Regrowth of coliforms and fecal coliforms in chlorinated wastewater effluent

Observations made both in the field in chlorinated effluent, and in laboratory experiments show that coliforms and fecal coliforms are capable of regrowth in chlorinated wastewater. Under field conditions regrowth of coliforms in chlorinated effluent held in a storage reservoir for about 3 days appeared inversely correlated to: (1) The residual chlorine in the storage reservoir and (2) The number of coliforms surviving chlorination. In the laboratory experiments regrowth occurred after initial doses as high as 11 ppm total chlorine even when there was no chemical inactivation of the chlorine. Fecal coliforms did not generally show regrowth to the same extent as coliforms. Regrowth occurred even when coliforms were not detectible in 10-ml of samples after chlorination.Since coliforms and fecal coliforms are capable of regrowth in chlorinated sewage effluent and admixtures of it, the sanitary significance of the number of coliforms after storage or in receiving bodies of water is difficult to interpret. Thus standards might be based on the number of coliforms, or fecal coliforms detected in effluents immediately after chlorination. However, this would not be justified if in addition to coliforms, pathogenic bacteria can regrow in chlorinated effluents.