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Wastewater Services

What We Do

The Wastewater Division is housed at two locations – Sutter Avenue; primary process and Jennings Road; secondary and tertiary processes. The Wastewater Division treats an average of 20 million gallons


Prior to1910, the City’s wastewater was discharged directly into the Tuolumne River at 9th Street. In 1919, the City purchased approximately 38 acres of land at the foot of Sutter Avenue and Martin Luther King (which is now known as the Sutter Wastewater Treatment Facility). An outfall line was installed rerouting flow to what was tantamount to a giant septic tank. The septic’s tank overflow was then discharged into the Tuolumne River at Sutter.

Over the years expansions took place: 1928 more acreage was purchased at Sutter, oxidation and percolation beds were constructed. In 1930 a 62ft clarifier was installed replacing the giant septic tank of 1919. In the years between 1937 and 1955 many improvements were constructed due to the stringency of the regulatory agencies. During the canning season, it was common practice to operate Sutter Facility differently than the off season, when most industry ceased production. This is a cycle that would continue for years.

In 1969 seven (7) miles southwest of Modesto property was purchased and a pond system (the largest in the US) was being developed for the secondary treatment process. Ultimately, discharge would be routed from the Tuolumne River to the San Joaquin River. This new system would become known as the Jennings Secondary Treatment Facility. In the years to follow the Sutter facility was converted to a dedicated primary process facility. The modifications included: the conversion of a clarifier to a gravity thickener and a bio filter to secondary clarifier. Moving forward through the decades massive changes were made to both facilities. This included improvements to effluent land application and river discharge, allowing the city to remove greater volumes of effluent. Mechanical aerators, chorine contact tank, chlorination and de-chlorination building, fixed film reactors and the land application system consisting of an irrigation fore bay, irrigation pump station and ranch reservoir. The City continued to meet new regulatory limits.

In 1998 the Cannery Segregation force main project; moving cannery waste flows from Sutter to Jennings had been accomplished. This provided complete isolation from the domestic influent flows allowing for full primary processing of domestic and screening of cannery at Sutter. Both flows in separate lines travel to Jennings where after secondary processing of domestic flows are blended and applied to land. Domestic flow that are not blended remain in the ponds until river discharge season. At this time it is chlorinated and de-chlorinated and flow proportionally delivered to the San Joaquin River.

Again, to meet the ever changing and more stringent effluent limits required by the State, the City has spent a decade and over 130M dollars on a state of the art Tertiary Biological Nutrient Removal (BNR) and Membrane system. High levels of nitrogen in wastewater cause eutrophication, a process where excess nutrients stimulate excessive plant growth such as algal blooms. The decomposition of the algae by bacteria uses up the oxygen in the water causing other organisms to die. So the BNR system uses nitrification which is a two-step aerobic process which typically takes place in aeration tanks. Denitrification requires anoxic conditions to encourage the appropriate biological conditions to form. The activated sludge process is often used to reduce nitrate to nitrogen gas in anoxic or denitrification tanks.

In May of 2018 we will use the Tertiary BNR system to create a ZERO effluent/discharge to the San Joaquin River. As partners in the North Valley Regional Recycle Water Project (; our tertiary effluent water supply will provide a new source of water for agricultural customers in the Del Puerto Water District (DPWD), whose supplies have been severely impacted by drought and environmental restrictions on pumping from the Delta.

Frequently Asked Questions

In an Emergency


Spills reporting 24/7:
(209) 577-6200
Non-Emergency Police Dispatch
(209) 522-2470

Contact Us

Wastewater Administration
Primary Plant
1221 Sutter Avenue
Modesto, CA 95351
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Report a Problem or Request a Service

(209) 577-6200 phone
(209) 577-6345 fax