaborate with regional OEMs to build larger plants.

Packaged RO solutions with capacities of up to 4,400 cubic meters per day (1.1 million gallons per day) of permeate production that include pre- and post-treatment capabilities are offered. Systems are available as capital equipment sales or as pay-by-the-gallon outsourced water treatment solutions (build-own-operate). Desalitech also proposes retrofits of existing plants.

Recovery rate is adjustable between 75 and 98 percent, while rejection of 95 to 99 percent is specified for feed total dissolved solids of  0 to 10,000 mg/L. System operating temperature is 45 deg C (36 to 113 deg F) and maximum pressure is 31 bar (450 psi).


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