N-nitrosodimethylamine (NDMA) Formation and Mitigation in Potable Reuse Treatment Trains Employing Ozone and Biofiltration

Ozone and chloramines are critically important for achieving stringent public health criteria and operational water quality objectives in potable reuse treatments trains. These disinfectants are also linked to the formation of N-nitrosodimethylamine (NDMA). NDMA has been successfully attenuated in full-scale biofiltration systems, but the associated mechanisms and design criteria are not well understood. In the current study, a pilot-scale ozone-biofiltration system was used to treat membrane bioreactor (MBR) filtrate from a full-scale water reclamation facility. Experiments were designed to assess the role of various operational parameters, including ozone dose and empty bed contact time (EBCT), on NDMA formation and removal. In ozonated biological activated carbon (BAC) and anthracite columns, EBCTs ≥ 10 min achieved >90% NDMA removal, while an EBCT of 2 min achieved only 30-40% NDMA removal. A non-ozonated BAC column achieved a maximum removal of ~45%, even with an EBCT of 20 min. Moreover, the typically non-ozonated BAC column still exhibited inferior performance when fed with pre-oxygenated or pre-ozonated MBR filtrate, thereby suggesting that pre-ozonation selects for a microbial community that is better adapted to NDMA biodegradation. The presence of monooxygenase genes (suspected to be involved in NDMA biodegradation) was also confirmed in the biofiltration columns. When subjected to final chloramination, pre-ozonation (but not biofiltration alone) was effective in transforming NDMA precursors and reducing NDMA formation by up to 96%. Monitoring of trace organic compounds (TOrCs) also highlighted potential concerns related to perfluoroalkyl acids (PFAAs) in potable reuse applications.