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.  Use this flow diagram  to see the full treatment process – from entrance into the treatment plant to discharge into the San Joaquin River. 


Headworks is the first treatment step as raw wastewater enters the treatment plant to remove large material (such as rags, sticks, plastics, small stones) before it enters the downstream treatment processes. This material must be removed before the raw wastewater enters downstream basins and biological treatment units where it would accumulate and decrease treatment volume.

Primary Settling Basins

The Primary Settling process physically removes wastewater solids. The low-velocity flow through the primary settling tanks allows heavy particles to settle to the bottom, where mechanical scrapers move them to the sludge hoppers. Floating materials are removed from the surface with water sprays and rotating collectors or skimmers.

Settling is enhanced by adding coagulants and polymers, which cause larger particles to form from smaller particles so they can easily removed. 

Bio-Oxidation Towers

After passing through Primary Settling Basins

Gravity flows the wastewater flows 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 the tower

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

After passing through the Biotower

The wastewater reenters the Biotower Pump Station, where it is re-combined 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 that has already passed through the primary pond.  The combined mixture of secondary settling tank liquid and primary pond liquid wastes is redistributed to the oxidation ponds where it undergoes aerobic, anoxic, and anaerobic reactions that reduce the Biochemical Oxygen Demand (BOD) and nitrogen in the wastewater. 


After passing through the Oxidation Ponds, the wastewater flows through two Wetlands, which have 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 that aid in settling materials and removing turbidity.

Each year a cycle of vegetation decay and regrowth renews the process. The Wetlands also attracts migrating birds that can be observed throughout the year.

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 that is easily removed by skimming.

Duel Media Tertiary Filters

The six Dual Media Filters are designed to remove the 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 it is discharged to the San Joaquin River. Each filter bed is 34 feet long by 25 feet wide.


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This City of Stockton web page last reviewed on --- 8/31/2017