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Drought-prone Paso Robles uses treated wastewater in irrigation, advancing water resilience goals

The City of Paso Robles

Paso Robles, CA

Paso Robles required other sources of irrigation water to substitute for groundwater after experiencing overdraft as a result of drought. New tertiary treatment facilities treat wastewater so it can be reused for irrigation purposes, advancing the city’s water resilience goals.

Topics Covered

Water Supply & Drought
Waste Management


Initial: 14.4 Million USD


General Fund/Existing Public Funds

Project Status

Operational since 2019

Problem Addressed

Located in drought-prone California, The City of Paso Robles needed to find new sources of irrigation water to reduce the overdraft of the city's groundwater basin and enhance water resilience.

The City of Paso Robles relies on a combination of surface water and groundwater supplies to meet the water demands of its community, with groundwater primarily coming from the Paso Robles Groundwater Basin. Much of the city’s wastewater was discharged into California’s Salinas River after undergoing treatment.

With overdraft occurring in the groundwater basin, plans were set to reduce reliance on the city’s already low groundwater supplies by further treating wastewater to irrigate public parks and golf courses. 2025 was set to be the year in which the local wastewater treatment plant would be modified with tertiary treatment facilities. However, drought in the state from 2011 to 2016 further taxed the Groundwater Basin, spurring the city to accelerate the project’s timeline.

Solutions Used

The city added new tertiary treatment facilities to the existing wastewater plant, creating a sustainable supply of recycled water for irrigation and preventing further overdraft of the groundwater basin.

With the help of engineering firm Black & Veatch, The City of Paso Robles added tertiary treatment facilities to its 4.9-mgd (million gallons per day) wastewater plant. Tertiary treatment applies a third, more rigorous level of treatment to water to make it safe for reuse in water-intensive processes.

The design of the new facilities utilizes gravity flow to move water through the various treatment stages, eliminating the need for pumping and reducing equipment and energy costs. Repurposed secondary sedimentation tanks that previously sat unused allow disinfection processes to operate continuously and at a more constant rate, and onsite ponds are used to store recycled water. If demand for recycled water is low, the plant can help replenish natural water resources by sending excess treated water to the Salinas River, reducing energy used for UV disinfection while still meeting discharge requirements.

“The project is a milestone in our long-term plan to create a resilient and sustainable water supply,” said Paso Robles Wastewater Resources Manager Matt Thompson. “We now can produce high-quality recycled water without building a purification plant. Black & Veatch helped us optimize existing assets and minimize capital and future operating costs in the process.”

The city also benefits from a nutrient management system that was built to prevent struvite buildup in pipes. The system keeps phosphorus, nitrogen, and ammonia from overloading and polluting the area’s waterways, and converts struvite into fertilizer the city can sell to subsidize its operating costs.

The addition of tertiary treatment to the facility enabled Paso Robles to produce California Title 22-compliant recycled water for irrigation of area parks, golf courses, and vineyards, reducing reliance on the Groundwater Basin and allowing it to recharge. The water previously discharged into the Salinas River is now being reclaimed in a sustainable and cost-effective manner.

With the new enhancements, the city benefits from the plant acting as a sustainable resource recovery facility where energy is harnessed through the plan’s processes and nutrients and water are recovered from waste streams.



Paso Robles can reuse treated wastewater to irrigate parks and golf courses, supporting the city’s water resilience goals


The city is better prepared for future drought conditions by investing in water reclamation


The plant’s nutrient management system converts byproducts of the treatment system into fertilizer the city can sell to offset operating costs


Paso Robles can produce high-quality treated water without the high cost of pumping as a result of the gravity flow design of the new treatment facilities


The new treatment facilities were built without expanding the plant’s footprint, allowing for a quicker project timeline

Something Unique

The project helped Paso Robles secure a $4 million Green Project Reserve grant for environmentally innovative projects. The project won the Global Water Awards’ 2020 Wastewater Project of the Year.

Who Should Consider

Municipalities looking for methods to become more sustainable by increasing water resilience in the face of drought.

Last Updated

Mar 23rd, 2022
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