The Town of Skowhegan WPCF is a conventional /contact-stabilization activated sludge secondary wastewater treatment facility which was designed to treat an average daily flow of 1.65 million gallons per day (mgd), preliminary/primary peak flow of 7.50-mgd and secondary peak flow of 4.68-mgd. Treated effluent is discharged to the Kennebec River.

The upgraded WPCF includes the following features: mechanically cleaned step screen and screenings wash press, grit removal system, septage receiving and handling system, primary clarification, secondary flow regulation, activated sludge secondary treatment, secondary clarification, secondary disinfection, storm flow disinfection, return sludge pumping, waste sludge pumping, scum pumping, sludge storage, sludge dewatering system, lime storage and feed system, and instrumentation and control systems, including Supervisory Control and Data Acquisition (SCADA) System.

The screening equipment consists of a step-type, mechanically cleaned fine screen (¼ inch) and a screenings wash press. A manual bypass bar rack (1½ inch) is provided for use during maintenance of the mechanical unit. All screening equipment, including the system control panel, is located in the Headworks Building. Screenings are raised out of the channel by the step action of the screen and are discharged directly into the screenings wash press. The washed and compacted screenings are discharged in a trash barrel located adjacent to the equipment. The screen may be lifted out of the channel for ease of maintenance.

The Grit Removal system consists of one aerated grit chamber with bypass channel, two positive displacement rotary lobe blowers and one overhead clam-shell bucket with hoist/trolley system. The grit chamber is an approximately 12,000 gallon (1,600 ft3) rectangular tank with a hoppered bottom that extends its length to facilitate collection and removal of the grit. The hopper can hold approximately 210 cubic feet of grit and is protected with steel rails to avoid damage from the bucket.

In aerated grit chambers, velocity is controlled to enable the heavier grit particles to settle out, while lighter organic solids remain in suspension. This control is created by introducing air along one side of the tank at approximately mid-depth in the liquid. The air rising through the wastewater imparts a spiraling motion perpendicular to the direction of flow as it passes through the tank. Grit, as it settles out, is swept into the hopper by this spiraling motion.

The air is supplied by blowers located in the septage receiving station drywell. It is delivered to the grit chamber through a 3-inch supply line which branches into two 3-inch headers at the tank. Butterfly valves in each header allow for the adjustment of air into either header. The normal operating range of air required in the grit chamber ranges from 68 to 135 cfm.

The air is fed into each header through vertical drop pipes connected to the header with quick-type connectors. At the bottom of each drop pipe is a horizontal diffusion header with a row of nonclog air diffusion nozzles located on the upper side. The entire assembly can be lifted out of a chamber through the use of a portable hoist to permit removal of the grit and to allow inspection of the assembly.

The electric hoist consists of two winches with a capacity of 1-1/2 tons, that is mounted on a motor-driven trolley which operates on the I-beam monorail. All of the hoist and the trolley functions are controlled by of the pendent pushbutton control station. The clamshell bucket has a capacity of 1/3 cubic yard, and is provided with drilled holes to facilitate draining water from the grit.

The Septage Handling facilities consist of a manual bar rack, an approximately 9,400 gallon septage storage tank, and a diaphragm pump for metering the septage into the WPCF’s influent.

Junction Chamber No. 1 distributes aerated grit chamber effluent to the primary clarifiers. This structure is also where waste sludge, primary scum, secondary scum, filtrate, floor drainage, tank drainage and on-site sanitary wastewater is discharged from the Operations Building Pump Room. These "recycle" flows are recorded by an electromagnetic flow meter (FE/FIT 50C) located in the Operations Building Pump Room. Junction Chamber No. 1 is equipped with two sluice gates which are used to isolate and control the flows to the primary clarifiers, as appropriate.

The primary clarifiers are two, 40-foot diameter, center feed type, circular clarifiers with 10 foot sidewater depths. The total volume of the two clarifiers is 187,898 gallons (25,120 ft3). At an average daily flow of 1.5 MGD the clarifiers have a detection time of 2.7 hours and an overflow rate of 668 GPD/sq.ft. At a peak hourly flow of 7.5 mgd the detention time is 0.60 hours, and the overflow rate is 3,000 gpd/sq. ft. Flow exits that clarifier through a v-notch weir plate located at the periphery of the tank. Sludge is scraped from the bottom toward a hopper in the center of the tank, and from there it is withdrawn with the primary sludge pumps, located in the basement of the Operations Building . Floating material is removed from the tank by a revolving skimmer arm which deposits the scum into the scum trough. From there it is discharged to the scum well.

Junction Chamber No. 2 distributes primary clarifier effluent to either the Secondary Treatment Facilities or the Storm Flow Treatment Facilities. During dry weather periods, primary clarifier effluent is directed to the Secondary Treatment System. During wet weather periods, the water surface in Junction Chamber No. 2 is surcharged and primary clarifier effluent is directed to both the Secondary Treatment System and the Storm Flow Treatment System. Storm Flow Influent flow is measured at Junction Chamber No. 2 via an ultrasonic flow element.

The Storm Flow Bypass Structure is located between Junction Chamber No. 2, the Secondary Treatment System and the Storm Flow Treatment System and contains the secondary influent flow meter (FE/FIT-10A), the secondary flow control valve (CV-10) and an isolation knife gate valve. The structure also has level sensor LSHH 10A to detect flooding. The secondary influent flow meter measures the flow to the Aeration Tanks and uses this flow signal to actuate the secondary flow control valve to control the flow rate to the Aeration Tanks to an Operator- adjustable valve. Excess flow causes surcharging in Junction Chamber No. 2 and results in flow being directed to the Storm Flow Treatment Facilities.

The WPCF aeration tanks consist of two aeration tanks, four mechanical aerators, three adjustable weir gates, three return sludge piping and mixing boxes, and two dissolved oxygen analyzers. The tanks are 30 feet wide, 55 feet long and 15 feet deep and have a combined capacity of 370,760 gallons (49,500 ft3).

Two dissolved oxygen probes (AE/AIT-30A and 30B) record dissolved oxygen levels in tanks and display send this information to the SCADA system.

One turbidity probe is used to monitor secondary effluent turbidity levels as a measure of overall process conditions under average and peak flow conditions.

Junction Chamber No. 3 distributes aeration tank effluent to the secondary clarifiers. Junction Chamber No. 3 is equipped with two sluice gates (SLG-1 and 2) which are used to isolate or control flow to the secondary clarifiers as appropriate.

Secondary clarification is accomplished with two, 50-foot diameter, center feed type, circular clarifiers with 12 foot sidewater depths. The total volume of the two secondary clarifiers is 352,487 gallons (47,125 ft³). At an average flow of 1.5 MDG the secondary clarifiers have a detention rate of 5 hours and an overflow rate of 432 GPD/sq. ft. At a peak flow rate of 5.0 MGD the detention time is 1.7 hours with an overflow rate of 1,290 GPD/sq. ft. Fluid and solid processes in the clarifiers are similar to that of the primary clarifiers except sludge is withdrawn with the return sludge pumps, and scum is accumulated in the secondary scum well.

Junction Chamber No. 4 distributes secondary clarifier effluent to the Chlorine Contact Tank.

The Town's discharge license requires seasonal (summer) disinfection with an effluent chlorine residual of 1 mg/L or less for both the Secondary Treatment System and the Storm Flow Treatment System.

Secondary Disinfection System
Sodium hypochlorite is injected into the disinfection mixing chamber of the Secondary Chlorine Contact Tank (CCT), where it is mixed with secondary clarifier effluent. Mixing is provided by a mechanical (flash) mixer. The Secondary CCT has two parallel trains, each providing 12 minutes of detention time during the peak hourly flow of 5.00-mgd provided both tanks are on-line (if the 5MGD was passed through just 1 tank then detention time would be cut to 6 minutes). Sodium bisulfite, if required, is injected into the Secondary CCT just upstream of each of the two effluent weirs, where it is mixed with final effluent. Mixing is provided by turbulence generated by a vertical free fall. Final effluent discharges to the Kennebec River through a 24-inch outfall pipe.

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Sodium hypochlorite is injected into the disinfection mixing chamber of the Storm Flow CCT, where it is mixed with primary clarifier effluent. Mixing is provided by a mechanical (flash) mixer. The Storm Flow CCT has one train, providing greater than 15 minutes of detention time during the peak hourly flow of 2.50-mgd. Sodium bisulfite is injected into the dechlorination mixing chamber, where it is mixed with storm flow effluent. Mixing is provided by a mechanical (flash) mixer. Storm Flow effluent discharges to the Final Effluent pipe through an 18-inch pipe.

Sodium Hypochlorite Storage and Pumping
Sodium hypochlorite is delivered as a bulk liquid and is stored in two, 1,200 gallon storage tanks located in the Operations Building Chemical Room. The tanks are equipped with ultrasonic level elements which are indicated at the Chemical Fill Panel and at the SCADA System. There are four, electronic metering-type sodium hypochlorite pumps, each rated for 0.3 to 25.0 GPH at 30 psi (maximum). Each pump’s primary duty is as follows: HYP-1, return and waste sludge chlorination; HYP-2, secondary treatment chlorination; HYP-3, secondary treatment chlorination; and HYP-4, storm flow treatment chlorination. All four pumps are connected to a common discharge header and can be used in the other applications if necessary.

Sodium Bisulfite Storage and Pumping
Sodium bisulfite is delivered and stored in 55 gallon drums. The sodium bisulfite for the Secondary Treatment System and the Storm Flow Treatment System is located in Dechlorination Building No. 1 and No. 2, respectively. There are two, solenoid-type sodium bisulfite pumps used to dechlorinate the Secondary Treatment flow and the Storm Flow Treatment flow.

The Scum Handling system consists of a primary and secondary clarifier scum arm and trough, primary and secondary scum wells, primary and secondary scum mixers, and primary and secondary scum pumps.

Scum created in the primary and secondary treatment processes is collected and directed from the respective clarifiers to one of two scum wells. Primary Clarifier scum is collected in the scum well on the west side of the Operations Building. Secondary Clarifier scum is collected in the scum well located on the east side of the Operations Building. Clarifier skimmings flow from the clarifiers into the respective scum wells.

Scum in the scum well is mixed by the dedicated mixer and then pumped from the respective scum well to Junction Chamber No. 1 by the respective scum pumps.

The Return Sludge Pumping system consists of three pumps, two principal pumps and one spare pump which also serves other functions. The return sludge pumps convey activated sludge from the bottom of the secondary clarifiers back to the aeration tanks. All three return sludge pumps are vertical, non-clog, screw impeller, centrifugal pumps, rated for 750- GPM at 35-feet TDH.

Though the RSL/DW-1 can pump return sludge if needed, it is also used to pump wastewater from the Operations Building Wetwell back to Junction Chamber No. 1. Flows to the Wetwell include filtrate, floor drainage; tank drainage and on-site sanitary wastewater. When pumping from the Wetwell, RSL/DW-1 is typically controlled by the level in the wetwell, as measured by a submersible level transducer. A LOW-LOW level float switch, also in the wet well, will disable the pump if the water level drops to its set point.

The Waste Sludge Pumping system consists of one pump (WSL-1) that pumps secondary sludge from the bottom of the secondary clarifiers to either the Sludge Storage Tank or to Junction Chamber No. 1. The waste sludge flow rate is measured by an electromagnetic flow meter. The waste sludge pump is a vertical, non-clog screw impeller centrifugal pump, rated for 150-GPM at 25-feet TDH (Sludge Storage Tank) or 125-GPM at 8-feet TDH (Junction Chamber No. 1). The waste sludge pump is manufactured by WEMCO-Hidrostall (Model D3K-S-DOS with 7.875 inch impeller).

The Sludge Storage Tank is an 80,000 gallon tank used to store waste sludge. Sludge is pumped to the tank by the waste sludge pump. The waste sludge aeration system includes low-pressure, coarse bubble-membrane diffusers and a rotary lobe, positive displacement blower. The diffused aeration system consists of 69, 9-inch diameter, EPDM membrane disc diffusers with a 13/64th inch orifice. The blower is located in the Garage Equipment Room and is rated for 150 to 456 ICFM at 7.5 psi (maximum). Tank levels are measured with a submersible level transducer that sends the information to the SCADA system. A high level float switch is interlocked to shut down WSL-1 to protect against overfilling the tank. The Operator also has the ability to decant supernatant from the tank with a telescoping, rack and pinion type decant valve equipped with scum baffle. The decant supernatant is discharged to the Operations Building Wetwell via an 8-inch decant line.

Sludge pumping is accomplished with three, positive displacement, duplex plunger pumps. Two pumps are used to pump primary (or co-settled) sludge from the primary clarifiers to the Belt Filter Press (BFP). The third pump is used to pump secondary sludge from the Sludge Storage Tank to the BFP. All three pumps are located in the Operations Building Pump Room and have two, 9-inch diameter plungers. Pump capacity ranges from about 0.492 gallons per revolution to 3.854 gallons per revolution. All three pumps are equipped with sampling valves on both their suction and discharge sides, as well as high pressure switches to shut down the pump in a "dead-head" situation. Sludge flow is measured by two 4-inch electromagnetic flow meters located in the Dewatering Room. The sludge pumps are manufactured by Komline-Sanderson (Model KS-9-2).

The Sludge Dewatering system is located in the Operations Building Dewatering Room on the upper level. Sludge dewatering is accomplished using a belt filter press with polymer sludge conditioning, a polymer mixing valve, and a sludge mixing valve. Dewatered sludge is conveyed using a shaftless screw conveyor. A potassium permanganate System is also available to help control odors. All Sludge Dewatering system components are located in the Dewatering Room.

The BFP is manufactured by Komline-Sanderson (Model GRS-2 Series III). The BFP has an active width of 2-meters, a 70-ft2 gravity zone, a 40-ft2 wedge zone and a 129-ft2 pressure/shear zone. The belt is monofilament polyester, modified satin weave. The BFP requires a minimum effluent water flow of 80 GPM and 85 psi. The polymer and sludge mixing valves were provided by Komline-Sanderson. The valves are in-line, adjustable, non-clog venturi-type mixing valves with a 4.50-square inch throat and with 6- inch end connections.

The dewatered sludge cake conveyor is a shaftless screw conveyor manufactured by JDV Equipment Corporation. The conveyor is 14 ft long and set at an angle of approximately 13 degrees.

The polymer system is a mechanically-mixed blending unit manufactured by USFilter-Stranco, (PolyBlendModel M601-D2.5AB, SP60101739). This unit feeds neat emulsion polymer into a mixing chamber using an electronic metering pump rated for 0.13-GPH to 2.5-GPH at 100 psi (maximum). The metering pump is manufactured by Liquid Metronics Inc. (Model B921-86PBA). The polymer System is provided with a dilution water supply via the Effluent Flushing Water system. The polymer system requires up to 600-GPH at less than 100 psi and provides for Primary Dilution (at 0 to 300-GPH) and Post Dilution (at 0 to 300-GPH). Adequate blending and dilution of the polymer is critical to dewatering performance.

The potassium permanganate system is a semi-automatic make-down system which consists of a potassium permanganate saturator tank, make-up water system, and two metering pumps. The 55-gallon saturator tank has a minimum and maximum capacity of 150-lbs to 350-lbs, respectively, of potassium permanganate crystals. The make-up water system is provided with a water supply from the Effluent Flushing Water system. The make-up water system requires up to 2-GPM at up to 50 psi and the system automatically adds water using a float/solenoid valve. The solenoid-type metering pumps are rated for 0.008-GPH to 8.0-GPH at 60 psi (maximum) and are manufactured by Liquid Metronics Inc. (Model B931-410SI).

The Lime Storage and Feed system is located in the Lime Silo (exterior) and in the Operations Building Dewatering Room. The Lime Storage and Feed System consists of a sludge lime blender, a lime day bin and appurtenances, a bulk lime storage silo and appurtenances, and means of conveyance. Lime is added to the dewatered sludge cake to stabilize the sludge prior to land application (Class B biosolids). All of these system components were provided by the Chemco Equipment Company.

Lime Silo
Bulk hydrated lime is delivered and stored in the Lime Silo (manufactured by the Pittsburgh Tank Corporation). The Lime Silo is designed to hold 1,600 ft3 of hydrated lime with a bulk density of 35 lb/ft3. Major appurtenances include:

• Lime Fill Panel located on the Lime Silo exterior.
• Dust collector (Model No. CECDC300) with a Cincinnati Fan and Ventilation Co. blower (Model PB-10A) rated for 647 CFM.
• Bin activator with vibratory motor.
• Manual 8-inch knife gate valve (DeZurik Model No. KGL-8).
• Heavy duty rotary airlock feeder (Meyer 6-inch x 6-inch) rated for a maximum of 1,000 lbs/hr.
• Positive displacement rotary lobe blower (Tuthill Pneumatic Model Competitor + 4005) rated for 360 ICFM at 1.5 psig discharge pressure and used to convey the lime from the Lime Silo to the Lime Day Bin.
• Two stage reciprocating vertical air compressor (Ingersoll-Rand Model C1053080VA) with an 80 gallon compressor tank and maximum air delivery of 17.4-CFM at 175 psi or 18.8-CFM at 90 psi.
• Solids flow switch (Monitor Technologies Model No. SDF-2) used to monitor lime flow from the Lime Silo to the Lime Day Bin.

Lime Day Bin
Lime is conveyed from the Lime Silo and stored in the Lime Day Bin (manufactured by Pittsburgh Tank Corporation). The Lime Day Bin is designed to hold 66 ft3 of hydrated lime with a bulk density of 35 lb/ft3. Major appurtenances include:

• Bin activator with vibratory motor.
• Pneumatic operated 8-inch knife gate valve (DeZurik Model KGL-8).
• Volumetric screw feeder (Chemco Model No. MH-2 2”) rated for a maximum of 546 lb/hr.
• Screw conveyor (Goodman - 6" Diameter Conveyor) rated for 550 lbs/hr. The conveyor is also equipped with a manual 4-inch knife gate valve (DeZurik Model KGL-4) that can be used collect lime from the system.

Sludge/Lime Blender
The lime is mixed with the dewatered sludge cake in the sludge/ lime blender. The blender is manufactured by Willowtech (Model D-270) and is rated for 1,500 lbs/hour continuous throughput capacity. Additionally, the blender is equipped with a Pepperl+Fuchs proximity switch (Model No. NBN-4-12GM60-WS). The switch is a safety feature which will prevent operation of the unit if the blender cover is removed.

The Storm Flow Treatment system allows for the WPCF to process flows of up to 7.5-MGD through the preliminary and primary unit processes during wet weather. Up to 5.0-MGD of that will be treated in the Secondary Treatment System, with the remainder being treated in the Storm Flow Treatment System. Secondary Effluent and Storm Flow Effluent (i.e. disinfected primary effluent) is blended in a sewer manhole in the outfall line, outside of the WPCF fence line, and is discharged to the Kennebec River as was approved and licensed by Maine Department of Environmental Protection. The components of the Storm Flow Treatment System include: the primary clarifiers, Junction Chamber No. 2, secondary influent flow meter, secondary flow control valve, chemical pumps, the Storm Flow Chlorine Contact Tank and mechanical mixers, Dechlorination Building No. 2 and Storm Flow Effluent Sampler.

Effluent Flushing Water System
The Effluent Flushing Water system provides chlorinated final effluent from the Chlorine Contact Tank for various on-site uses including: plant wash water, carrying water, equipment process water (belt filter press, polymer, permanganate, mechanical screen and wash press), and seal water. Components of the system include two horizontal, non-clog, centrifugal pumps rated for 150-gpm at 260-feet TDH, a discharge electromagnetic flow meter, discharge pressure element rated for 0 to 150-psig, suction differential pressure element rated for 0 to 200-inches water column (used to measure the pressure differential across the basket strainer), a 6-inch duplex basket strainer, and a 40-gallon hydropneumatic tank (ASME Rated to 150 psig).

The Effluent Flushing Water Pumps are manufactured by Gould’s Pumps (Model 3756L with a 7-¾-inch impeller). The 3-inch electromagnetic flow meter is a Rosemount (Model 8705-P). The pressure transducer is manufactured by Endress+Hauser (Model No. PMP41-PC23PBJ21G1). The differential pressure transducer is manufactured by Endress+Hauser (Model No. PMD 235-WU4F9EHID). The duplex basket strainer is manufactured by Hayward (Model 50). The hydropneumatic tank is manufactured by CompTec Composite Water Systems (Model CPV-40T).

The Foam Spray system provides final effluent from the Chlorine Contact Tank to the foam spray header piping within the aeration tanks for foam control. The system consists of two horizontal, non-clog, centrifugal pumps, foam control piping around the perimeter of the aeration tanks and a series of spray nozzles (approximately every 5 feet). Each tank's foam spray can be regulated by gate valves located just inside their west wall. This System is cross-connected to the Effluent Flushing Water system via a ball valve in the basement located in the piping between the two sets of pumps.

There are four automatic sampling locations at the WPCF, as follows:

• AS-1 (Influent Sampler): Headworks Building , downstream of mechanical screen.
• AS-2 (Effluent Sampler): Dechlorination Building No. 1, downstream of dechlorination.
• AS-3 (Primary Effluent Sampler): Junction Chamber No. 2, process control samples.
• AS-4 (Storm Flow Effluent): Dechlorination Building No. 2, downstream of dechlorination.

All of the automatic samplers are manufactured by American Sigma (Model 900 Refrigerated Sampler).