Eugene/Springfield Water Pollution Control Facility 1984-1991

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-•i L Page 26 Section D: PLANT OPERATIONS • Co-thickening Operators have not used the gravity thickener for waste sludge thickening. Instead, they have used a process of co-thickening with raw sludge in the primary clarifiers. This process was not planned for in the new facility. It was a solution that reduced costs by avoiding the use of the flotation thickener. The process cannot be used in warm weather months because of odors produced from gassification. PERFORMANCE: Co-thickening, lim- ited to use during cold wastewater tem- peratures (<17 C), has proved a better and less expensive method than the flo- tation thickener process. The average concentration of co-thickened waste sludge and raw sludge has been greater than that produced by the flotation thick- ener. Thicker sludge has meant less vol- urne,allowing a longerdetentiontime in the digesters and greater operating effi- ciency. SLUDGE HANDLING Solids are reduced and stabilized through anaero- bic digestion. The process has consistently re- duced solids by more than 30 percent. Sludge holding tanks receive and store digested sludge until it is pumped to the Sludge Management Facility. The tanks also provide storage for meth- ane gas. The complex also houses the methane- powered co-generation equipment, which offsets energy costs. A boiler serves as backup to the engine generators to meet plant water heating needs. • Digesters The anaerobic digestion process uses three gas- mixed digesters and two sludge holding tanks. Each digester holds one million gallons. Digesters are operated at a temperature range of 95 to 100 degrees Fahrenheit to maintain temperature re- quirements for mesophilic bacteria. Three spiral heat exchangers maintain the temperature. Raw primary sludge and thickened waste-activated sludge are fed to each digester alternately at con- centrations of three to six percent solids at a rate of 10 minutes each. Operators monitor the diges- tionprocess to maintain desired temperature, pH, volatile acid content, and alkalinity. Digester gas productionandcarbondioxide contentaretracked daily. The sludge digesters require cleaning every three years. By contracting out the cleaning, the plant saves money in staff time and equipment down- time and reduces the cleaning time to one week, causing minimal disruption to the digestion pro- cess. The waste material removed from the digesters during cleaning was originally pumped to the plant sludge lagoon. The waste contains a considerable amount of hair and plastics, which must be shred- ded before it is removed from the lagoon. Shred- dinghas not always sufficiently reduced the size of plastics, so manual skimming has been used to remove them from sludge targeted for the land application program. PERFORMANCE: The anaerobic di- gesters have performed at an acceptable level byreducingvolatile solids atleast 38 percent -the level required for land application. The plant's average reduc- tionhas been 51 percent. Digester gas has produced enough fuel to operate one en- gine generator full time. • Sludge Holding Tanks The two sludge holding tanks have capacities of 360,000 gallons each. During the construction of the new plant, both tanks were retrofitted with floating domes, each able to store up to 25,000 cubic feet of gas. The gas storage allows the enginators to operate continuously, which makes up for fluctuations in digester gas production. Ex- cess gas is sent to the waste gas burner through a pressure control valve, where it is burned off. Sludge is now pumped each day from the plant's holding tanks to the Sludge Management Facility. Digested sludge is further stabilized in sludge la- goons before land application. Before the new facility was completed, sludge was pumped to a temporary sludge lagoon at the east end of the plant.