Dechlorination Chemical Options and Considerations

Dechlorination of wastewater effluent discharges to surface waters is a requirement in California and many other locations. Sulfur dioxide, sodium bisulfite, calcium or sodium thiosulfate or ascorbic acid has been utilized with comparative advantages and disadvantages.Calcium thiosulfate has found increasing used as an alternate dechlorination agent for wastewater treatment plants. Calcium thiosulfate when used as a dechlorination agent typically results in less cyanogen chloride formation than sodium bisulfite dechlorination, this is especially important for wastewater plant effluents that exceed California Toxics Rule of 5.2 μg/l for total cyanide. Other benefits of calcium thiosulfate are it is non-hazardous and does not crystallize at low temperatures.This paper will review the benefits and disadvantages of dechlorination with calcium thiosulfate and sodium bisulfite. The mechanisms in the reduction in cyanide formation will be reviewed and design requirements for adequate chemical mixing, contact time and other water quality effects will be presented. The finding from a case study using sodium bisulfite and calcium thiosulfate to disinfect a tertiary effluent will be presented.