The NSERC Chair was part of a US based team of researchers and consultants who collaborated on a Water Research Foundation project (WRF 4966) entitled, “Biological Filtration: NDMA Control or Source of Precursors?” The project has been completed and the final report has been published. WRF is an organization that facilitates research in all areas of drinking water, wastewater, stormwater, and water reuse.
The project focused on the occurrence and removal of nitrosamine precursors, more specifically N-nitrosodimethylamine (NDMA) precursors during biofiltration. Nitrosamines are disinfection byproducts that are formed when precursors react with chloramines in drinking water treatment plants and distribution systems. NDMA is the most commonly detected nitrosamine in drinking water, and since there are potential health risks associated with it, NDMA has a maximum acceptable concentration of 9 ng/L in Ontario. Biofiltration is increasingly practised in North America and NDMA precursor removal or formation during biofiltration has been reported to be highly variable among different locations and also within a location.
The survey confirmed that NDMA precursor occurrence in biofilter influents varied widely among utilities that is NDMA precursors either remained constant, increased or decreased across biofilters. Both dissolved and particulate NDMA precursors were detected, and often the particulate precursors increased, when an increase in NDMA precursors was observed during biofiltration. Overall, no correlations were observed between biofilter operating conditions/design parameters and NDMA precursor formation/removal during biofiltration.
Bench-scale testing conducted at 3 locations established that source water was the most important parameter affecting NDMA precursors in biofiltration. Please see our previous blog post summarizing a paper we published about these bench-scale results. uwaterloo.ca/nserc-chair-water-treatment/blog/post/chair-team-co-authored-manuscript-changes-ndma-precursors
Pilot scale studies focused on a water source that had a substantial increase in NDMA precursors upon biofiltration. Biofilters with GAC media performed somewhat better (i.e. lower NDMA precursor formation) than anthracite media biofilters. Operationally, elimination of chloraminated backwashing and an increased backwash frequency reduced NDMA precursors in the biofilter effluents.
Based on extensive bench and pilot scale tests, it seems that organic compounds (i.e. pre-precursors) in the source water can be transformed during biofiltration to NDMA precursors. However, the nature of these pre-precursors is as of yet unknown.
Overall, it is recommended that utilities engaging in biofiltration conduct a thorough sampling program to understand the potential of their source water and upstream treatment processes to form NDMA pre-precursors and/or precursors. Sampling should be conducted over multiple seasons to account for a range of source water qualities, upstream (ozonation and pH softening) and downstream (disinfection) treatment processes, and operational conditions experienced throughout the year.
Finally, operational and design modifications to minimize NDMA precursor formation may include the use of GAC filter media, non-chloraminated backwash, and increased backwash frequency. It goes without saying that these strategies should be tested for efficacy based on site-specific water quality and operational conditions.
Please contact us at water-chair@uwaterloo.ca if you would like to find out more about this report.
