The North Outfall Sewer carries wastewater to LAG for treatment from commercial and residential sources in the eastern portion of the San Fernando Valley. About 95% of the 20 million gallon per day flow comes from commercial and residential uses and only about 5% from industrial uses..
At the Headworks, bars and screens remove the largest solids – such as branches, plastics, and rags. Grit, which is sand, rocks and small debris, is also removed. Screening, along with grit removal, is called preliminary treatment.
The influent add additional energy to the wastewater, so that it will flow through the Plant’s treatment processes without the need for further pumping.
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. |
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 dual-bed or tetra denite sand filters to remove any remaining solids. To improve filtering, ALUM is used as a coagulant to increase the size of the remaining solids.
Disinfection
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 or discharged to the LA River.
Of the 20 million gallons processed each day at LAG, about 4.5 million gallons are used for reclaimed purposes by DWP and the City of Glendale. Some is utilized at the plant for treatment processes and landscape irrigation. About 600,000 gallons per day is used as cooling water by the Glendale Steam Power Plant. Other uses include irrigation at Griffith Park, in freeway landscaping, local cemeteries and at nearby golf courses. The remainder flows into the Los Angeles River where it supports the river’s riparian habitat.