New Search
Section D: PLANT OPfiRATIONS <br />Page I9 <br />pors. If dangerous concentrations of combustible <br />vapors occur, the meter triggers alarms so operators <br />can implement safety measures. <br />• Flow Measurement <br />Four Parshall flumes allow operators to measure <br />influent flow immediately before grit removal. <br />Any combination of up to four independent trains <br />may be used at this point in the process. Each train <br />consists of a Parshall flume, a grit removal cham- <br />ber, and a preaeration chamber. <br />The Parshall flumes have gauges that measure up <br />to 65 MGD. The original flow measurement equip- <br />ment used conductivity strips sensors. The strips <br />.didn't allow for accurate flow measurement be- <br />cause of the turbulence along the sides of the <br />flumes, so they were replaced with pressure trans- <br />ducers. <br />• Grit Removal <br />The process removes many of the inorganic mate- <br />rials, such as gravel, sand, and eggshells, from raw <br />wastewater before it undergoes further treatment. <br />Removing the grit prevents excessive wear of pumps <br />and other equipment. The grit removal process <br />consists of a mechanically cleaned grit chamber, <br />grit slurry pumps, and grit classifiers. Grit that has <br />beenremoved is disposed of at the ShortMountain <br />Landfill. <br />Influentflow enters the grit chambers immediately <br />downstream of the Parshall flumes. The four grit <br />chambers are equipped with aeration devices and <br />baffles to enhance grit settling. The wastewater is <br />aerated by four 150-HP aeration blowers. <br />Settled grit is mechanically removed by a longitu- <br />dinal collector and dumped into a hopper. This <br />slurry is pumped by Marlow grit pumps to one of <br />four grit cyclones. The cyclones separate organics <br />from the grit. The discharged gritfalls into two grit <br />classifiers where volatile matter is washed away. <br />The grit is transferred to a 10-yard dump truck and <br />taken to the Short Mountain Landfill weekly. <br />PERFORMANCE: The average re- <br />moval rate for aerated grit removal sys- <br />tems is 1 to 10 cubic feet per MG of flow. <br />The average amount of grit removed in <br />our aerated grit removal system since <br />June 1986 has been 23 cubic feet per day <br />or 0.75 cubic feet per 1 MG of flow. <br />In addition to grit removed by the aerated grit <br />removal system, an estimated 500 to 800 cubic feet <br />of grit is removed each year from the raw sewage <br />screw pump wet wells. Also, 1,400 to 2,200 cubic <br />feet of grit is removed each year from the lagoons <br />as a result of digester cleaning. This means that. l8 <br />to 26 percent of the grit does not reach the removal <br />system. <br />• Preaeration <br />The last step of the pretreatment process is <br />preaeration.Two 150-HPblowerssupplyair(which <br />provides additional dissolvedoxygen) to thewaste- <br />water before it enters the primary clarifiers. The <br />aeration also helps remove odorous gases from the <br />raw wastewater. <br />The original system was equipped with coarse air <br />bubble diffusers, and later one chamber was retro- <br />fittedwith fine air bubble diffusers. The fine diffus- <br />ershave ahigher oxygen transfer efficiency but, at <br />the plant, have had a greater tendency than the <br />coarse diffusers to accumulate rags. So, any opera- <br />