Investigation of the formation of NDMA_UFC across biofilters

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.