Production of N-Nitrosodimethylamine Precursors by Biofilters Is Highly Dynamic and Affected by Filter Media Type and Backwashing Conditions

Pilot-scale granular activated carbon (GAC)-sand and anthracite-sand biofilters were operated in parallel to study the effect of media type and operating conditions on N -nitrosodimethylamine (NDMA) precursor change across the biofilters. Precursor concentrations were assessed by quantifying NDMA formation under simulated distribution system (NDMA SDS ) conditions in preliminary testing and under uniform formation conditions (NDMA UFC ) in all subsequent tests. NDMA SDS and NDMA UFC concentrations in the biofilter influent and effluents were highly variable and typically increased across the biofilters, by as much as 177 ng/L. Backwashing with nonchloraminated filtrate proved to be beneficial in decreasing NDMA UFC production compared to chloraminated backwash water, especially for the GAC-sand biofilters. In addition, the GAC-sand biofilters produced significantly lower NDMA SDS and NDMA UFC concentrations than the anthracite-sand biofilters. The change in NDMA SDS concentration across the biofilters linearly correlated with ATP concentration in both GAC-sand and anthracite-sand biofilters, suggesting that biological activity is important for promoting precursor increase through biofilters. Finally, the fraction of particle-associated NDMA precursors increased across the biofilters, suggesting that biofilm or biofilm-associated compounds are an important contributor to NDMA precursors in the filtrate. Overall, this study provided novel insights into the effects of filter media type and backwashing conditions on NDMA precursor production across biofilters.