Treatment ProcessAerial Photo of RWCF

The Stockton Regional Wastewater Control Facility is located in the southwest area of the city of Stockton, California. The facility includes primary, secondary, and advanced tertiary treatment of wastewater.  A flow diagram shows the full treatment process from entrance into the treatment plant to discharge into the San Joaquin River. 



The Headworks is the first treatment step for raw wastewater entering the treatment plant.  The purpose for the Headworks is to remove large material (such as rags, sticks, plastics, small stones, etc.) from the raw sewage 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 consists of the physical removal of wastewater solids.  The low velocity flow through the primary settling tanks enables heavy particles to settle to the bottom, where mechanical scrapers move them to the sludge hoppers.  The floating materials are removed from the surface by means of water sprays and rotating collectors or skimmers.

Settling is enhanced by the addition of coagulants and polymers, which cause larger particles to form from smaller particles that can then be easily removed. 


Bio-Oxidation Towers

After passing through the Primary Settling Basins, the wastewater flows by gravity 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 recombined with fresh settled wastewater and is 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 is settled 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 then 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 there is a cycle of vegetation decay and regrowth that renews the process.  The Wetlands is a sanctuary for migrating birds that can be observed during different times of the year.


Nitrifying Biotowers

The Nitrifying Biotowers are 166 feet diameter by 26 feet in height and are necessary to meet NPDES permit requirements, specifically concerning ammonia removal.  Wastewater is pumped from the Wetlands to the Nitrifying Biotowers, which contain organisms that will consume ammonia.  During the summer months, the Oxidation Ponds are so effective removing the 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 causes the suspended matter to bond together and 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 is intended to ensure very low suspended solids content in the final liquid waster before it is discharged to the San Joaquin River.  Each filter bed is 34 feet long by 25 feet wide.



The post-Chlorination system provides final disinfectant of the treated wastewater approximately 35-feet upstream of the effluent control structure, where essentially all residual chlorine is removed from the wastewater.  Following dechlorination, the treated effluent is pumped into the San Joaquin River. 



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This City of Stockton web page last reviewed on --- 2/3/2016