The removal of n-nitrosodimethylamine formation potential in drinking water treatment plants

A long-term (>18 months), systematic investigation was conducted to examine (1) the removal of N-nitrosodimethylamine (NDMA) formation potential (FP) by different treatment processes under various operational conditions at nine water treatment plants in South Carolina, and (2) the occurrence of NDMA in their distributions systems. Average NDMA FP removal by alum clarification ranged from 12 to 30% during different seasons and weather conditions. Powdered activated carbon addition improved the removal of NDMA FP, especially at high doses (>4 mg/L). The use of oxidants (i.e., chlorine [Cl₂] and chlorine dioxide [ClO₂]), especially simultaneous application, enhanced the removal of NDMA FP and lowered the NDMA concentration in the distribution systems to <10 ng/L. However, simultaneous application of ClO₂ and Cl₂ led to the formation of elevated levels of chlorate. The average NDMA FP reduction by reverse osmosis and microfiltration was 81 and 7%, respectively. The overall NDMA FP removal efficiencies between raw and finished water ranged between 40 and 59%.