In 2015, the State Water Resources Control Board (SWRCB) conducted a municipal wastewater recycling survey in conjunction with the Department of Water Resources (DWR). The purpose of this survey was to estimate the quantity of municipal recycled water produced and beneficially reused statewide.[1] Cities in Tulare County reported 18,537 AF of recycled water used for agricultural irrigation, primarily secondary undisinfected wastewater effluent. The estimated potential for tertiary treated municipal recycled water is 33,500 AF per year.[2]

For health reasons, undisinfected secondary effluent is used primarily for groundwater recharge or for agricultural irrigation.

California’s Water Code limits application of undisinfected secondary effluent to non-food crops or crops in which the water has no direct contact with the edible portion of the plant. Undisinfected secondary effluent must be applied in a manner that does not allow people to come into direct contact with the effluent. For this reason, undisinfected secondary effluent cannot be used to displace many types of uses of potable water for nonpotable uses, such as for irrigating parks and playgrounds, school yards, residential landscaping, and unrestricted access golf courses.[3]

Photo credit: Almond trees on sand, Shutterstock 439945252
Photo credit: Almond trees on sand, Shutterstock 439945252

The four largest urban wastewater treatment plants (Cities of Visalia, Porterville, Tulare, and Dinuba) treat 80% of the County’s wastewater, a combined volume of 13.4 billion gallons annually. Until recently, 90% of the wastewater was treated to secondary undisinfected quality.

Increased awareness of the need to build local supplies for drought resilience have resulted in many urban areas now treating their wastewater to tertiary standards, at a minimum, to enable using recycled water to displace use of valuable potable water supplies for non-potable purposes.  Tertiary treated effluent can be used in urban areas with frequent human contact, such as to irrigate parks and golf courses. It can also be used for both food and non-food agricultural irrigation and groundwater injection.

In 2017, the City of Visalia completed the County’s first tertiary wastewater treatment plant that now treats 33% of the County’s wastewater effluent. Secondary undisinfected effluent still accounts for the largest volume (57%), but the cities of Porterville and Tulare plan to upgrade their systems to tertiary. When those upgrades are complete, nearly 90% of the County’s urban wastewater will be tertiary quality.

Technological advances in water filtration and disinfection have led to a fourth “purification” stage with advanced filtration and ultraviolet disinfection of tertiary treated wastewater.[4] The SWRCB is considering new regulations that would allow this new “purified” water resource to directly augment potable water supplies.

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Direct potable reuse (DPR)

There are two forms of DPR. In the first form, purified water from an advanced treatment facility is introduced into the raw water supply immediately upstream of a water treatment plant. In the second form, finished water is introduced directly into a potable water supply distribution system, downstream of a water treatment plant.

Indirect potable reuse (IPR)

In IPR, purified water from an advanced water treatment facility is introduced into an environmental buffer, such as a water body upstream from the intake to the drinking water facility, for a specified period of time before being withdrawn for potable purposes (see also de facto potable reuse).

De facto potable reuse

The downstream usage of surface waters as sources of drinking water that are subject to upstream wastewater discharges (e.g., unplanned potable reuse).

Source: State Water Resources Control Board (SWRCB)

[1] Municipal Wastewater Recycling Survey. State Water Resources Control Board’s website: https://www.waterboards.ca.gov/water_issues/programs/grants_loans/water_recycling/munirec.shtml.

[2] Roberts, J., The Role of Wastewater in Drought Resilience. July 2018. Syzergy, Inc.

[3] California Code of Regulations, Title 22, § 60304. Use of Recycled Water for Irrigation.

[4] Orange County Water District’s Groundwater Replenishment System and the City of San Diego’s Purewater System are examples of supplies that already apply advanced filtration and additional disinfection to tertiary treated wastewater. Sources: “GWRS – new water you can count on”, Orange County Water District website: https://www.ocwd.com/gwrs/ and “Pure Water San Diego”, City of San Diego website: https://www.sandiego.gov/water/purewater/purewatersd.