The wastewater treatment process is essentially the same as occurs naturally in lakes, rivers, or streams. The function of a wastewater treatment plant is to speed up this natural process by creating ideal conditions for breaking down waste products and purifying water. Like most treatment plants, the Water Pollution Control Facility (WPCF) operates 24 hours a day. We are committed to protecting public health and the environment while operating the plant in the most efficient and cost-effective manner possible.
Wastewater treatment at the WPCF is a combination of physical, biological, and chemical processes:
- At the Influent Pump Station, Screw Pumps lift raw sewage from the sanitary sewer system into the plant.
- Preliminary (or Mechanical) Treatment removes inorganic material by using Bar Screens to exclude large objects such as sticks, rags, and trash. The Grit Basin allows heavy, sandy, or abrasive matter to settle out. These materials are collected by Grinders and Classifiers and discarded at the landfill.
- Secondary Treatment creates optimum conditions for the growth of naturally occurring microorganisms, which digest organic material and remove nutrients, in the Oxidation Ditches. Organic solids are removed by settling to the bottom of Secondary Clarifiers. A return Sludge Pump re-circulates active microorganisms back into the system (like starter in sourdough bread). After secondary treatment 90% to 95% of solids have been removed from the wastewater.
- Tertiary (or Advanced) Treatment is required because we are subject to very stringent effluent discharge limits. In tertiary treatment, effluent from the secondary treatment process is chemically conditioned and directed through Cloth Media Filters to remove particles.
- Disinfection destroys pathogenic organisms in the tertiary effluent before it is discharged to the receiving water. This step helps protect the public from exposure to disease-causing organisms and further insures that the environmental impact of our discharge will be minimal. The plant has two Ultraviolet (UV) Light Disinfection Channels that inactivate bacteria and viruses prior to discharge. The final effluent is tested to confirm full treatment and to demonstrate compliance with the City's discharge permit.
Completing the Treatment Process
Two additional steps to complete the treatment process:
- Solids Processing makes the solids which were removed from the secondary treatment process suitable for beneficial use or for disposal at a landfill. Waste sludge from the secondary treatment process is directed to a series of Facultative Sludge Lagoons (over 200 acres) for stabilization and drying. Wind action and the growth of naturally occurring algae minimize odors and further break down the sludge. After drying, the waste solids are tested to confirm full treatment before being trucked offsite.
- Overflow Pond. During emergencies, water can be sent to the 43 acre Overflow Pond instead of being discharged to the environment.
Monitoring, Reporting & Discharge Permits
- Process Monitoring - Each step in the treatment process has its own sampling and monitoring requirements and we use a variety of laboratory tests to make sure everything is running smoothly. Plant operators use process monitoring data to optimize plant performance.
- Discharge Monitoring and Reporting - Since 1972, treatment plants that discharge directly water have been required to have a National Pollutant Discharge Elimination System (NPDES) permit issued by the USEPA or an approved State agency. In California, NPDES permits are administered through the Regional Water Quality Control Boards. Our NPDES permit (PDF) tells us how often to monitor and report on the quality of the water we're discharging. It dictates approved testing methods and even some operational parameters. Under the NPDES program, the state can establish even more stringent requirements than those applied by federal EPA. California's Basin Plans are the most stringent water quality regulations in the country.