AWWA WQTC60762

When inland communities undertake large scale water desalting projects, disposal of thebrine concentrate can present a major problem. In the southwestern United States, thepresence of silica in the groundwater exacerbates the brine concentrate problem becauseit limits the extent to which water can be extracted from the brackish water supply, resultingin the generation of large volumes of waste concentrate. This project investigates ways to reduce the volume of silica-saturated reverse osmosis rejectconcentrate.Two processes were studied in this investigation: nanofiltration of the concentrate(for removing hardness) followed by reverse osmosis treatment of the nano permeate; and,lime-treatment of the concentrate (for removing silica) followed by reverse osmosisof the lime-treated water.The results showed that the membrane processes could reduce the concentrate volume byup to 55%, but at recoveries greater than this, membrane fouling was observed. This isprobably because the nanofiltration pretreatment step did not remove all of the hardnessfrom the concentrate, resulting in silica precipitation in the subsequent RO process whenthe silica concentration in its reject stream reached about 300 mg/L. Lime treatment ofthe concentrate was shown to be very effective for removing silica, with the overallprocess described as a first-order reaction. However, no silica removal occurs until thelime dosage exceeds the lime-equivalent of the alkalinity.In this study, lime treatment was shown to be more cost-effective than the best "throwaway"option of deep-well injection. At a lime dosage of 750 mg/L, there would be a netsavings of $1.6 million per year compared to injection, and 5 MGD (0.218 m³/s) ofproject water would be produced at a cost of $1.43 per thousand gallons ($0.375/m³). Includes 3 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

18

File Size:

1 file , 680 KB