Located in Washington County the Borough of Charleroi is situated on the west bank of the Monongahela River, near Lock No.4. The Borough was incorporated in 1895 and has served as the major marketing, shopping, and professional center for the industrial “Mon Valley” since the turn of the century. Manufacture of glassware (World Kitchen) has historically been the Borough’s primary sources of employment.
Before 1963, all wastes produced in Charleroi and surrounding boroughs were discharged without treatment to the Monongahela River at a point just downstream of Lock No.4. On November 18 1963, the Sanitary Water Board ordered the Borough to take immediate steps toward abatement of the discharge of raw wastes to the river and established November 18,1965, as the compliance date. In compliance with that order, the Borough of Charleroi had plans and specifications for intercepting sewers and treatment facilities prepared and were issued a permit for their construction and operation June 21,1965. However construction was not immediately undertaken, and on November 18,1965, the Sanitary Water Board undertook to enforce its order.
The Authority of the Borough of Charleroi was created in 1942 to finance and operate water treatment and distribution facilities to serve the Borough and its surrounding area. In 1965 the Boroughs of Charleroi, North Charleroi, and Speers commissioned the Authority to arrange the financing of wastewater treatment facilities to serve those boroughs and, in addition, the Borough of Dunlevy.
The Authority believed it was in the best interest of the taxpayers to have the facilities redesigned, and after consultation with the Department of Environmental Resources (DER), authorized The Chester Engineers to undertake the preparation of plans and specifications.
The November 18, 1963 directive called for primary treatment (35% BOD removal) of wastes before their discharge to the river. Plans for facilities to provide that degree of treatment were completed, and an application for a permit was filed with the Sanitary Water Board in September 1966. A permit was issued on February 17,1967.
Realizing that in the not-too-distant future a higher degree of treatment would be required, the Authority undertook on its own to upgrade the degree of treatment. Accordingly, The Chester Engineers was ordered to design additional facilities to provide secondary treatment (85% removal of BOD). In February 1968, plans and specifications for the additional facilities were submitted to the DER with the request to vacate the then existing permit and to issue a new one for the secondary treatment facilities. Permit No. 468-8-021, dated May 8,1968, was issued.
In 1966, the Authority began planning and design to provide for the conveyance and treatment of sewage from Charleroi, North Charleroi, Speers and Dunlevy. Plans and specifications were prepared and construction of a sewage treatment plant, pumping stations and an interceptor sewer began in 1968. The new conveyance and treatment facilities were placed into operation in December 1970. These facilities have been in continuous operation since that time with only minor changes, mostly consisting of maintenance. In 1986, Fallowfield Township completed the extension of a sewer to provide service to a small portion of the township. Since that time sewage service has been extended to approximately 80% of Fallowfield Township residents. The Charleroi sewage treatment plant is designated as the treatment site for sewage collection in Fallowfield Township and all collected flow is treated at the Charleroi plant.
The facilities have two unique features. Except during periods of high river levels, raw wastes enter and flow through the facilities by gravity. During periods of high river levels, the effluent is pumped. The original ground was at the elevation of the existing combined sewer and just above normal water levels in the river. The original clay soils were unsuitable for the support of the structures; and, therefore, regardless of the elevation at which structures would be constructed, piling driven to bedrock would be required.
The cost to excavate the entire site to its original grade was almost offset by the savings in the cost of the additional pile length required to construct the facilities at the existing grade. More importantly, if constructed at the lower elevation, pumping of flow would be required only when the river was in flood stages, holding pumping costs to a minimum.
During 1977, the effluent pumps operated only 41 days and during 1976.43 days. Therefore, pumping costs for these years were only about 10 percent of what they would have been had the facilities been constructed at the higher level. Although gravity flow of effluents is not an uncommon means of reducing pumping costs, the risks of deep excavations to achieve that savings are not usually taken.
Description /The Sewage Treatment Plant
All wastes reach the plant by gravity via a 24-inch influent sewer from a junction manhole on the original combined interceptor.
Wastes enter screen channels through a pneumatically operated sluice gate. The operation of the sluice is controlled by the water level in the grit chamber influent flume. The flow entering the plant is limited to not more than 9.00 mg (3.0 x the dry weather flow). Flows in the interceptor sewer greater than 9.00 mg are diverted to the river via the original outfall over a diversion weir in the junction chamber.
Wastes normally pass through either of a pair of mechanically cleaned bar screens which intercepts and removes the rags and other debris. Screenings are discharged to containers and disposed by land filling.
After passing through the screen, the wastes flow by gravity to a grit removal facility consisting of a mechanical mixer in a conical tank. The grit is removed from the conical tank by a centrifugal pump discharging to a dewatering screen where grit is deposited in containers and water returned to the treatment process. The grit chamber influent and effluent channels are so arranged that the conical tank can be by passed. Screened and degritted wastes then flow by gravity to primary treatment facilities.
The primary sedimentation facilities consist of two rectangular tanks. The heavier solids which settle out in the tanks are crowded to hoppers by conventional flight and chain conveyors. The solids are removed from the hoppers by sludge pumps which discharge them to the gravity thickener.
The light solids which tend to float on the surface or the sedimentation tanks are conveyed to a manually operated, rotating pipe-type skimmer. The skimmer discharges to a hopper in which the grease is concentrated and pumped to the digester or discharged to containers.
The activated sludge process is used for biological treatment of the settled sewage. Aeration facilities consist of four tanks equipped with fine bubble type aerators with air supplied by any one of three electrically driven centrifugal blowers.
The design of the biological process was based on the procedure presented by T. R. Hasetine in “A Rational Approach to the Design of Activated Sludge Plants.” The facilities are arranged so that the conventional, contact stabilization or step-aeration modes can be practiced. Air for the process is furnished by three, variable-speed centrifugal blowers, one of which is solely for standby.
Solids separated from the aerated mixed liquor in the final tanks are discharged to a pumping facility consisting of three centrifugal pumps. The return sludge flows are then pumped to the aeration tanks, and the excess solids, to a holding tank.
The aerated mixed liquor discharges over weirs in the aeration tanks and flows by gravity to four center-feed square final tanks equipped with circular, suction-nozzle-type solids removal equipment.
The treated sewage flows by gravity from the final sedimentation tanks to two chlorine contact tanks. In addition, the contact tanks serve as wet wells for the effluent pumps. Sodium Hypochlorite is dosed to the contact tanks in water solution at controlled rates by variable speed positive displacement pumps located in the administration building.
During periods of normal river water, treated and disinfected wastes flow by gravity to the river. During flood stages, the effluent is pumped to the original outfall.
The effluent pumping facilities consist of three vertical pumps, which take their suction from the chlorine contact tanks. The pump controls are arranged so that when the water level in the chlorine tanks reaches a predetermined level above the weir, the pumps begin operating.
Back flow of river waters through the low-river stage gravity discharge sewer is prevented by a backwater valve and a manually operated sluice gate in the chlorine contact tank’s effluent channel. Storm water runoff from the site is conveyed by gravity sewers to the effluent channel of the chlorine contact tank and either flows by gravity with the treated effluent to the river or is pumped by the effluent pumps.
The primary solids are withdrawn from the sedimentation tank hoppers and conveyed to the gravity thickener by either of two variable speed centrifugal pumps or a variable speed positive displacement pump. Grease removed from the primary sedimentation tanks is pumped to the thickener and/or digesters by a centrifugal grease pump. Excess solids produced by the activated sludge process can be wasted as either return sludge or mixed liquor by gravity to a holding tank. Centrifugal pumps transfer the excess biological solids from the holding tank to the gravity thickener. The operation of the transfer pumps is controlled by the water level in the holding tanks.
The combination of thickened primary and activated sludge solids is transferred by centrifugal pumps from the thickener to either of two primary anaerobic digesters. Supernatant from the digesters flows by gravity to the effluent channel of the primary sedimentation tank. The primary digesters have gas circulation equipment, and their contents are circulated on a time cycle through a heat ex changer by centrifugal pumps. Each primary digester is equipped with a floating cover. Partially digested solids are transferred from the primary digesters to one secondary digester by either gravity or centrifugal and positive displacement pumps. The secondary digester is equipped with a gas collection cover, and the gas is used to heat the primary digester and building.
The digested solids are withdrawn by gravity from the secondary digester to a day tank in which they can be thickened by decanting. Positive displacement pumps transfer the solids from the day tank to the belt filter press. The cake resulting from the dewatering of the solids on the belt filter press is discharged to trucks or containers by a belt conveyor. The ultimate disposal of the solids is to a municipal landfill.
A two-story administration building houses on the upper level an office, laboratory, electric controls, sodium hypochlorite storage and feeding equipment, digester heat ex-changer, and belt filter press. The lower level has the digester pumps and piping, digested solids day tank, a shop area, building heating equipment, locker room, and the screening facilities.
The lower lever is at the same elevation as the top of the treatment tanks. Both levels are accessible by vehicles. Primary sludge pumping equipment is housed in a gallery at the effluent end of the primary tanks, ant all piping, pumps, etc., are housed in interconnecting tunnels between the treatment units. An operator can walk from the administration building to the chlorine contact tank without going outside.
The air blowers and return sludge pumps are housed in a one-story superstructure located between the chlorine contact tanks.
The effluent pumps are in a two-level pump station. The upper level, where pump controls are located, is at an elevation well above the highest river flood level recorded.
A pressure effluent water system supplies wash up, lawn sprinkling, and other non potable water uses. An air-gap pressure seal water system uses potable water as its source of supply. A compressed air system furnishes pressurized air for the operation of air tools, paint sprayers, etc.
The Authority constructed a garage and storage building and a separate building which houses an emergency generator. A Westinghouse security system was installed, which not only protects the plant against unauthorized entry but also permits an operator to immediately alert the control center in the event of trouble. In the event a single operator is on duty, the control center is alerted if the operator activates a remote signaling device. Response by a police car to the system has been less than five minutes.
In 1997 and 1998 the Authority expanded the sewage treatment plant as part of its Corrective Action Plan (CAP). The expanded sewage treatment plant has an average daily permitted flow of 3.0 mgd with a peak hydraulic capacity of 9.0 mg and all process piping throughout the plant have been modified or replaced to be able to convey a minimum peak flow of9.0 mg.
The clarifiers are designed at a peak hour overflow rate of 1,200 gallons per day per square foot (gpdsf). At a peak flow of 9.0 mg and four new clarifiers have been constructed.
The existing clarifiers were converted to new chlorine contact tanks. At the peak flow rate, these tanks are capable of providing a detention time of nearly 28 minutes.
New fine bubble aerators were installed which provide for both aeration and mixing. This not only improves oxygen transfer efficiency but also reduces power consumption costs..