Treatment ProcessAerial Photo of RWCF

The Stockton Regional Wastewater Control Facility is located in the southwest area of Stockton. The facility includes primary, secondary, and advanced tertiary treatment of wastewater.


First treatment step as raw wastewater enters the treatment plant to remove large material (rags, sticks, plastics, small stones) before it enters the downstream treatment processes, where it would accumulate and decrease treatment volume.

Primary Settling Basins

To remove wastewater solids.The low-velocity flow through the primary settling tanks allows heavy particles to settle to the bottom to be moved to sludge hoppers. Floating materials are removed with water sprays and rotating collectors or skimmers.

Bio-Oxidation Towers

After passing through Primary Settling Basins

Gravity flows wastewater through a 60-inch line to the Biotower Pumping Station. Here, it is combined with recycled wastewater that has already passed through the biotowers and is pumped to the top of the tower for treatment.

At the top of tower

The wastewater is evenly dispensed across the top of the tower media. As wastewater trickles down through the media, naturally occurring bio-organisms metabolize the organic material in the wastewater, converting it to carbon dioxide, water, and new biological material.

After passing through Biotower

The wastewater reenters the Biotower Pump Station to re-combine with fresh settled wastewater and recycled to the top of the biotowers for further treatment. In a continuous loop, the wastewater is recycled through the biotower until it overflows the discharge weir and flows to the Secondary Settling Tanks, where additional material settles out.

Oxidation Ponds

This liquid waste from the Secondary Settling Tanks is pumped from the main plant to the Oxidation Pond Supply Channel and mixed with wastewater already through the primary pond. The combined mixture of secondary settling tank liquid and primary pond liquid wastes is redistributed to the oxidation ponds for aerobic, anoxic, and anaerobic reactions that reduce the Biochemical Oxygen Demand (BOD) and nitrogen in the wastewater. 


Then the wastewater flows through two Wetlands with a combined capacity of 135-acres. The Wetlands are planted with vegetation to remove turbidity, BOD, and slight removal of some metals. Each Wetland area has varying water levels to aid in settling materials and removing turbidity. Each year a cycle of vegetation decay and regrowth renews the process.

Nitrifying Biotowers

The Nitrifying Biotowers are 166 feet diameter by 26 feet high and meet NPDES permit requirements, specifically concerning ammonia removal. Wastewater is pumped from the Wetlands to the Nitrifying Biotowers, which contain organisms that consume ammonia. 

During the summer months, the Oxidation Ponds are so effective removing ammonia that ammonia has to be added later for proper disinfection. During the winter months, the Nitrifying Biotowers remove most of the ammonia instead of the Oxidation Ponds.

Dissolved Air Flotation Thickening

The Dissolved Air Flotation (DAF) thickeners dispense tiny air bubbles and coagulant into the water from the Wetlands. This process bonds suspended matter together to rise to the surface of each DAF thickener. A floating blanket is formed and easily removed by skimming.

Duel Media Tertiary Filters

Six Dual Media Filters are designed to remove remaining suspended solids and algae particles from the DAF thickener flow. This final polishing of the wastewater ensures low suspended solids content in the final water before discharge to the San Joaquin River. Each filter bed is 34 feet long by 25 feet wide.


External Links

There are currently no external links.

This City of Stockton web page last reviewed on --- 2/27/2018