Release of organic matter in a discontinuously chlorinated drinking water network

The effects of discontinuous chlorination on the characteristics of the water in a pilot drinking water distribution network were investigated. The release or consumption of organic matter (as dissolved organic carbon, DOC) following chlorination and non-chlorination periods were estimated, as were changes in bacterial cell production. In each unchlorinated network 0.3 mg DOC l −1 was consumed and the average cell production was approximately 1.3×10 5 cells ml −1. In discontinously chlorinated networks (chlorine treatment: 3.3 mg Cl 2 l −1, chlorine residual: 0.1 mg Cl 2 l −1) the DOC release (DOC out−DOC in) was between 0.1 and 0.2 mg C l −1. Biomass production (cells out−cells in) during this chlorination period was lower (approximately 2×10 4 cells ml −1). The delay before DOC was released in chlorinated networks appeared to be less than 24 h, which corresponds to one hydraulic residence time. Likewise, when chlorination was stopped, 24 h or less were required before an efficient DOC removal was resumed. When chlorination was prolonged the observed release of DOC was progressively reduced from 0.2 mg l −1 to zero, thus after 6 weeks of continuous chlorination the DOC in was equivalent to the DOC out.