Eugene/Springfield Water Pollution Control Facility 1984-1991

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a Pttge 18 Section D: PLANT OPBRATIONS ment, flow sampling, grit removal, preaeration, and odor removal. These components provide control and measurement of flow, conditioning of wastewater for further downstream processes, and elimination of odors. • Raw Wastewater Receiving Points Raw wastewater from the collection system enters the pretreatment process at two receiving points. Almost one-third to one-half of the Eugene area wastewater enters the plant through a 72-inch gravity sewer that discharges into the raw sewage wet well where it is pumped to the upper level of the pretreatment structure, allowing it to flow by gravity through the rest of the plant. The second receiving point is on the upper level of pretreatment where two 54-inch pressure mains bring raw wastewater to the plant. These two mains deliver wastewater from areas east of the Willamette River, including Springfield, West Eugene, and River Road/Santa Clara. The force mains save energy because the wastewaterdoes not have tobepumpedagainwhen it reaches theplant. • Raw Wastewater Pumping Raw wastewater is pumped from the raw sewage wet well to the upper level of pretreatment by four 21-MGD screw lift pumps. The pumps have afive- foot-diameter helical screw that transports waste- water at a 30-degree incline. The total vertical lift to the discharge point is 35 feeC. All screw pumps operate at a fixed speed, powered by 200-HP mo- tors. An instrument control system for the pumps has an automatic staging option. When on automatic control, a conductivity strip senses the level of raw sewage in the wet well and determines how many on-line pumps are needed. The pumps were de- signed to turn on and off automatically as the level in the wet well changes. Plant operators have had problems with the automatic staging controls and always control the pumps manually. One problem with the auto controls is that influent gates must remain open to all pumps designated for operation. This means pumps must start at 100 percent flow capacity, causing extreme flow spikes through ev- erydownstream process, resulting in excessive hy- draulicloading. • Comminution The pretreatment process relies on five comminutors that can meat 39 MGD each to reduceparticulate matteranddebris toasizesmaller than 3/8 of an inch. Shredding instead of screening ouC the larger materials eliminates the need to handle and dispose of screened materials. These materials often can be major contributors to odors. Three of the comminutors were installed as new units. The other two were removed from the old Eugene plant and rebuilt for the new operation. The comminutors require frequent repair because of problems such as jamming on cloth-like materi- als, failures in the gear box, and screens bent by rocks. As the Maintenance work section became more familiar with the comminutors, they were able to make adjustments and modifications that reduced some problems. However, many comminutor parts are obsolete and locating parts continues to be a problem. • pH and Combustible CJas Monitoring An in-line pH meter provides continuous moni- toring of influent pH immediately downstream of the comminutors. The pH level is tracked by the plant computer, which sounds an alarm if pH conditions are occurring that could harm the bio- logical treatment process. Operators can then in- vestigatewhether or not the problem is caused by an industrial spill and can decide if it is necessary to isolate incoming flows by channeling them into empty basins. Industrial monitoring staff can also track the influent source if it is caused by an industrial spill. A combustible gas meter was in- stalled to monitor the atmosphere under the cov- ered pretreatment structure for combustible va- Sewage enters the treatment plant through "headworks" containing four screw lift pumps.