Virus removal in advanced wastewater treatment systems.

The Sanitation Districts of Los Angeles County operate five biological secondary treat ment plants that discharge chlorinated effluent into the Rio Hondo and San Gabriel River flood control channels, as shown in Figure 1. These plants, which use the activated sludge process, together treat 300 000 m3/d (80 mgd). During much of the year, the effluent from these plants is the only flow in these channels. Recently the public has been per mitted access to the channels for limited recre ational activity. Although the intended recre ational use of the channels does not include body contact, swimmers have been observed on many occasions. The California State Department of Health has established quality and treatment stan dards for wastewater discharged into impound ments where there is human contact with the water. One of the objectives of the stan dards is to prevent transmission of viral dis eases. Because viruses are normally present in low concentrations in treated wastewater and because methods for detection of viruses are complex, a limit on virus concentration is not specified in the standards. In lieu of virus monitoring, the standards require tertiary treatment. Reclaimed water for recreational