Wastewater Treatment Processes

BRIEF HISTORICAL BACKGROUNDThe City of Marietta first began treating wastewater in the mid 1950’s when sanitary sewer interceptors were installed along the Muskingum and Ohio Rivers and storm sewers were separated from sanitary sewers. The City’s original wastewater treatment plant consisted of primary sedimentation and chlorination prior to discharge to the Ohio River. In the 1980’s, the City renovated the former primary treatment and added activated sludge for secondary treatment. In 2021, the City completed a renovation of the entire wastewater treatment which was a multi-year, multi-scope and phased total renovation.

COLLECTIONS SYSTEMMarietta currently has around 81 miles of gravity sewer collections system and 13 lift stations which convey the wastewater to the treatment plant. The City utilizes a SCADA software system to monitor the status of the lift stations. The City has a collections system crew that maintains and repairs the sanitary sewer collections system. The crew consists of equipment operators, a wastewater service technician and laborers. The department routinely uses backhoes for excavation and repair. Operators also use a vacuum truck and a high pressure sewer jet for unplugging sanitary sewer collections system. A mainline CCTV system and a push CCTV system are used for evaluating the collections system. The City utilizes a web based geographical information system for mapping of the collections system and asset management. Any collection system problems may be reported to the treatment by calling 740-373-3858, ext. 1.

CURRENT PLANT DESIGNThe City’s Wastewater Treatment Plant is designed for a population of 17,490 persons with a population equivalent of 45,146 persons. The plant is designed for an average daily flow of 4.0 mgd (million gallons per day) and a 12.0 mgd peak hourly flow. The design biochemical oxygen demand, BOD, is 226 mg/l concentration and a loading of 7,544 lbs/day BOD. The design TSS, total suspended solids, is 196 mg/l concentration or 6,276 lbs/day TSS. The plant design for ammonia is 26.5 mg/l concentration or 885 lbs/day.

The plant treats wastewater from the City of Marietta as well as some areas in Washington County adjacent to the City. The City has an Industrial Pretreatment Program to protect the collections system and wastewater from wastes that could lead to problems with treatment, cause upset, pass through, interference to treatment or cause damage to the system or treatment plant. The City currently has excess treatment capacity available, has a septic receiving station to process septic waste and is able to treat trucked in waste from smaller area treatment plants.

PRELIMINARY TREATMENTThe flow enters the influent pump station of the plant through a four inch manually operated bar screen into an approximately 70,000 gallon wet well. There is one 2.0 mgd, 34 HP, controlled jockey pump utilized for low flow periods and four 4-MGD, 70-HP controlled non-clog submersible pumps which pump the wastewater from the wet well through the plant for further treatment.

The City utilizes an automatic perforated plate fine screen that removes solids greater than 6 mm or 0.25 inch followed by a grit removal system and then a pre-aeration basin that aids in bio-flocculation of TSS and scum removal. This is considered the preliminary treatment. Screenings and grit are hauled to a sanitary landfill for disposal.

PRIMARY TREATMENTFollowing preliminary treatment is primary treatment which consists of three rectangular tanks of approximately 135,000 gallons per tank. Primary treatment is a physical process which lowers the velocity of the wastewater to 0.03 ft./sec. This typically allows for removal of 90-95% of the total solids, 40-60% removal of the total suspended solids and 10-15% removal of the total solids. There is also 25-35% removal of biochemical oxygen demand. The dissolved and colloidal solids are treated in the activated sludge process. The solids are pumped to the aerobic digestion system for further stabilization. The floatable solids on the primary settling basins are removed by two rotating drum screens and trucked to a sanitary landfill for final disposal.

Flow exiting the primary treatment is next applied to a biological anoxic selector system. The purpose of the biological anoxic selector system is to create a high food to microorganism ratio in the range of 3 to 8 inverse days to encourage the proliferation of flocculation forming bacteria and to discourage the formation of various low food to microorganism filamentous bacteria which do not settle or separate well.

SECONDARY TREATMENTThe City uses the Activated Sludge Process for secondary treatment. There are 3 aeration basins with an individual capacity of 513,000 gallons per tank that allow 9.2 hours of detention at 4.0 mgd of flow. There are 3-150HP turbo blowers that supply compressed air to the membrane disc diffusors in each aeration basin. This City cultivates natural bacteria under enhanced conditions that utilize the dissolved and colloidal organic solids as food in order to reduce the biochemical oxygen demand of the wastewater to the levels required to meet OEPA and USEPA requirements. Some of the bacteria is returned to the process at the biological anoxic selector system and the remainder is wasted to the City’s aerobic digestion system for further stabilization. The liquid in the aeration basins is referred to as mixed liquor which is a blend of microorganisms and the wastes which are consider as food to the bacteria. Basic controls are the oxygen levels, solids concentrations, wasting rates, return rates and time in the system. The City optimizes the controls to allow full nitrification.

The activated sludge process is coupled to two 85 ft. diameter x 15.75 feet final or secondary clarifiers with a volume of 668, 600 gallons each. There are 3 return activated sludge pumps at 700-1500 gpm each which are controlled that supply the settled sludge back for the process or for wasting to the aerobic digestion system. The purpose of the final or secondary clarifiers is to allow separation of the biological solids from the treated effluent.

DISINFECTIONThe City utilizes two non-contact Ultraviolet light modules in two separate channels which are controlled, that allow ultraviolet light disinfection of the effluent prior to discharge to the Ohio River receiving stream. The 254 nm ultraviolet light disrupts the DNA of pathogenic organisms to prevent them from reproducing in the environment. Pathogenic organisms affected are enteric (intestinal) bacteria, viruses and parasites. Diseases of concern that are addressed by disinfection are salmonellous, typhoid fever, paratyphoid fever, cholera, gastroenteritis, e. coli, bacillary dysteria, hepatitis, polio, coxackievirus A and B, echoviruses, adenovirus, and recovirus, among others.

HIGH RIVER PUMPING STATIONThe City has a high river pumping station consisting of a wet well and two 12-inch submersible pumps with 50 HP motors that allows the City to pump flows out into the Ohio River when the river level is over the final outfall to allow full treatment and prevent damage to the plant up to the 100-year flood level.

AEROBIC DIGESTIONPrimary sludge and waste activated sludge are further stabilized by aerobic digestion. The City has 3 aerobic digesters and is capable of digesting a total of 1,110,950 gallons at full capacity. The City utilizes a pumped mixing system for mixing the aerobic digesters as has each digester equipped with an airline, diffusors and mixing pump. Three rotary piston PD blowers supply air to the fine bubble diffusors. The fine bubble diffusors serve as the oxygen source for the aerobic bacteria which further stabilizes the sludge prior to dewatering and sanitary landfill.

The City has two centrifuges. One is for thickening purposes to provide recuperative thickening for the purpose of maintaining the desired conditions of in-tank sludge concentrations and to reduce operating costs. Feed rate for the thickening is in the range of150 gpm and it is capable of handling solid concentrations from 0.5% to 2.5%. The purpose of the dewatering centrifuge is to take feed concentrations in the range of 1.5-4.5% and process in the range of 150 gpm to produce a sludge cake containing at least 25% solids and suitable for landfill.

STAFFING OF TREATMENT PLANTThe operations staff of the wastewater plant consists of twelve employees. The plant is staffed 24 hours a day, 365 days per year.