Isolating Viruses From Finished Water

Reduction of enteroviruses and rotaviruses averaged 81 and 93 percent, respectively, at a full‐scale 205‐mgd (776‐ML/d) plant whose treatment train includes chemical flocculation, sand filtration, and chlorination. The highest reduction of enteroviruses occurred during prechlorination‐flocculation and filtration, whereas the highest reduction of rotaviruses occurred during prechlorination‐clarification and final chlorination. Enteroviruses or rotaviruses occurred in 24 percent of the finished water samples, which had >;0.2 mg free chlorine/L and met coliform bacteria (1/100 mL) and turbidity (1 ntu) standards. Although major plant deficiencies may have been responsible for the occurrence of viruses in the finished water, the results of this study indicate that finished water, with measurable levels of free residual chlorine and meeting standards for coliform bacteria and turbidity, cannot be assumed to be virus free.