All of the solids removed from the wastewater must somehow be disposed of. The sludge that is pumped from the Primary Clarifiers (“raw sludge”) is rich in organic matter and if not properly treated would quickly turn septic (a state of decomposition characterized by strong, foul odors). The sludge also contains many disease-causing microbes, everything from hepatitis-B to dysentery to giardia.(The HIV, or AIDS, virus cannot survive in domestic wastewater.) Thus the sludge must be converted to something safer and stabler before it can be disposed of.
This facility uses anaerobic digestion to stabilize the raw sludge. As the name implies, the process takes place in an oxygen-free atmosphere. The Cortland WWTF digesters consist of three, 36 foot diameter concrete covered vessels. Two of these (on the left and in the back in the photo) are heated to 99º F by spiral heat exchangers and kept fully mixed. These are called Primary Digesters and the heating and mixing are what make this a high-rate system. A two-stage biological process occurs in the Primary Digesters. Acid-forming bacteria convert the raw organic content of the sludge to hydrogen sulfide gas and volatile acids. Then methane-forming bacteria convert these volatile acids into methane, carbon dioxide and biologically stable sludge.
The output of the Primary Digesters is transferred to the Secondary Digester (on the right in the photo). The Secondary Digester is neither heated nor mixed, and very little additional digestion takes place in it. Rather, it acts as a clarifier, the stabilized solids settling to the bottom to be dewatered, and the relatively clear liquid returning to the head of the works to be retreated. The Secondary Digester also has a floating, gas-holding dome cover. Quite a bit of methane is produced in the digestion process. The collected methane is used to fire gas-burning boilers which help to heat the Primary Digesters and the plant buildings.