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Private Sewers & Septic Systems
Streamlining Treatment Plant Operations: Tillman Project Flowcharts
The Additional Valley Outfall Relief Sewer and East Valley Interceptor Sewer carry wastewater to Tillman from about 70% of the San Fernando Valley. The plant can treat up to 80 million gallons per day. About 40% of the 80 million gallon daily flow comes from commercial uses and 60% from residences.
Wastewater from the main sewer lines flow through grit channels where sand, rocks and grit are removed. These items settle to the bottom of the grit channels by gravity and centrifugal force.
Influent Pumping Facility
After grit removal, the wastewater is lifted 30-feet by eight large-diameter screw pumps to the screening facility in the Headworks building. Screw pumps are more energy efficient than centrifugal or other types of pumps. The wastewater then flows by gravity through all the remaining treatment processes at the Plant.
At the Headworks, bars and screens remove the largest solids – such as branches, plastics, and rags. Screening, along with grit removal, is called preliminary treatment.
Primary treatment – taking out the sludge
Most of the solids (sludge) are removed here after they settle to the bottom of the covered primary tanks. All of the primary sludge flows back into the main sewer system where it is sent to the Hyperion Treatment Plant for further processing. The remaining wastewater then flows by gravity to the secondary treatment system for further treatment. The primary tanks are covered to reduce odors.
Secondary Treatment – Aeration Tanks
Bacteria are added to the aeration tanks for the nitrification-denitrification process. The bacteria feed off the organic wastes in the wastewater. Oxygen is added in the nitrification process to speed up the bacteria’s rate of decomposition. The nitrification-denitrification process reduces the amount of nitrogen in the plant’s effluent. The wastewater, rich in activated sludge, then flows to the secondary clarifiers.
Secondary Treatment – Clarifiers
The second stage of secondary treatment involves the settling of activated sludge by gravity in the final settling tanks, or secondary clarifiers. A portion of this settled activated sludge is returned to the aeration tanks (returned activated sludge, or RAS) to maintain biological equilibrium in the aeration tank, while the remaining portion is discharged/ wasted (waste activated sludge, or WAS) to the sewer. All of the waste activated sludge flows to the Hyperion Treatment Plant for further processing.
After secondary treatment, the wastewater flows through diamond-shaped cloth filters to remove any remaining solids. To improve filtering, a cationic polymer is used as a coagulant to capture and remove any remaining colloidal-sized solids.
Disinfection is a two-step process. Liquid bleach (sodium hypochlorite) is added to the wastewater, which kills any remaining pathogens or disease-carrying organisms. The chlorine-containing water remains in the last set of holding tanks at the plant for about two hours The water is then dechlorinated (with sodium bisulfite) to protect fish and other aquatic wildlife. The treated water is then reclaimed to one of three lakes, used onsite, or discharged directly to the Los Angeles River.
Over 25 million gallons per day of reclaimed water is produced at DCT. About 2.5 million gallons per day are recycled at the plant for treatment processes, landscape irrigation, cooling of plant equipment, air conditioning, and other applications. Over 23 million gallons per day are recycled to the three nearby lakes, the Japanese Garden Lake, the Wildlife Lake and the Balboa Recreation Lake. The remainder of the Plant’s treated water is discharged directly to the Los Angeles River. The plant’s discharge, combined with the outfall from the three lakes, provides a minimum of 20 million gallons per day to the Los Angeles River for support of the river’s riparian habitat.