A portion of the water supplied by Metropolitan Water District (MWD) of Southern California is diverted to Las Virgenes Reservoir in the wintertime to augment supply when demand is high in the summertime. Although the water is fully treated to drinking water standards, federal regulation prohibits serving it to customers because it has been stored in an open reservoir. The Westlake Filtration Plant, located at the reservoir site, provides the necessary treatment to comply with the regulation that went into effect in 1989. The plant is also available for operation during maintenance shutdowns by MWD or water supply interruptions in emergencies.

Water is filtered using diatomaceous earth (or DE). The water then goes through primary disinfection, using sodium hypochlorite (or bleach), to kill or inactivate bacteria, viruses, and other potentially harmful organisms in drinking water.  As a final step, ammonia is added to produce chloramines as a secondary disinfectant to maintain water quality as it moves through pipes to consumers. Chloramines as a secondary disinfectant produce fewer disinfection byproducts than chlorine.