After leaving the trickling filters, the wastewater flows to an intermediate screw pump station, similar to the influent screw pumps, but smaller in size. These pumps are 36 inches in diameter and only 25 feet long. They are driven by 30 horsepower motors through a gear reducer which reduces the 1440 rpm shaft speed of the motor to 29 rpm for the screws. The intermediate screw pump station is required to lift the wastewater so that it can flow through the activated sludge system by gravity.




The first step we use in the activated sludge process is to provide oxygen to feed the microscopic bacteria in the wastewater by injecting streams of fine bubbles that rise through the wastewater in the aeration tanks. The bacteria feed on the organic material in the wastewater and convert it to water and carbon dioxide. This photo shows the surface of the wastewater with the bubbles rising to the top.






To provide the air needed, we use centrifugal blowers that can supply air at about 3,000 cubic feet per minute (cfm) at a pressure of 8 pounds per square inch (psi). The air is forced through the white pipes until the air reaches one of the black diffusers shown in this picture. Each diffuser has a large number of small holes. The air is forced out of these holes in the diffusers, and then the air floats up through the wastewater to provide oxygen for the microscopic bateria in the tanks. Each diffuser discharges approximately 5 cfm. This environment of food and oxygen helps generate high concentrations of microscopic bateria that purifys wastewater.

Wastewater leaving the aeration tanks is routed to the secondary clarifiers for further treatment.





The effluent from the activated sludge tanks comes up into the center of these secondary clarifiers. The flows slowly travels out to the perimeter of the secondary clarifier tanks where the water on the surface eventually flows over wiers. As the wastewater flows outward, heavier biological mass settles to the bottom of the clarifier. At the bottom of the clarifier tank, collector arms rotate to collect the biological mass that has settled. This biological mass contains active healthy growing bacteria that developed in the aeration tanks.

Wastewater requires 25 to 30 days to develop enough bacteria to remove ammonia nitrogen from the wastewater. Unfortunately, the wastewater flows through the tanks in four hours or less. So to grow nitrifying bacteria, we recycle a percentage of the flow back to the aeration tanks which enables some of the biological mass to reach the desired age of 25 to 30 days. We call this biological mass Return Activated Sludge (RAS). The remainder of the settled solids from the secondary clarifiers, called Waste Activated Sludge (WAS), is sent to a gravity thickening tank where the sludge is thickened from 0.5 to 3% solids to reduce its volume. These settled solids are sent to aerobic digesters for stabilization.

A gravity thickener is used to thicken the solids by settling the solids to the bottom which are pumped to anaerobic digesters. The liquid portion of the flow is transported to our sand filters for what we call effluent filtration, a form of tertiary treatment.

1. Preliminary Treatment
2. Primary Treatment
3. Roughing Filters
4. Activated Sludge
5. Effluent Filtration
6. Effluent Disinfection
7. Sludge Digestion
8. Sludge Dewatering
9. Sludge Storage
10. Other Unique Features