Oso Water Reclamation Plant Improvement Project

Santa Margarita Water District is upgrading the Oso Water Reclamation Plant to meet increasingly stringent water quality standards, improve the water quality in nearby Oso Creek, and increase its supply of sustainable, local water.


Latest News

  • Informational Meeting & Tour on Oct. 17 @ 4 PM RSVP here
  • Phase 1 - completed
  • Phase 2 - begins Nov. 2023

Project Overview

  • 2022 - 2025
  • $53 million investment 
  • 83% increase in capacity 
  • 20% smaller footprint
  • Improved water quality in Oso Creek 
  • Increased local water supply 
  • Enclosed, underground portions of the plant will meet the visual appeal of Mission Viejo's Core Area Vision Plan

The Improvement Project

The improvement project includes a largely enclosed, sophisticated water treatment plant that can treat up to 3.3 million gallons of wastewater per day to recycled water standards; an advanced water treatment facility – like the one used to fill Lake Mission Viejo - that can treat one million gallons of Oso Creek urban runoff water to improve the water quality; and new buildings for administration, warehouse space, and parking.  The architecture and color elements will blend with the surrounding landscape and fit in with the visual character of Mission Viejo.    


  • Demolition of existing treatment plant and buildings
  • Increased noise is expected while excavators break up the concrete structures and dump trucks haul away the debris

Phase 2: November 2023 - May 2025

  • Treatment plant construction
  • The nearby creek berm will be removed
  • Increased noise during berm work
  • Crews onsite from 7 AM - 5 PM
  • No weekend, holiday, or night work is anticipated 

Phase 3: March 2024 - May 2025

  • Administrative and warehouse construction 
  • Paving and restoration 

SMWD is also working with the City of Mission Viejo as it begins the development of its Core Area Vision Plan for the shopping center located on the west side of Oso Creek opposite the treatment plant location.  

About the Treatment Plant 

The Oso Treatment Plant was built in 1975 and treats 1.8 million gallons of wastewater each day to recycled water standards. During the cooler months, when Mission Viejo and the surrounding communities do not require as much water, the recycled water is pumped to the Upper Oso Reservoir located near Los Alisos and the 241 for storage until warmer, dryer weather arrives, and water demands are higher. 

There are three main steps to treat raw wastewater to safe recycled water standards:   

  1. Primary, or the physical processing where the solids are separated from the liquids
  2. Secondary, or the biological processing substantially degrade the biological content of the waste through aerobic biological processes.
  3. Tertiary, or recycled water is the processing of treating the water to Title 22 standards through chlorination. 

It takes roughly 24 hours to treat wastewater and convert it to usable recycled water. Recycled water has been used safely in the United States since 1929. It’s used for watering common areas, medians, parks, sports fields, and refilling Lake Mission Viejo. 

Nearly all our drinking water in south Orange County imported from hundreds of miles away through a sophisticated system of pipelines, reservoirs, pumps, and more. It’s far too valuable to use just once. One day, recycled water may be a source of safe drinking water for the region.

This is one of four treatment plants owned and operated by the Santa Margarita Water District. 

Oso Treatment Plant