Solar evaporation ponds produce salt in Tavira, Portugal.
Click here to enlarge imageDuring desalination, a waste concentrate stream is generated in addition to the desalted product water. Management of this concentrate is often the most challenging aspect of a desalination project. Common options for handling concentrate are direct discharge to surface water, deep well injection or further treatment to achieve zero liquid discharge (ZLD).
At coastal facilities, concentrate is typically discharged to the ocean. This option is not available at inland facilities, however, and the need to protect surface water and groundwater sources may preclude disposal into the environment. The alternative is ZLD, in which the concentrate is further treated to produce desalinated water and essentially dry salts.
The most common ZLD application in operation today is treatment of power plant cooling water using thermal desalination and/or evaporation ponds. Although design innovations over the years have optimized energy efficiency, thermal desalination remains an energy-intensive process due to the thermodynamic properties of water.
In previous research conducted by Black & Veatch for the Southern Nevada Water Authority (SNWA) on a brackish groundwater, it was estimated that the cost for thermal treatment of reverse osmosis (RO) concentrate using a mechanical vapor compression concentrator and a crystallizer was $3 per thousand gallons of total product water. This represented 70% of a total treatment cost that included production wells and RO membranes.
Energy requirements for evaporation ponds are minimal, but the protective liner and monitoring system required for groundwater protection result in construction costs that typically range from $100,000 to $200,000 per acre, excluding land cost. Furthermore, even in an arid climate, large land areas are required. An SNWA study estimated that handling one million gallons per day (mgd) of concentrate would require a 350-acre evaporation pond at a capital cost of $70 million - clearly not a viable solution. Another disadvantage is that water is lost in evaporation ponds. In the SNWA pilot study, the RO recovery was 63%, so 37% of the well production would be lost with the concentrate discharged to an evaporation pond.