Solving nitrate contamination.
Nitrate is a real curse for a municipal water supply and a foul word to the people responsible for delivering safe drinking water. It is fairly common to find nitrates in drinking water supplies that exceed the 10 mn/L (as nitrate-nitrogen) Maximum Contaminant Level (MCL). Found in both groundwater and surface water supplies, nitrate contamination is a result of natural geologic formations or the activities of man. Major contributors to nitrates in groundwater include fertilizers, industrial waste chemicals, seepage from septic systems and animal manure where there are high populations of livestock. Nitrate contamination can affect any size community, from small farming towns to large metropolitan areas.
More than 50 percent of the U.S. population obtains its potable water supply from groundwater supplied by 48,000 community water systems and approximately 12 million individual wells.
Thirty-four of the nation’s 100 largest cities rely completely or partially on groundwater. A community with a nitrate problem can consider the following three alternatives as a remedy (in order of preference):
* develop a new water supply;
* construct a blending facility; or
* treat the existing supply.
Treating the water to below the maximum contaminant level (MCL) can be the least costly in some cases and may be the only alternative available. The process currently most used for nitrate removal in the United States is the ion exchange process; specifically – anion exchange.
This process has been demonstrated and proven. Ion exchange has been successfully employed for removing nitrate from drinking water in the United States in a number of places. Nitrate is an anion (negatively charged [electrically] ion). Accordingly, an anion/ion exchange process can be used to remove nitrate. In ion exchange, the nitrate ion is exchanged for another more desirable (or less undesirable) anion such as chloride.
On the surface, the ion exchange process for nitrate removal appears quite simple. The nitrate-laden water is passed through a bed of resin. The resin consists of a matrix to which “functional groups” are attached. The functional groups are “exchange sites,” which, in the case of nitrate removal, are usually loaded with chloride ions. As the nitrate laden water passes through the resin bed, the chloride ions are desorbed from the resin, and the nitrate ions are absorbed on to the exchange sites. Eventually, the resin is “exhausted;” that is, most or all of the exchange sites are loaded with nitrate ions.
The resin must then be “regenerated” with salt water (brine). During this regeneration process, the nitrate on the resin is replaced with chloride ions, and the nitrate laden spent brine is discharged to waste and the resin bed rinsed and placed into service again.
A common misconception is that nitrate removal by ion exchange is like softening by ion exchange. A “softening” design will remove nitrate, but the result is a process that uses substantially more salt and produces considerably more wastewater than does an ion exchange nitrate removal process. This has been proven many times, but the misconception is still wide spread.
Disposing of the wastewater is a critical concern with the nitrate removal process. The process does not destroy the nitrate, so it ends up in a more concentrated form in the waste streams. The ion exchange process produces a brine. Disposal problems can arise from the quantity of salts in the waste streams. The brine includes a considerable amount of salt (sodium and chloride ions) as well as the nitrate (and sulfate) removed by the process.
Sometimes it is possible to send the brine concentrate to a conventional wastewater treatment plant for further treatment and disposal.
This article was written by Thomas Brown, Senior Civil Engineer, Boyle Engineering Corp., Bakersfield, Calif.