Marietta Wastewater Treatment
Treatment Process
Marietta currently has 79 miles of gravity sanitary sewer lines and 9 lift stations of varying sizes. The wastewater treatment plant is designed for an average daily flow of 3.34 mgd, 8.25 mgd peak flow; a population of 17,940 persons with a population equivalent of 41,743; a BOD concentration of 249 mg/l and loading of 6,966 lb/day; a TSS concentration of 156 mg/l and loading of 4,358 lb/day.
PRELIMINARY TREATMENT
Flow enters the wastewater plant in a 42" diameter line, passes through a 4" manual bar screen and then a Wieseman automatic bar screen that removes solids greater that 1/4" prior to entering the wet well. Three Worthington centrifugal 2200 gpm and one 3200 gpm pumps send the flow to the grit removal facility. The grit removal facility, placed into operation in 2004, provides removal of grit sized to 2.56 specific gravity, 35 mesh, by centrifugal acceleration. The Wastetech vortex grit removal system is designed for a maximum flow of 14 mgd. The Wemco pumps supply constant speed with a classifier and screw auger to dewater the grit which is trucked to a nearby landfill.
The flow next enters the 10,400 gallon preaeration basin with a detention time of 45 minutes at 3.34 mgd flow and a design aeration rate of 21 cfm/1000 cu ft of volume. A Roots blower supplies air at 300 cu ft/min at 7psi to freshen septic wastewater.
PRIMARY TREATMENT
The flow proceeds to three 20'x90'x10' primary settling tanks with a design detention time of 2.9 hours. Primary sludge is routinely pumped to the primary anaerobic digester by two 50 gpm Marlow diaphragm type pumps and scum is pumped by two 150 gpm centrifugal pumps to two Don Tech Rotary drum screens with 1/8" slots for dewatering. The scum is trucked to a landfill.
SECONDARY TREATMENT
Flow from the primary settling tanks is routed to three 32'x134'x16' aeration basins operated in a series flow mode with the first basin operating as an anoxic selector to reduce filaments and recover alkalinity. Bacteria in the activated sludge process break down the dissolved and colloidal organic solids. Aeration is provided by four Hoffman centrifugal blowers rate at 2750 cfm each at a pressure of 7.8 psi and driven by 150 HP motors. Air is evenly distributed in the basins via WYSS tube type diffussors. The activated sludge leaves the bioreactor and flows to the two 66' diameter final clarifiers with a depth of 12' and a volume of 0.307 MG each that have a design detention time of 4.4 hours. Return activated sludge is removed by the Worthington 3500 centrifugal pumps driven by 20-HP motors and returned to the bioreactor to supply fresh, hungry "bugs" to the wastewater. The waste activated sludge is removed by two Worthington 250 gpm centrifugal pumps driven by 2-HP motors pumped to the gravity sludge thickener.
The gravity thickener is 40' diameter with an 8' sidewall depth and has a capacity of 75,160 gallons and is designed for a mass loading of 5 gal/hr/sq ft. The gravity sludge thickener pre-thickens the waste sludge before it is pumped to the primary anaerobic digester by the two Marlow 50 gpm thickened sludge pumps.
DISINFECTION
The final clarifier effluent flows to the Infilco Degremont ultra violet light disinfection system that has a detention time of 15.1 minutes at 8.25 mgd. There are two parallel channels, 20'x28.5'x10.7', with 4 modules of 40 bulbs each mounted vertically that disinfect the wastewater prior to its discharge into the Ohio River. Marietta is required to disinfect all year long.
SLUDGE TREATMENT
Sludge from the primary settling tanks and the gravity sludge thickener is pumped to the 336,400 gallon primary anaerobic digester which is designed for a loading rate of 74.2 lbs volatile solids/day'/1,000 cu ft of volume. Methane gas produced by anaerobic digestion is used to heat the primary digester to 96-98 degrees F and also heats the plant control building. The primary digester is mixed by a Walker Process PEARTH gas mixing system that compresses the gas and releases it through a system of lances in the digester that are alternated in operation and provide a rolling mixing motion to the digester contents. The 314,600 gallon secondary digester is neither heated nor mixed and is used for solids separation. The supernatant is returned to the plant headworks and sludge is transferred to the 426,360 gallon sludge storage tank until it is processed by belt filter press.
Two Moyno progressive cavity pumps rated at 150 gpm and controlled by a variable frequency drive units supply liquid sludge to the two meter Komline Sanderson belt filter press, installed in 2001, at around 83 gpm during normal operations. The press is designed to produce 1,200 lb/hr of cake solids. The pressed sludge cake exits the press onto a serpentine conveyor system and enters the cake storage area. In 2008, the City landfilled 199.44 dry tons of sludge in a sanitary landfill and transferred 103.97 dry tons of sludge to another NPDES facility for further treatment to class A sludge which is an enviromentally responsible "green" option that allows for beneficial reuse.