Treatment Process

Preliminary Treatment

Wastewater from the Regional System arrives at the treatment plant and first undergoes preliminary treatment to remove unwanted solids from the water.  These solids may damage equipment or have no benefit to the treatment process.  Large debris such as plastic, wood, etc. is first removed by passing through large screens.  These screens capture anything larger than ¼” and remove it from the water.  Heavier material such as rocks and sand are then settled out in pre-aeration tanks.  The solids removed in this phase are washed, dewatered, compacted, and disposed of via the landfill. 

Remember, treatment starts at home, and you can help the plant by only disposing of the 3 P’s in your sewer system: Pee, Poop, and (toilet) Paper.

Primary Treatment

Wastewater leaving Preliminary Treatment enters the Primary Clarifiers to begin Primary Treatment. Within these tanks, the water is slowed down to allow the heavier organic solids to sink to the bottom and the lighter organic solids to float to the top. The heavier solids, now called sludge, are pumped off the bottom, and the lighter solids, now called scum, are scraped from the top. These solids are sent to the Anaerobic Digesters for further treatment.  The now cleaner water leaves the Primary Clarifiers and continues to Secondary Treatment.  At this point in the process, roughly 60% of what originally came into the treatment plant has been removed. 

Treatment Plant2_web

Secondary Treatment

Secondary Treatment begins in the Aeration Basins.  A naturally occurring biological growth of aerobic microorganisms called activated sludge are suspended within the Aeration Basins.  These microorganisms are selectively cultivated by Operations staff to further clean the water as required by permit limitations.  Oxygen is provided to the Aeration Basins via large blowers which allow the microorganisms to thrive, consume and manipulate pollutants and wastes leftover from Primary Treatment.  

Secondary Water Treatment Facilities
Treatment Plant_web

The activated sludge must then be removed from the water which occurs in the Secondary Clarifiers.  In the same fashion as the Primary Clarifiers, the water here is slowed down allowing the heavier activated sludge to sink to the bottom and any scum to float to the surface.  The activated sludge is pumped back to the Aeration Basins so it can begin treating new incoming sewage in a continuous loop.  A small portion of the activated sludge is wasted (removed) from the system each day to control the population of the microorganisms in order to accomplish treatment goals.  This sludge is thickened to remove excess water and is sent to the Anaerobic Digesters for further treatment.  At this point in the process, roughly 98% of what originally came into the treatment plant has been removed.

Tertiary Treatment

The water leaving the Secondary Clarifiers is then conveyed to the Filters.  The Filters contain a four-foot bed of anthracite coal. This small, black, granular coal (like the type used in some fish aquariums) acts as a filter to trap fine suspended solids and some potential pathogens, or disease-causing organisms. As the bed of coal becomes plugged with solids, it gets harder for water to pass through.  Operations staff periodically backwash the filters to remove trapped solids and wash the filter.  These trapped solids are returned to the Aeration Basins for retreatment.  The water leaving the filters has a high level of clarity and is ready for the final phase of treatment. At this point in the process, roughly 99.9% of what originally came into the treatment plant has been removed.


The final stage of treatment is disinfection.  The treatment plant utilizes Ultraviolet Light (UV) to inactivate disease-causing microorganisms remaining in the water.  As the microorganisms are subjected to the UV light, their DNA is altered in a manner that prevents them reproducing or replicating, thereby ending their impact to downstream users and the environment.

The City is currently undergoing a $75M project to replace its aged UV system with a brand-new unit.   The aged system is expensive to operate and is no longer supported by the manufacturer.  The new system will use roughly half the electricity of the older system, increase plant capacity, and allow Operations staff more redundancy for maintenance activities.  Construction is underway and is expected to be completed in 2025.

Water that leaves the UV system now meets Title 22 Disinfected Tertiary Recycled Water regulations and can be beneficially reused in the City’s reclaimed water systems and in the production of geothermal energy at the Geysers.  100% of the water that enters the facility is turned into Recycled Water, a rare feat in the wastewater industry at large.  On average, the City recycles roughly 7 billion gallons of wastewater each year.  During exceptionally wet years, water is occasionally discharged to the Laguna de Santa Rosa when storage capacities are at 

Tertiary Water Treatment Facilities

Trucked Waste Program

The City has an elaborate trucked waste program.  Wastes such as septic, greywater, and rinse water are trucked to the facility and discharged to the plant for treatment.  Many wastes are too high in strength to be discharged to the treatment plant and are instead brought to the plants High Strength Waste facility.   Wastes such as Inedible Kitchen Grease from restaurants, food processing facility wastes, breweries, wineries, etc. are offloaded to dedicated tanks and pumped directly to the Anaerobic Digesters for treatment.

Solids Treatment

The many solids mentioned in the steps above are all pumped to the Anaerobic Digesters for treatment.   The Anaerobic Digesters can be compared to one’s stomach; they contain anaerobic (no oxygen) microorganisms, they are heated to ~99ºF, and the microorganisms within break down the solids into gas and a stabilized sludge.

The gas produced form the digestion process is rich in methane.  This methane rich gas is cleaned and sent to the plants Combined Heat & Power (CHP) facility.  There, the gas is combusted in engine driven generators which produces 1/3-1/2 of the plant’s electrical needs.  The water used to cool the engines (thereby heating up) is sent to the Anaerobic Digesters to heat them up to the appropriate temperature.

The stabilized solids are then sent to the Dewatering Facility.  There, coagulants are added to the sludge and the sludge is squeezed to remove excess water.  The sludge, now called cake, can now be used as a soil amendment in agricultural fields.

Schematic of the Process

Water Treatment Schematic