Context. Changes in drinking water treatment can affect the biofilm in the drinking water distribution system (DWDS) and enhanced monitoring is recommended to guarantee drinking water safety after substantial process modifications. Ultrafiltration was introduced in 2016 to the drinking water treatment plant Kvarnagården in Varberg, Sweden, achieving total removal of bacteria cells. This improved treatment led to the decision to remove the disinfectant monochloramine in 2020. 

Gap. Several studies compare the bacteria in different DWDS with and without disinfectants. To minimize complicating variables, this study examined the effects of the removal of the disinfectant in a single full scale DWDS.

Aim. This study describes flow cytometry (FCM) monitoring initiated to follow bacteria in the water of the DWDS during removal of monochloramine. 

Methods. Water samples from 15 points in the DWDS were analysed by standard FCM, with data analysis using custom scripts in R for data analysis.

Findings. By comparing FCM data to baselines, distinct changes in the bacterial community were observed following removal of monochloramine. Percentages of high nucleic acid bacteria (HNA) decreased at all sampling points, from 55 – 90 % to 45 – 60 %. Total cell counts (TCC) increased only in parts of the DWDS that had been exposed to monochloramine before the removal. Principal component analysis visualized the changes (Figure 1). The bacterial community at DWDS locations that used to differ largely regarding TCC and %HNA started to become more homogenous, emphasizing the value of FCM monitoring, as these same changes were not observed with traditional heterotrophic plate counts. The bacterial communities stabilized in summer 2020 and began to follow previously observed seasonal trends.

Utilization. This study demonstrated the application of FCM monitoring of the impact of monochloramine removal on bacterial water quality, and the importance of having robust baselines for multiple locations within a DWDS for comparison.