Wastewater Treatment


The City of Shasta Lake’s wastewater treatment plant is designed to treat a dry weather flow of 1.3-MGD (million gallons per day). Wastewater is pumped to the plant from pump stations located throughout the city’s sewer collection system. The plant utilizes an Activated Sludge form of treatment and consist of several treatment processes and structures which include: 


The first step in the treatment process is the headworks. Raw sewage enters the plant headworks from several raw sewage pump station throughout the city. Influent wastewater flow is metered and screened by a mechanically cleaned bar screen. The bar screen captures and removes coarse solids in the wastewater. Removal of large solids and debris helps prevent damage to downstream pumps and other mechanical equipment and increases the efficiency of the plant by reducing solids buildup in the downstream basins. The solids (screenings) are deposited into a dumpster by the mechanically cleaned bar screen and removed from the plant site to a local landfill.

Aeration Basins

The aeration basin structure houses three Bardenpho Process aeration basin trains. Each train is comprised of an anaerobic zone, several anoxic zones, a swing zone and several aerobic zones. The swing zone and aerobic zones are aerated. The anaerobic, anoxic and the swing zones have mixers. 

The bulk of organic material present in the wastewater is removed in the aeration basins. It is assimilated by bacteria and other microorganisms in the Activated Sludge. The special arrangement of the anaerobic, anoxic, and aeration zones of the Bardenpho Process provides nitrogen and phosphorus removal. 

Secondary Clarifiers

The secondary clarifiers provide a quiescent environment to separate the activated sludge solids from the treated wastewater. The clarified water overflows the secondary clarifier weir and flows by gravity to the filters via the effluent junction box. Settled sludge is removed from the secondary clarifiers through sludge suction piping that is attached to the lower rotating clarifier mechanism. The sludge removed is either returned to the aeration basin as Returned Activated Sludge (RAS) or is removed and sent to the Aerobic Digester as Waste Activated Sludge (WAS).

Aerobic Digester

The aerobic digester biologically reduces the amount of volatile matter in the WAS. A floating surface aerator supplies oxygen to the process to maintain an aerobic environment. The end result of the process is a stable and less putrescible sludge.  After digestion, the stabilized sludge flows to the sludge stabilization basins.


Secondary effluent flows by gravity to the effluent filters. The effluent first flows through the polymer mixing zone and flocculation basin where polymer can be added to agglomerate fine suspended particulates. The flocculated water then flows to the filters where the flocculated particles are removed as the water flows through the filter media.

The filter has an automatic backwashing mechanism that permits filter backwashing without taking the filter offline. The backwash water is returned to the treatment process by the RAS pump station.

UV Disinfection

The filter effluent travels through two UV disinfection channels and is discharged to the recycled water basin. The primary purpose of UV disinfection is to disinfect the plant effluent by inactivating the pathogenic organisms that survived the other treatment processes.

The disinfected plant effluent flows through the recycled water basin where the water is provided to the recycled water pump station. At the outlet of the basin, a flow distribution structure distributes plant effluent to the Churn Creek outfall.