Arsenic in the Southwest
by TJ Stroebl
Arsenic continues to be a concern throughout the Southwest as many communities have levels just above the maximum contaminant limit of 10 micrograms per liter (µg/L). These cities are looking at Tonka’s co-precipitation filtration process paired with Simul-Wash™ for an efficient, cost-effective way to remove arsenic, while conserving water and reducing operating costs.
Iron is removed from water by oxidation, precipitation and filtration, a process where Tonka has particular expertise and rich history. Arsenic, in its oxidized state, will bind and coprecipitate with iron, which can then be removed through conventional media filtration.
Many plants originally designed for only iron removal may be already removing arsenic, which co-precipitates with existing raw water iron. In the Southwest where iron is not typically found in raw water, iron for arsenic absorption can easily and safely be added in the form of ferric chloride. The process of adding chemicals can be as simple as adding ferric chloride and a trace of chlorine to oxidize the arsenic. PH reduction can increase the efficiency of the process, and we’ve found that in some waters, arsenic removal requires the addition of a polymer or filter aid.
Pilot study proves the process
Recent pilot studies in the Southwest have reduced raw water arsenic levels from 39 µg/L down to 1 µg/L and lower. One fact is certain: every water is different, which is why Tonka recommends conducting a pilot study to optimize the chemical requirements.
Water savings with Simul-Wash™
Nowhere is water conservation more important than in the Southwest, which makes Tonka’s Simul-Wash™ a critical feature in the treatment process in that region. As the iron and arsenic are filtered out in the media bed, periodic backwashing is required. Simul-Wash™ employs a sustained air and water backwash at sub-fluidized water rates which provides optimal cleaning of the filter, while using about half as much water as a conventional backwash. Tonka is currently involved in several arsenic projects in the Southwest, one of which is a 10.8 MGD plant that has a projected savings of 1.2 billion gallons of water over the estimated 30-year plant life.
Best Available Technology (BAT)
It should be noted that Tonka’s iron co-precipitation process is a USEPA Best Available Technology (BAT) for arsenic removal, and Tonka has proven it to be effective and cost efficient across the country. Tonka has a variety of equipment types available that will accommodate this process and meet the specific needs and requirements of any job.
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