Impact of zinc on biologically mediated monochloramine decay in waters from a field based pilot scale drinking water distribution system

Abstract Zinc is a heavy metal that is essential for human health at trace levels. Zinc also has antibacterial properties and these were investigated on microbiological mediated monochloramine decay (measured as a microbial decay factor, Fm) occurring in chloraminated drinking water. Water samples displaying rapid chloramine decay were obtained using a field based novel design pilot-plant distribution system (PDS), located at the Tailem Bend water treatment plant, South Australia. The PDS comprised 1 kL tanks each holding 900 m of polyethylene tubing with sampling points at 300 m intervals. Flow rates were applied to achieve hydraulic retention times (HRTs) that simulated those of the full-scale distribution system. Total (k t ) and microbiological mediated (k m ) decay rates were determined for chloraminated waters as these passed through the PDS. Highest Fm values (2.1 ± 0.7) were found for waters collected at the outflow of the PDS pipework, demonstrating increase in microbiological mediated monochloramine decay with increased HRT. Zinc (0.6–60 mg/L) added to water samples was found to lower k t , k m and consequently Fm, as its concentration was increased. Mathematical models (R 2  > 0.9 and T-test value > 0.8) were developed that describe reductions in monochloramine decay rates with the zinc addition. Reduction in k t and Fm in response to zinc were found to have an inverse correlation with initial microbial activity in the chloraminated waters. Zinc concentration below drinking water aesthetic limits (