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

Abstract

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 (NDMASDS) conditions in preliminary testing and under uniform formation conditions (NDMAUFC) in all subsequent tests. NDMASDS and NDMAUFC 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 NDMAUFC production compared to chloraminated backwash water, especially for the GAC-sand biofilters. In addition, the GAC-sand biofilters produced significantly lower NDMASDS and NDMAUFC concentrations than the anthracite-sand biofilters. The change in NDMASDS 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.

Investigation of the formation of NDMAUFC across biofilters

“…Bench‐scale and pilot‐scale column experiments were also conducted to support the research objectives in a more controlled environment. The methods for the bench‐scale and pilot‐scale column experiments are discussed in detail elsewhere (Evans et al, 2019; Hozalski et al, 2020; Ma et al, 2020; Zhang et al, 2019). The paper focuses on the full‐scale results.…”

“…The Facility C pilot had very low biofilter influent and effluent NDMA UFC concentrations of <7 ng/L regardless of whether the column tested contained GAC or anthracite media or whether backwashing was conducted with chloraminated versus nonchloraminated water. However, at the Facility Q pilot (Hozalski et al, 2020), where higher NDMA UFC concentrations were observed, a greater increase in NDMA UFC was observed across the pilot biofilters with anthracite media compared to GAC media (pairwise t ‐test, p < .05). The lower NDMA UFC following biofiltration with GAC media could have been due to sorption of precursors by the GAC.…”

“…The lower NDMA UFC following biofiltration with GAC media could have been due to sorption of precursors by the GAC. These limited data (Hozalski et al, 2020) suggest that media type should be further considered for facilities observing an increase in the potential to form NDMA across biofiltration.…”

“…The goal of Water Research Foundation project #4669 was to develop a better understanding of the impact of biofiltration on NDMA UFC concentrations and provide guidance to utilities to reduce the impact of biofiltration on downstream formation of NDMA. The results of testing conducted at bench‐scale and pilot‐scale have been previously summarized in Hozalski et al, 2020 and Zhang et al, 2019. Zhang et al (2019) found no correlation between conditions that could promote nitrification and an increase in NDMA UFC across bench‐scale biofilter columns.…”

“…Zhang et al (2019) found no correlation between conditions that could promote nitrification and an increase in NDMA UFC across bench‐scale biofilter columns. Hozalski et al, 2020 demonstrated lower NDMA formation under both simulated distribution system (SDS) and UFC conditions post‐biofiltration by GAC‐sand biofilters compared to anthracite‐sand biofilters. Hozalski et al (2020) also found that filter backwashing with non‐chloraminated water resulted in lower NDMA UFC concentrations compared to using chloraminated backwash water, particularly for the GAC‐sand biofilters.…”