Westlake Filtration Plant

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Potable water stored in the Las Virgenes Reservoir has been treated extensively by the Metropolitan Water District of Southern California (MWD). However, once potable water has been stored in an open reservoir, whether previously treated or not, federal and state regulations require additional disinfection and filtration before it is served to customers. LVMWD accomplishes this additional disinfection and filtration at the Westlake Filtration Plant, adjacent to the reservoir.

Water from the reservoir is pumped to a concrete tank in the plant, called a raw water reservoir. It is then treated by ten filtration units using diatomaceous earth (or DE) as a filter media. DE consists of microscopic plant skeletons called “diatoms.” Each diatom has hundreds of jagged edges that interlock with other diatoms to form a tightly woven filter capable of “catching” extremely small materials. Algae, which can cause taste and odor problems, is the primary material removed from reservoir water. The filtration plant’s process is mostly automated, but it can be manually operated if necessary.

Disinfection is accomplished using chloramines, which maintain the safety and quality of water as it travels through the distribution system. Chloramine disinfection is preferred for water with organic materials present because it does not produce potentially harmful disinfection by-products.

Filter CatwalkWestlake Filtration Plant can process up to 15 million gallons of drinking water per day (MGD). Water processed at the plant is tested regularly. It consistently meets or tests better than all state and federal drinking water standards. Westlake Filtration Plant only operates when water is drawn from the reservoir in the high-demand summer months, when there are planned maintenance shutdowns by MWD or in the event of emergencies, when the supply of water from MWD may be unexpectedly interrupted. The plant, however, is always in “standby mode” and can be on-line within hours. While in operation, the filtration plant is controlled by state certified, skilled treatment plant operators and a complex automated monitoring system, to ensure safe and consistent water quality

Filtration tanks use DE as filter media. These eight filters can process up to 15 MGD.


The filtration plant is necessary to disinfect and filter water from Las Virgenes Reservoir before it is served to customers.

Las Virgenes Reservoir in Westlake Village

This plant was completed in 1990, at a cost of $9 million, to meet federal and state regulations for drinking water quality.


  • A 20,000 sq ft. facility capable of disinfecting and filtering up to 15 MGD
  • Uses 10 diatomaceous earth (DE) filters as the primary treatment process
  • Disinfection is accomplished using chloramination



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