Investigation of the formation of NDMA _UFC across biofilters

Abstract Full‐scale sampling at 16 facilities illustrated that NDMA precursor concentrations, measured by adding chloramine under Uniform Formation Conditions (NDMA UFC ), increased across biofiltration at 7 of 16 facilities (by 3–48 ng/L or 12%–296%) but stayed the same or decreased (by up to 5 ng/L or 24%) at the other nine facilities. Increases in NDMA UFC concentrations were attributed to both particulate and soluble precursors. Only two facilities had an increase in NDMA UFC greater than 10 ng/L. However, NDMA UFC concentrations in the biofilter effluent from five facilities exceeded 10 ng/L during one or more sampling events. For these facilities, testing at multiple scales showed that mitigation steps could include pretreatment with ozone (which resulted in overall lower NDMA UFC ), increased free chlorine contact time prior to ammonia addition, and/or optimized biofilter design and operation.