WASTEWATER TREATMENT PROCESSES
Marietta currently has around 80 miles of gravity sanitary sewer lines and 13 lift stations of varying sizes and construction. The Wastewater Treatment Plant is designed for an average daily flow of 3.34 million gallons per day (MGD), 8.25 MGD peak flow, a population of 17,940 persons with a population equivalent of 41,743 persons, a biochemical demand (BOD) of 249 mg/L and loading of 6,966 lb/day BOD and a total suspended solids (TSS) concentration of 156 mg/L and loading of 4,358 lb/day TSS. The City currently has additional wastewater treatment capacity available. The plant is currently under renovation and following construction will have a average daily flow designed for 4.0 MGD.
Flow enters the Wastewater Treatment Plant in a 42-inch diameter line, passes through a 4-inch spaced manually operated bar screen and then through a Wieseman automatic bare screen that removes solids greater than ¼-inch prior to entering a wet well. Three Worthington centrifugal 2,200 gallon per minute (GPM) and one 3,200 GPM pumps send the flow to the grit removal facility where inorganic materials sized to 2.56 specific gravity, 35 mesh are removed by centrifugal acceleration. The Wastetech vortex grit removal system is designed for a maximum flow of 14 MGD. The two Wemco pumps supply constant speed with a classifier and a srew auger to dewater the grit which is then trucked to a sanitary landfill for disposal. The flow next enters the 10,400 gallon preaeration basin with a design detention time of 45 minutes at 3.34 MGD flow and a design aeration rate of 21 cfm/1000 cuft of volume.
The flow proceeds to the three 20 feet’x90’x10’ primary sedimentation basins that have a design detention time of 2.9 hours. This process removes the settleable solids in the wastewater. Primary sludge is routinely pumped to the gravity sludge thickener and then following thickening to the aerobic digestion system by two Waste Corp Mudsucker 50 GPM pumps. The floating scum is pumped by two 150GPM centrifugal pumps to two Don Tech Rotary drum screens with 1/8’ slots for dewatering. The scum is collected and sent to a sanitary landfill for disposal.
BIOLOGICAL ANOXIC SELECTOR SYSTEM
Following primary treatment the flow is delivered to the Biological Anoxic Selector System which is designed to create a high food to microorganism ratio for short periods of time with an anoxic condition in order to select against low food to microorganism filamentous bacteria that previously resulted in poor separation in the final clarifiers including at time violations of the City’s permit to discharge into the Ohio River. Two Flyght mechanical mixers under anoxic conditions mix the primary effluent and return activated sludge. The selector is 13 feet wide by 66 feet 8 inches long with a side water depth of 10.89 feet and an effective volume of 60,000 gallons. The design basis is to create a food to microorganism ratio of 3-8 inverse days to encourage the proliferation of flocculation forming bacteria and to discourage the generation of S. Natans, Type 1701, type 021 N and N. limicola types I and II which previously proliferated in the activated sludge process which were determined to originate from industrial sources and often led to non-compliance with Ohio EPA discharge requirements. Since the selector has been installed there have been zero non-compliance related issues related to this particular problem.
The flow leaves the biological anoxic selector system and enters a flow splitter box which is used to determine which aeration basin(s) it will be applied to for further treatment. The splitter box allows the operator to select what aeration basins are in use and determine if treatment loads require one, two or three aeration basins in parallel or series mode. It also allows the operator to select between conventional activated sludge, modified step fee or contact stabilization modes.
ACTIVATED SLUDGE SECONDARY TREATMENT
Flow from the flow splitter box is routed to three 32’x134’x16’ aeration basins which may be operated in series or parallel flow mode. Bacteria which is grown in the activate sludge process within the aeration basins break down the colloidal and dissolved solids in the incoming wastewater. Aeration is provided by four Hoffman centrifugal blowers rated at 2750 cfm each at a pressure of 7.8 psi and driven by 150-HP motors. Air is evenly distributed in the aeration basins by WYSS tube type diffusers. Each aeration basin has a capacity of 513,000 gallons with a design loading of 30.2 lbs BOD/1000 cuft. The activated sludge process utilizes aerobic and facultative bacteria to break down dissolved and colloidal solids organically and convert the biochemical oxygen demand to a level where it is permitted to be released into the water environment, once the secondary clarification and disinfection has been completed. Return activated sludge is returned to the biological anoxic selector for reintroduction of bacteria to the treatment process and waste activated sludge is pumped to the gravity sludge thickener in preparation for stabilization in the aerobic digestion system.
SECONDARY OR FINAL CLARIFIERS
The activated sludge leaves the aeration basins and flows to the two 66 feet diameter final clarifiers for the purpose of separating the biological sludge from the treated wastewater. The final clarifiers have a depth of 12 feet and a volume of 0.307 MG each with a design detention time of 4.4 hours under average daily design flow conditions. The return activated sludge is pumped with two Worthington 3500 gpm centrifugal pumps driven by 20-HP motors and returned to the biological anoxic selector to supply fresh bacteria to restart the biological process over again. The waste activated sludge is pumped by two Flyght centrifugal pumps to the gravity sludge thickener where it is co-settled with the primary sludge for further processing in the aerobic digesters. The final clarifiers are of the two bro design and do not currently meet the Ten States Standards for Wastewater Treatment Plants and therefore will be replaced with new and larger final clarifiers in Phase III Scope IV renovations.
GRAVITY SLUDGE THICKENER
The gravity sludge thickener is a 40 feet diameter circular tank with an 8 feet side water depth and a capacity of 75,160 gallons. It is designed for a mass loading of 5 gal/sqft/hr. Currently the primary sludge from the primary sedimentation basins and the waste activated sludge are co-settled and thickened to reduce volume in the thickener tank and then the combined sludge is pumped on timers to the aerobic digesters for further treatment prior to disposal.
The final clarifier effluent flows to the Infilco Degremont ultra violet light disinfection system which has a detention time of 15.1 minutes at 8.25 MGD. There are two parallel channels, 20’x28.5’x10.7’ with four modules of 40 bulbs each mounted vertically that disinfect he wastewater prior to final discharge into the Ohio River receiving stream. The City disinfects all year around.
Sludge from the gravity sludge thickener is pumped into the 336,400 gallon primary aerobic digester. The sludge is transferred from the primary digester after the appropriate biological detention time into the secondary aerobic digester for the appropriate biological detention time. From the secondary aerobic digester the sludge is applied to a Flottweg thickening centrifuge to further reduce liquid volume and then transferred to the sludge storage/load out digester where it waits until the plant is ready to dewater it for trucking to a sanitary landfill for final disposal.
The total available digestion capacity is 1,140,950 gallons. The primary aerobic digester is 55 feet in diameter with a 24 foot side water depth and a conical bottom. The secondary digester is a 45-feet diameter circular tank with a water depth of 24.5 feet. The sludge storage tank is a 45 diameter circular tank with a 27 feet side water depth capacity. During the Phase II renovations the former anaerobic digestion system was retrofitted into the current aerobic digestion system which may be operated in batch or series mode. The digesters are mixed with a Vaughn Roto Mix pump mixing system to conserve energy and aerated with Sanitairre fine bubble disc diffusers.
The City utilizes two Flottweg decanter centrifuges, model C5E-4/454 that were installed during the phase II renovation. One centrifuge is dedicated to thickening sludge either prior to dewatering or may be utilized for recuperative thickening in any of the digesters as desired. A liquid cationic polymer is utilized with both the thickening and dewatering centrifuges. The other centrifuge is dedicated to dewatering sludge prior to hauling it to a sanitary landfill for final disposal. The dewatering centrifuge increases the sludge per cent solids from around 3% solids to 26-30% solids which is then hauled in a more solid form to the landfill and reduces sludge disposal costs significantly due to the lower volume of sludge with the higher per cent solids cake. The sludge hauling is currently contracted to an outside hauling provider that hauls it to the Rumpke Beech Hollow Landfill for sanitary landfill disposal which is the least costly disposal method currently available to the City.