Investigating and Managing Biofilm and Material Accumulation in Chlorine and Chloramine Drinking Water Distribution Systems

Biofilms grow within drinking water distribution systems (DWDS), accumulating organic and inorganic materials at the pipe wall, which if mobilised can adversely impact water quality, threatening public health and regulatory compliance. While disinfection residuals are added to limit planktonic microorganism regrowth during distribution, biofilms are ubiquitous and the impact of disinfection and other management interventions on biofilms remains uncertain. This is particularly true for chloramine with the majority of research being confined to chlorine residuals and/or undertaken in laboratory or bench-top scale reactors that are unrepresentative of real operational drinking water distribution systems (DWDS).

This study determined the impact of different water qualities (including disinfection residuals) and management interventions on biofilms, material accumulation and water quality response when mobilised. For the first time, a combination of fieldwork flushing and pipe loop experiments were performed simultaneously at the extremities of two operational DWDS (one chlorine and one chloramine). Pipe loops were fed by DWDS water and therefore were representative of the real systems, whilst enabling a level of laboratory control. Biofilms were grown for 3 months before undergoing a management intervention. Network fieldwork focused on flushing whereas pipe loops underwent a combination of flushing and chlorine burning. To assess the impacts of/on biofilms, physicochemical water quality, cell counts (flow cytometry), bacterial community composition (DNA analysis) and biofilm chemical composition (XRF analysis) were performed.

Results showed chloramine biofilms contained more cells and presented a greater discolouration risk than chlorine counterparts, thus the perception that chloramine offers superior control in all DWDS does not hold true. The impact of management interventions (both flushing and chlorine burning) is short-lived with biofilm cells and discolouration material returning to previous levels within 3 months. Recovery time is not solely influenced by disinfection residual type, bulk water loading (biological and chemical), also drives biofilm and material accumulation but is complex affected by supply and process rates interactions which may lead to maximum storage capacities in the biofilm. The insights gained from this study will support improved management of both chlorine and chloramine DWDS to preserve safe drinking water quality and compliance from source to tap. Industry should carefully consider the use of chloramine disinfection residuals, and move towards simultaneously managing DWDS operations and treated water quality.