Impact of Orthophosphate as a Corrosion Inhibitor and Chloramine Disinfectant on Drinking Water Biofilm Communities

A drinking water distribution system (DWDS) must maintain conditions within quality standards which assure the effective and safe transport of finished drinking water from treatment plants to the household tap. Although safe to drink, finished water is not sterile, and may contain hundreds of microorganisms in a single milliliter. These microorganisms are present from the source waters, such as lakes, rivers and aquifers, and have passed through early treatment steps. Final treatment steps, such as the maintenance of disinfectant residuals, are used to further minimize viable cells present and focus on the reduction of harmful organisms. Microbial cells entering the distribution system have the ability to attach to pipe walls, and possibly use what little nutrients are present for growth. Over time, these microbial communities may impact the quality of treated drinking water. As many Canadian DWDSs contain lead service lines, which may slowly solubilize over time, the allowable amount of dissolved lead in treated water has come under restrictive scrutiny, to lower amounts and minimize health concerns. As lead service lines are common, corrosion inhibitors, such as orthophosphate, have been increasingly utilized by municipalities as a means of controlling lead solubility. However, the impact of these corrosion inhibitors on the microorganisms present in a DWDS is not fully understood. This research aims to better understand the impact of orthophosphate, in the presence of a chloramine disinfectant, on bacterial communities in a simulated DWDS. Annular reactors (Ars), fed with treated drinking water, were established in a flow-through design, which allowed for routine microbiological sampling, including an assessment of both genetic and metabolic profiles of suspended and attached microorganisms. During the initial flow-through experiment, ARs dosed with three concentrations of orthophosphate were monitored for microbial profiling over the course of 12 weeks. Viable cell counts for both bulk and coupon-associated microorganisms, along with biofilm reformation potential, increased in the presence of orthophosphate at doses above 1 mg/L. Genetic and metabolic diversity of coupon-associated communities increased in the presence of orthophosphate at concentrations of 2 and 4 mg/L. In the second 12-week experiment, ARs contained a high (3 mg/L) or low (2 mg/L) concentration of chloramine, both in the presence and absence of orthophosphate at 2 mg/L. Bulk phase and coupon-associated samples again exhibited higher viable cell counts and biofilm reformation potential in the presence of orthophosphate, with only minor differences being dependent on chloramine dose. Metabolic activity and diversity was highest for coupon-associated microorganisms exposed to orthophosphate. Genetic diversity was lower for communities exposed to chloramine in the absence of orthophosphate, as well as those exposed to the higher chloramine concentration in the presence of added orthophosphate, indicating the selective pressure associated with this disinfectant residual. An opportunity to assess microorganisms collected at the end of a longterm pipe loop study of different corrosion inhibitors indicated that orthophosphate increases viable cell counts of microorganisms collected from pipe walls, when compared to sodium silicate alternatives. Overall, this research shows that the use of orthophosphate as a corrosion inhibitor at or above 2 mg/L allows for microorganisms in a DWDS to replicate to higher levels and, as a consequence, reform more biomass as biofilm material. These OP-exposed biofilms have shown an increased resistance to the chloramine disinfectant residual and are likely to impact the profile of bulk phase microorganisms in a distribution network.

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