Secondary Treatment
 		 Primary Treatment
 		 Solids Handling
 

Biological Filters

Biotower Top

  During the secondary treatment process, effluent from the primary sedimentation basins is pumped to the top of two biological filters. The water is sprayed down over plastic media in the filter. The media provides a surface for bacteria to grow and consume most of the organic material in the primary effluent.

Aeration Basins

After biofiltration, wastewater is pumped into aeration basins. Here, fine bubble diffusers introduce air that is necessary for the flocculation of particles. Flocculation converts the organic solids into heavier clumps that settle more easily. These clumps can be removed by sedimentation in the downstream secondary clarifiers.  

Secondary Aeration Basin

Secondary Clarifiers

Secondary Clarifier

  In the secondary clarifiers, bacteria and fine organic substances settle out of the wastewater. The majority of the settled sludge is returned to the aeration basins and is reused in the biological process. The excess sludge, or "waste activated sludge," can be pumped to gravity belt thickeners or wasted to the plant sewer system so it can co-settle in the primary sedimentation basins.

Disinfection

During the disinfection process, the secondary effluent is treated with sodium hypochlorite and sodium bisulfite. A backup system uses ultraviolite treatment. With this system the effluent is passed over ultraviolet light bulbs to kill the harmful bacteria. Final effluent is discharged to Carquinez Strait via a 2.5-mile-long force main pipeline.  

Ultraviolet Disinfection Unit

Trickling Filters

Both the primary and secondary clarifiers use trickling filters to remove organic matter from wastewater. Trickling filters enable organic material in wastewater to be adsorbed by a population of microorganisms (bacteria, fungi, algae and protozoa) that attach to the medium as a biological film or slime layer. As the wastewater flows over plastic
honeycomb layers in the tanks, microorganisms already in the water gradually attach themselves to the plastic and form a film. As the layer of microbial growth thickens, anaerobic organisms develop. The microorganisms near the surface lose their ability to cling to the plastic, and a portion of the slime layer falls off the filter. This process is known as sloughing. The sloughed solids fall to the bottom of the clarifiers and are removed are from the wastewater.

 

Wet Weather Diversion

During the rainy season, or any period when wastewater entering the plant exceeds 30 mgd, additional plant facilities are brought on line. These facilities ensure flows up to 60 mgd are properly treated before being discharged.

Grit Chamber

Once screened, flows in excess of 30 mgd divert to a third grit chamber on the plant's east side. Here, gravity settles out grit and sand.

 

Primary Sedimentation Basins

Diverted wastewater then enters eight rectangular primary sedimentation basins. The effluent in excess of 30 mgd is mixed with sodium hypochlorite to kill harmful bacteria. The effluent is then mixed with sodium bisulfite to remove residual hypochlorite that may have a toxic effect on aquatic life. These basins also contain scrapers that remove sludge from the tank bottom.

 

Junction Box

Following treatment, both primary effluent streams meet at a junction box that backs up flows in excess of 30 mgd into the plant's main chlorine contact tank. The remaining 30 mgd of flow enters the secondary treatment process.

 

Flow Splitting Structure

After secondary treatment, effluent enters a flow splitting structure. Here, a portion of the secondary effluent is channeled back into the main chlorine contact tank and blended with the backed-up primary effluent. This process is computer controlled to divert secondary effluent into the main chlorine contact tank until the tank reaches its maximum capacity. The blended primary and secondary effluent is pumped into Carquinez Strait.

 

Disinfection

The remaining secondary effluent enters a separate disinfection process. First, water is chlorinated with sodium hypochlorite and channeled into two chlorine contact tanks. Next, it is dechlorinated with sodium bisulfite before it is pumped into Mare Island Strait.  

 

© 2005 Vallejo Sanitation and Flood Control District