Cyanide Interferences in Effluent and How to Avoid It - Case Studies

Presentation Description: Cyanide is a priority pollutant that all municipal wastewater treatment plants (WWTPs) must monitor if they discharge to surface water. Detections of cyanide in effluents from several WWTPs have resulted in these facilities being issued special study requirements or effluent limitations in their National Pollutant Discharge Elimination System (NPDES) permits. Despite widespread environmental compliance monitoring, total cyanide analyses are subject to several significant and problematic matrix interferences that, if not properly resolved during the time of sampling and/or analysis, can result in positive and/or negative bias in the reported result. Sources of interference can include chlorine, thiocyanate, thiosulfate, nitrate+nitrite, aldehydes, and sulfide when total cyanide is analyzed by the acid distillation method (e.g., SM 4500 and EPA 335). The standard sodium hydroxide preservative is also a source of interference. Regardless of the specific mechanisms at play, the potential for introduction of bias and error in cyanide analyses of treated wastewater samples by acid distillation is highly significant, and the accuracy of any cyanide analysis result should be considered carefully. This presentation will discuss recent cyanide studies conducted at several WWTPs in California to evaluate false positive cyanide results. The types of interferences and study results differed among facilities, meaning there was not a one-size-fits-all solution. Nonetheless, we determined sampling protocol modifications to minimize the potential for false positives at each WWTP. The sampling method modifications were consistent with requirements in 40 CFR part 136 and based on studies conducted according to ASTM guidance. Studies were conducted in coordination with the applicable California Regional Water Quality Control Board and we determined that effluent limitations and additional routine monitoring for cyanide were not required in the NPDES permits.