Enteric virus and indicator bacteria levels in a water treatment system modified to reduce trihalomethane production.

A drinking-water treatment plant with high concentrations of trihalomethanes in its finished water and large numbers of viruses in its source water was located. This plant was used to study the effect of an alteration in the point of chlorination from the first to last step of water treatment on the biological and chemical qualities of its finished water. This alteration caused some reduction in trihalomethane production, but trihalomethane concentrations still exceeded the prescribed limit of 100 micrograms/liter. No viruses or bacterial indicators were ever isolated from the finished water of the modified plant. Total virus removal by the treatment steps before chlorination (coagulation, sedimentation, and sand filtration) averaged ca. 90%, whereas removal of bacterial indicators by these processes averaged between 88 and 98%. Recoveries of viruses and bacterial indicators in the source water were generally negatively correlated.