The fate of phosphate (and chlorine) in drinking water distribution systems

Chemicals are added to treated drinking water to protect water quality during long residence times within ageing drinking water distribution systems (DWDS). The impacts and benefits of this, phosphate dosing to mitigate lead exposure risks in particular, in operational systems, are not fully understood, and hence dosing practices may not be optimal. Regulatory sampling data from the water treatment works (WTWs) outlet and at customers' taps from six contrasting systems for 6 years were analysed (2014–2020). Results showed the expected variation in chlorine decay throughout the DWDS as a function of the source water, network infrastructure and residence time. Phosphate concentrations had no measurable change detected from the WTW to the customer tap, irrespective of the seasonal dose. As phosphate concentrations at the WTW increased seasonally, at times doubling, the phosphate concentrations at the tap followed suit, questioning the effectiveness of this increased seasonal dosing. This is significant given the increasing costs, decreasing availability and wider environmental concerns around phosphate.