Wastewater Treatment

Proposed Wastewater Treatment Plant Upgrades

The City is in the process of completing environmental review for proposed upgrades to the Wastewater Treatment Plant.

A Planning Commission public hearing is scheduled for February 19, 2015, to obtain public input and consider adoption of a Mitigated Negative Declaration for the proposed project.

Additional Information can be viewed here: 

Initial Environmental Study and Technical Appendices


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 headworks is the location where all wastewater enters the plant. Influent into the plant is measured and the water travels through the headwork structure in one of two channels. One channel is equipped with a mechanical bar screen which removes large solids such as rocks, plastics, paper and other material. This material is placed by the mechanical device in to a Dumpster for landfill disposal. The other channel is equipped with stationary screens that must be cleaned by plant personnel. This channel is used only when the mechanical screen is out of service. From the headworks flow can be sent to two locations. The oxidation ditch is the normal destination for water leaving the headworks, but the oxidation ditch can be by-passed if necessary sending water to the ERB (emergency retention basin).

Oxidation Ditch

The oxidation ditch follows the headworks and is the heart of the plant's Activated Sludge process. In this process wastewater is mixed with biological organisms. A population of these organisms is maintained and controlled to provide the best removal possible of suspended and dissolved solids in the wastewater. These organisms consume or attach to the suspended or dissolved solid particles in the wastewater. These solids would be very hard to remove without the aid of the organisms since they would not settle out on their own. Mixing of the wastewater and the organisms is accomplished by four surface aeration brushes located around the ditch. The brushes also provide oxygen for the organisms to live. Once the organisms have consumed or have attached themselves to the particles they come together to form a flock called Activated Sludge. The sludge is made up of Mixed Liquor Suspended Solids or MLSS. This solution then leaves the oxidation ditch and is elevated by way of three Mixed Liquor pumps so that it can gravity flow to the next stage in the process which is the secondary clarifiers.

Secondary Clarifiers

The plant's two secondary clarifiers receive flow from the oxidation ditch by way of three mixed liquor pumps. The clarifiers are circular basins in which no mixing or oxygen is provided. In this environment the activated sludge which is now heavy with suspended particles settle to the bottom of the clarifiers. As the sludge settles, clean water remains and flows out of the clarifier over a weir at the edge of the clarifier. This water travels on to filtration. Eight 4" draft tubes that circulate slowly at the bottom of the clarifier remove the sludge, once settled. The sludge flows from the clarifier to the RAS/WAS structure where it can go to one of three locations.


The RAS/WAS structure acts as a transfer station for sludge leaving the secondary clarifier. Sludge leaving the RAS/WAS structure can go to three locations. It can gravity flow to the Headworks where it is blended with in-coming sewage before flowing to the oxidation ditch. Sludge can also be returned directly to the oxidation ditch from the RAS/WAS structure. Sludge returned to the oxidation ditch directly or via the Headworks is called RAS (return activated sludge). The whole process of sludge flowing to the clarifiers and being returned to the oxidation ditch again is called the Secondary Loop System. However, in order to maintain control of the organism population in the oxidation ditch, removal of sludge from the system may be required. At this time, one of two pumps can be used to pump sludge from RAS/WAS structure to the Aerobic Digester. Sludge pumped to the digester is called WAS (waste activated sludge).

Aerobic Digester

The plant's digester works similar to both an oxidation ditch and a clarifier combined. Activated sludge is mixed and aerated in a circular basin much like it is the oxidation ditch. The difference is that sewage is not added. Besides oxygen, only scum and sludge from the clarifiers is added. In this environment, the lack of sewage (food for the organisms) causes the organisms to reduce their volume by consumption of their own mass. As the mass of the sludge decreases, water is left behind. Occasionally the aerator/mixer is turned off and the sludge then settles to the bottom of the digester. The water that remains at the top of the digester is decanted out of the digester through a floating drainpipe. This water is returned directly to the oxidation ditch for further treatment. As more sludge is added to the digester from the RAS/WAS structure, there will be less room for the sludge to settle. At some point sludge must be removed from the digester and sent to the Sludge basins.

Sludge Basins

The sludge basins are the last process stage for activated sludge that has been removed from the secondary loop system. Sludge arrives at the basins from the aerobic digester. The sludge is aerated and will continue the process of digestion started in the digester. Once there is a large amount of sludge in these basins, all water is decanted off and the sludge is dried and removed. The most likely destination for the sludge is a landfill. However, the sludge is usually of good enough quality that it could be used as a fertilizer for landscaping or other plants.


Water leaving the secondary clarifiers travels on to filtration. The plant has two traveling bridge filters. Each filter has a compartmented filter bed, which contain a sand media used to filter out solids. Water from the clarifiers enters from a central channel between the filters, flows down through the filters and exits to channels on the outer edge of the filters. As solids build up in the sand media it becomes harder for the water to pass through the filter. This causes the water level on the filter to rise. When the water level reaches a preset point, a backwash or cleaning of the filter begins. The bridge located on top of the filter begins to move from one end of the filter to the other. As it travels on special tracks located on each side of the filter, filtered water is pumped back through the bottom of the filter one compartment at a time. This allows the other compartments of the filter to remain in service. As the water and dirt is pushed up through the filter, it is pumped out through a hood. This hood is connected to the bridge and stretches the length of the filter compartment. Water and dirt removed from the filter is then pumped back to the oxidation ditch for treatment. The bridge will continue moving from one compartment to the next until it reaches the opposite end of the filter. Then if the water level has dropped to the correct level the backwash will stop until levels again reach the high set point. Filtered water leaves the filters and move on to the contact basin for chlorination.

Chlorination and Dechlorination

Chlorine gas is used to disinfect the water leaving the filters. Disinfecting kills any remaining disease causing bacteria or organisms left over from treatment. Chlorine gas is injected into a treated water supply. This chlorine/water solution is then piped to a contact basin and added to water coming from the filters. A contact basin is used to give the chlorine enough contact time with the water for disinfecting to take place. Water enters at one end of the basin and must travel through a series of channels before it reaches the other end of the basin. After disinfection, treated water can be discharged to several locations. During winter months water is discharged to Churn Creek if water levels in the creek are high enough. A dilution of 10:1 is required for discharge to the creek. Water going to Churn Creek must be de-chlorinated by the use of SO2 gas. The gas is added in the same way that chlorine is added. The SO2 solution is added just before the water leaves the plant and removes the chlorine. The SO2 is also used up in this process. During the summer months water goes to different locations. Most water goes to a 410 acre-foot reservoir where it is stored for later use. Water from the reservoir is used by the city to irrigate 60 acres of pastureland surrounding the treatment plant. At this time water is also supplied to a local fast food restaurant for landscape irrigation. Treated water is also pumped from the treatment plant to a nearby logging mill where it is used to spray log decks. In the future, it is hoped that the water will be used for other irrigation purposes such as golf courses and roadside landscape.