Water Pollution Control Facility (WPCF)

The City of Woodland owns and operates the Water Pollution Control Facility (WPCF) which treats wastewater from throughout the City’s residential, commercial, and industrial users.  A two-year project to upgrade and expand  the WPCF began in 2005.  From 7.8 million gallons per day (MGD), the facility has expanded to its current 10.4 MGD.  It also has upgraded into a tertiary (advanced) treatment system.  The tertiary system requirements were implemented in order to maintain strict water quality standards as the WPCF treated effluent is discharged into the Tule Canal. The expansion, along with the higher water quality standards, resulted in designating the WPCF as a class V facility – the highest treatment rating in California standards.

What is wastewater treatment?

It's cleaning used water and sewage so it can be returned safely to our enviornment.

How do treatment plants protect out water?

Wastewater treatment plants: 

  • Remove solids-everything from rags and plastics to sand and smaller particles found in wastewater;
  • Reduce organic matter and pollutants--naturally occurring helpful bacteria and other microorganisms consume organic matter in wastewater and are then separated from the water; and,
  • Restore oxygen--the treatment process ensures that the water put back into our rivers or lakes has enough oxygen to support life.

Where does wastewater come from?

Homes--human and household wastes from toilets, sinks, baths, dishwashers, garbage disposals, clothes washers and indoor drains.
Industry, Schools, and Business--chemical and other wastes from factories, food-service operations, school activities, hospitals, shopping centers, etc.
Incidental Storm Water Infiltration and Inflow from Runoff and Groundwater--water that seeps into the sanitary sewer system during a storm, as well as groundwater that enters through cracks in sewers.  The City of Woodland has one system for wastewater from homes and businesses (sanitary sewers) and a separate system for storm water runoff (storm drains).  In general, stormwater runoff remains separate from sanitary sewer flows and is not treated before it is discharged to local waterways.

On the average, each person in the U.S. contributes 50-100 gallons of wastewater every day.  If you include industrial and commercial water uses, the per person usage of water is as high as 150 gallons per day.

How does our wastewater treatment plant work?

The 5.5 million gallons per day (average) entering the facility is conveyed by over 250 miles of sewer pipe containing over 14,000 sewer connections.  The Utilities O&M Section of Public Works is responsible for the maintenance and repair of all public sewer mains and manholes.

The type of wastewater treatment used in the Woodland WPCF is called the extended aeration activated sludge process.  This is a biological process in which naturally occurring living microorganisms (bacteria, protozoa, tiny plants and animals) are maintained at a very high population level.  They quickly consume the dissolved and suspended organic material carried through the city sewer system and into the WPCF as a source of food.  This process promotes the formation of biological masses that clump together by adhesion and settle to the bottom forming "sludge."

Wastewater treatment basically takes place in three stages:

  1. Preliminary treatment, which removes the large solids and inorganic matter.
  2. Secondary treatment, which removes over 95% of the pollutants and completes the process for the liquid portion of the separated wastewater.
  3. Filtering / Disinfection Removes any additional solid materials and disinfects the water for discharge or reuse as “recycled” water.
  4. Effluent Discharge.  Discharge to the facility’s receiving waters.

Preliminary Treatment

The Woodland preliminary treatment occurs in a facility called the “Headworks”.  The facility at the beginning or “head” of the process is intended to remove the larger organic material and inorganic material that can create damage to the WPCF systems.


Screens let water pass, and capture trash (such as rags, diapers, etc.).  There are two bar screens located inside the Headworks Building that capture the material that can clog and damage costly wastewater equipment.  The trash is collected and properly disposed of.  The screened wastewater them moves on to the second step of preliminary treatment – grit removal. 


Two Aerated Grit Basins allow sand, pebbles, eggshells, and other heavier inorganic particles called “grit” to settle from wastewater by gravity. This material would accelerate the wear of the costly wastewater equipment. The aeration keeps the organic particles in suspension for the next phase of treatment.  The grit is collected from the bottom of the basin, washed and disposed of properly.

Secondary Treatment

The Woodland WPCF consists of aeration basins and settling tanks called clarifiers.  This system promotes the lifecycle of the microorganisms.  The aeration basins provide them with oxygen and food and the clarifiers allow them to settle out and be “returned” to the aeration basin for constant seeding of the basins.


Four Aeration Basins supply large amounts of air to the mixture of wastewater and helpful bacteria and other microorganisms that consume the organic matter.  The growth of the helpful microorganisms is sped up by vigorous mixing of air (aeration) with the concentrated microorganisms (returned activated sludge) and the wastewater.  Adequate oxygen is supplied to support the biological process at a very active level.  The ratio of food (organic matter) to organisms to oxygen is continually monitored and adjusted to meet daily variations in the wastewater. 


Four Final Clarifiers allow the clumps of biological mass (the microorganisms) to settle from the water by gravity.  A portion of this mixture, called "activated sludge," is returned to the aeration basins to help maintain the needed amount of microorganisms.  The remainder is pumped “wasted” to the sludge holding ponds.  The ponds allow the waste solids to further break down.  The remaining inert material is removed periodically from the ponds and disposed of properly.

Filtering and Disinfection

After clarification, the resulting “effluent” is filtered through a series of cloth filters.  Although the effluent is now 99% cleaner than the sewage coming into the WPCF, it must be filtered prior to the Ultraviolet Disinfection.  This process will provide a high-quality final effluent that can be safely returned to the environment.


The secondary effluent leaves the clarifiers and is pumped through a filter covered with a cloth fabric.  This filter removes practically all of the remaining solid matter.  From here the clear water flows onto the next portion of this process – disinfection.  A small portion is sent to the sludge ponds to provide a water “cap” for odor control and to optimize biological activity.


All filtered effluent then passes on to the last treatment process to completely meet National Pollutant Discharge Elimination System (NPDES) water quality criteria.  Ultraviolet (UV) disinfection disables any remaining microorganism.  The disinfection process enables the completely treated effluent to be safely returned to the environment.

Laboratory Analysis

The influent, final effluent and numerous other treatment points are monitored for a variety of parameters.  WPCF laboratory staff performs sampling and analysis for process control and NPDES compliance. Lab staff becomes certified for numerous analysis by facing rigorous compliance requirements.  All analyses face stringent quality control procedures to ensure a high level of accuracy.

Industrial Pretreatment Program (IPP)

The IPP staff provide an extremely valuable service to all the Public Works staff.  By regulating and monitoring the discharge activities of local industrial and commercial dischargers, the IPP staff works to ensure the safety of the collection system and WPCF staff.  They also ensure that  the sewer collection system is protected from damage and that the WPCF facility is safe and can remain fully in compliance with all regulations.

Administration Services

The WPCF Administration provides administrative and clerical services to the WPCF staff.  As with any highly regulated facility, data gathering and reporting is a continuing process.  The industrial nature of the facility also requires numerous purchases for materials, parts and contracted services.  Administration services provide continuous support to all activities of the WPCF staff.

WPCF Summary

The 10.4-MGD facility has four oxidation ditches with revolving rotors to provide oxygen to microorganisms in the wastewater.  Microorganisms need oxygen to consume the organic matter in the wastewater.  Next, the four clarifiers allow the solids and liquids to separate.  Coagulants are added at the clarifier sites, where solids settle.  After the clarified wastewater passes through synthetic, cloth media filters, it is ready for ultraviolet (UV) light disinfection.  Following UV treatment, the disinfected WPCF water discharges into the Tule Canal.

Licensed operators with Grades II to IV certificates, issued by the State Water Resources Control Board, operate the WPCF.  For operational control, a newly integrated Supervisory Control and Data Acquisition (SCADA) system fully automates the plant, both on site and through the internet.  SCADA also facilitates plant operation 24 hours a day, 365 days per year.

The WPCF Operations, Maintenance, Laboratory, IPP, and Administration staff are fully committed to their service to City of Woodland residents.  By efficiently and effectively providing these services, they ensure the health and safety of every Woodland resident and provide a long-term sustainable future to all our natural resources.

The staff at the Woodland Water Pollution Control Facility will be happy to arrange a tour for you and/or your group. To arrange a tour call 530-661-2069.

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