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Section D: PintvT OP$xnTtON8 Page 21 <br />k <br />,_ <br />~~ <br />The primary sludge pumping system was built with <br />an in-line sonic density meter to continuously <br />monitor the density of thickened sludge. A com- <br />puterprogram isavailable that can scan the meter <br />readings and automatically regulate the sludge <br />pump stroke rates. However, the automatic feature <br />is not used because rags tangling on the in-line <br />probe and accumulated gas bubbles in the sludge <br />affect meter readings. Also, the meter could not <br />sample individual clarifiers. <br />PERFORMANCE: Primary clarifiers in <br />domestic wastewater treatment systems <br />are designed to remove 25 Co 35 percent <br />of the influent BOD and 50 to 60 percent <br />of TSS. Plant clarifiers have consistently <br />achieved an average of 32 percent of <br />BOD removal and 56 percent TSS re- <br />moval. This removal rate is for periods of <br />operation to clarify raw wastewater only <br />and does not take into accountperiods of <br />co-thickeningwithwasteactivatedsludge. <br />The primary clarifiers initially removed scum at a <br />marginally acceptable rate. When rake arms were <br />modified, scum removal improved. The scum is <br />removed from the surface by a rake arm mechanism <br />that pushes the scum up a beach and into a trough <br />that leads to a scum pit. The scum pits are pumped <br />each day during the swing shift. <br />PRIMARY TREATMENT ODOR <br />CONTROL <br />Primary clarifiers have been the source of odors <br />from time to time. Odors have occurred when <br />clarifiers were taken off=line during low summer <br />flows and during the co-thickening of waste acti- <br />vatedsludge. <br />In the past, taking clarifiers off-line to drain and <br />clean them caused odors when operators left the <br />clarifiers on until the settled sludge was pumped <br />out. The process took several days, and clarifier <br />contents became septic. Now operators prevent <br />odors by increasing the sludge pump stroke rates <br />two or three days before taking them off-line and <br />by completely draining them soon after they are <br />shut down. <br />Odors also have been caused in the past when <br />summer flows dipped to 6001iters per second (1/s ), <br />or 12.5 MGD, between 3 and 6 a.m., causing a <br />longer primary clarification detention time and a <br />greater depletion of dissolved oxygen. In 1988, <br />operations began spraying a chemical polymizer <br />agent in the launder area to counteract odors <br />during low-flow periods. <br />Another effective odor control measure has been <br />to throttle the primary effluent gates to raise water <br />levels in the launders of the primary clarifiers. <br />Reducing the distance of the effluent fall over the <br />weirs into the launders has lowered turbulence and <br />minimized odors. <br />Co-thickening of waste activated sludge with raw <br />sludge in the clarifiers also has produced odors <br />during warm weather months. To prevent this <br />problem, operations now stops co-thickening in <br />early spring when raw wastewater temperatures <br />reach 17 C. <br />SECONDARY TREATMENT <br />.Secondary treatment converts the unsettleable <br />fine particulates, colloidal, and dissolved solids <br />into a settleable biological floc. The key compo- <br />nents of secondary treatment are aeration basins, <br />aeration blowers, secondary clarifiers, and return <br />activated sludge pumps. <br />Additional equipment supports and protects the <br />secondary system and is tied into other plant pro- <br />1,000-hp aeration blowers provide oxygen fm- the biological <br />treatment process. <br />