North City Water Reclamation Plant
4949 Eastgate Mall, San Diego, CA. 92121
The City of San Diego imports approximately 90% of its water supply. To meet the future potable water demands while decreasing our dependence on imported water, the City has developed and implemented innovative water management strategies.
The North City Water Reclamation Plant (NCWRP) is the first large-scale water reclamation plant in San Diego's history and part of the single largest sewerage system expansion in the area in more than 35 years. This state-of-the-art facility can treat up to 30 million gallons of wastewater per day which is generated by northern San Diego communities. Wastewater entering the plant undergoes a series of treatment and purifying steps using the latest technologies to supplement the water supply of the region.
Reclaimed water produced at the NCWRP is distributed throughout the northern region of San Diego via an extensive reclaimed water pipeline. More than 79 miles of distribution pipelines are installed in Mira Mesa, Miramar Ranch North, Scripps Ranch, University City, Torrey Pines, Santaluz and Black Mountain Ranch to provide reclaimed water to our customers for irrigation, landscaping and industrial use. NCWRP also provides reclaimed water for the City of Poway. Reclaimed pipelines, sprinkler heads, meter boxes and other irrigation equipment are color-coded purple to distinguish reclaimed water pipes from drinking water systems.
The NCWRP Operations Building includes the plant operations staff which monitors and controls every phase of the treatment process through a central control room. The Operations Building also includes process laboratories where samples of wastewater from every stage of treatment are analyzed. From 12:30 a.m. to 5:30 a.m. control of the North City plant is transferred to Public Utilities' Communications Center in Kearny Mesa and an operator from NCWRP is on call in the event of an emergency.
Untreated wastewater (influent) enters the plant through an 84 inch pipeline beneath the I-805 and flows into North City's Influent Pump Station. Four pumps at the influent pump station pump the wastewater up a 90 feet rise to the Headworks. From the Headworks, through the remaining treatment process, the majority operates through gravity flow.
At the Headworks wastewater passes through large rake-like bar screens to remove solid debris and floating material called rags. Rags are comprised of cloth, wood, plastic and vegetable matter. Once the rags have been removed from the wastewater they are dewatered and trucked to the landfill. The screened wastewater then passes through aerated grit chambers where heavier solids such as sand, gravel, coffee grounds and eggshells are settled out and removed. The grit is then dewatered and taken to the landfill.
Primary Sedimentation occurs when solids sink to the bottom of the tanks and "scum" (grease and cooking oil) float to the surface. "Raw Sludge" that has settled to the bottom of the basins is collected by a chain and flight system then pumped to the Blended Sludge Pump Station. Similarly, the scum is removed from the surface of the sedimentation tanks, dewatered and taken offsite for disposal. Primary Sedimentation takes approximately 90 minutes removing 99% of the settleable solids, 50% of the suspended solids and 25% of the Biological Oxygen Demand (BOD).
Aeration is found in two different zones: anoxic zones where oxygen is depleted and aerobic zones where oxygen is consumed. Both zones have bacteria that eat soluble BOD. The anoxic zone is designed to control filaments while the aerobic zone is designed for the organisms to ingest and digest the organic solids while increasing in number and density. The organisms are called mixed liquor.
Odor Control is an important part of the influent, headworks and primary wastewater treatment process. Odor is caused primarily by hydrogen sulfide gas. Throughout the plant, Odor Control "Scrubbers" draw the foul air (and odors) off the flow of wastewater. The foul air is drawn into the "scrubbers" where it passes through a bleach solution spray at a pH of 9.0 which neutralizes odor-causing sulfide compounds. The "scrubbed" air then passes through carbon filters which removes any additional foul air before being released into the atmosphere.
Secondary Clarification occurs when the mixed liquid settles to the bottom of the basins and is collected by a chain and flight system. The solids (return activated sludge) are then pumped to the aeration basin influent channel. A small portion of the RAS is also sent to the blended sludge pump station thru the low capacity waste pumps. The water has now been treated to Secondary effluent quality. The secondary effluent can either be returned to the sewer system, Point Loma Wastewater Treatment Plant or to Tertiary Treatment for reclaimed water applications.
Blended Sludge Pump Station sends the combined primary sludge, secondary scum and low capacity waste to the Metro Biosolids Center for further processing.
In Tertiary Treatment secondary effluent flows into anthracite coal beds where it is filtered of remaining solids.
Chlorine Contact/Final Effluent: Filtered Effluent is then chlorinated in chlorine contact tanks for a minimum of 90 minutes for proper disinfection. At this stage the utility water is ready to use for plant irrigation and equipment operations.
Demineralization: A portion of the filtered effluent is sent to the Demineralization Facility which uses an electro-dialysis reversal (EDR) process to reduce the salinity of the reclaimed water.
Chlorine Contact/Reclaimed Water: Filtered Effluent is blended with demineralized water then chlorinated in chlorine contact tanks for a minimum of 90 minutes for proper disinfection. The demineralized water is now available for industrial purposes for irrigation or agricultural purposes that require low salinity.
Cogeneration: All the power required to operate the North City Water Reclamation Plant is provided by an on-site Cogeneration facility operated by Minnesota Methane. The cogeneration plant is powered by methane piped from the Miramar Landfill and MBC digesters. Approximately 75% of the power produced is used for plant electrical needs with the remainder of the power sold to the local electric grid.