The heavy solids removed from the primary sedimentation tanks and waste activated sludge (WAS) from the secondary clarifiers are routed to anaerobic digesters for treatment.








We have two anaerobic digesters that stabilize the sludge. Each digester is 60 feet in diameter, 20 feet deep and holds about 500,000 gallons of sludge. During the digestion process, methane gas is produced. This digester has a floating dome that moves up and down to provide additional storage room for the methane gas as it's generated. The pipe in the center of the dome moves the methane gas to heat exchangers. The methane gas is used as a fuel source to heat the sludge and maintain a temperature of about 90 degrees fahrenheit that is needed by the anaerobic bacteria in the anaerobic digesters to thrive.



This is one of the heat exchangers. In the heat exchanger, the methane gas is used as a fuel source to heat sludge that is continually recirculated through the heat exchangers. Normally, the digesters produce enough methane gas to heat all the sludge. Sometimes during cold weather periods, we need to supplement heating fuel needs with natural gas. Control valves in the gas supply lines automatically switch over to natural gas, when necessary.





During hot weather, we sometimes have too much methane gas, and the storage capacity of the floating dome digester can be exceeded. To eliminate this excess gas, it is routed to a waste gas burner where it is burned off. Routing of the methane gas to the waste gas burner is controlled automatically by the gas pressure under the floating dome. If you happen to be near the plant on a hot summer night, you might be able to see the flame from the waste gas burner.




We also have a sludge storage tank used for holding the sludge prior to dewatering. It has submersible mixers to keep the sludge in suspension. This storage tank has an odor control system that draws air from the top of the tank and sends the air to a biofilter. This biofilter has lava rock on the top (visible in this photo). Below the lava rock media, there is a mixture of wood chips and peat moss that foster the development of microscopic bacteria that remove odors from the air. Sludge stored in this tank is drawn off for the dewatering process.

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