The Encina Water Pollution Control Facility (EWPCF) treats about 22 million gallons per day (MGD) of wastewater, with a capacity of over 40 MGD. The wastewater is collected from six communities in North San Diego County, relying on gravity and active pumping. EWA’s commitment is to meet or exceed the requirements of its ocean-discharge permit 24-hours-a-day, 365 days-a-year. This is demonstrated by over 3,000 consecutive days of compliance.
Wastewater initially enters the Screenings Building, which contains four mechanically cleaned bar screens, three grit cyclone separators, a hydraulic press, and other equipment to remove inorganic debris such as rags, rocks, wood and grit that is detrimental to downstream equipment.
Next, wastewater flows through primary sedimentation tanks to remove readily settleable solids (primary sludge) and floating material (grease). Chains are used to collect primary sludge from the bottom of the tanks while skimmers remove grease from the surface. These materials are pumped to anaerobic digesters for treatment.
The EWPCF utilizes an activated sludge secondary treatment process. The wastewater flows through aeration tanks where dissolved oxygen and activated sludge (solids removed from later in the treatment process) are introduced. The activated sludge contains microorganisms, which break down the organic material in the wastewater through a biological process.
Secondary Sedimentation Tanks
The wastewater is then directed into secondary sedimentation tanks to allow for settling of microorganisms and degraded organic material from the secondary treatment process. The tanks are equipped with rotating arms at the bottom and surface to remove solids. To balance microbiology in the secondary treatment process, some of these solids (activated sludge) are returned to the aeration tanks and some solids are wasted. Waste activated sludge is pumped to sludge thickening tanks before being sent to the anaerobic digesters.
Effluent Pumping Station
Treated wastewater from the secondary sedimentation tanks flows by gravity to the effluent pumping station before being discharged into the Pacific Ocean through the ocean outfall pipe, which extends 1.5 miles offshore. The pumping station is equipped with systems to permit gravity discharge to the outfall when tidal conditions permit. Effluent pumping is required during combinations of high flow and high tidal conditions.
Waste solids from the secondary sedimentation tanks are pumped into one of three dissolved air flotation tanks. Air is introduced into the bottom of the tanks, which bubbles up through the sludge. Polymer is injected to coagulate the solids, which are pushed to the surface by the air bubbles, thus helping separate them from the water.
Solids removed from the primary sedimentation tanks and the sludge thickeners are treated in anaerobic digesters prior to dewatering. Solids are held in the digesters for 15-20 days at 95-98˚F (heated with waste heat from the EWPCF’s cogeneration facility) to reduce pathogens. The digestion process produces methane gas, which is utilized by the EWPCF’s cogeneration facility to produce electricity.