Lower Allen Township WWTF Gravity Thickener Improvements

Key Accomplishments

• Rehabilitated two existing 40-foot-diameter gravity thickeners, each enclosed by a fiberglass dome, while maintaining operation of the active Lower Allen Township Authority wastewater treatment facility.
• Removed the existing drive motors and gear-drive units from Gravity Thickener No. 1 and No. 2 and installed new motors and gear-drive assemblies supporting the existing bottom scrapers, center wells, and skimmer systems.
• Removed and replaced the center column in each thickener and modified the existing steel center cages and walkway deck plates to accommodate the new drive assemblies while maintaining future maintenance access.
• Replaced the existing sludge feed piping within each center column with new 10-inch Schedule 40 stainless-steel sludge feed lines.
• Added a second scum skimmer arm to each thickener, modified and rebalanced the skimmer assemblies, and installed new stainless-steel bottom scraper squeegees.
• Sandblasted and recoated the carbon-steel thickener equipment, including rake arms, center cages, center columns, drive units, influent wells, walkways, decking, guardrails, skimmer arms, and supports.
• Removed and reinstalled the existing access bridges as necessary for drive replacement, including coordination of bridge-mounted electrical conduit, wiring, lighting poles, local controls, and spray-nozzle system components.
• Coordinated work so only one gravity thickener was out of service at a time, with each shutdown limited to a maximum of 45 calendar days after draining. The first unit was required to operate successfully for at least 48 hours before the second thickener could be taken offline.
• Maintained operator access and existing wastewater-treatment operations throughout construction, including protection of fiberglass dome panels, existing tank coatings, underground utilities, overhead odor-control ductwork, and electrical infrastructure.
• Completed equipment startup and performance verification through torque-switch testing, dry and wet mechanical operation, adjustments to drive mechanisms and skimmers, a 48-hour operating test under sludge flow, and independent vibration testing of the new drive units.
• Coordinated manufacturer's field services for installation inspection, startup, performance testing, and instruction of the Owner's operating personnel, along with equipment certification and O&M documentation.
• Johnston Construction Company served as Prime Contractor for the Lower Allen Township Authority Wastewater Treatment Facility Gravity Thickener Improvements project in New Cumberland, Pennsylvania. The project focused on comprehensive rehabilitation of the facility's two existing gravity thickeners to improve the reliability and maintainability of the sludge-thickening process.
• Additional process improvements included replacement of the existing 10-inch sludge feed lines with Schedule 40 stainless-steel piping, installation of additional scum skimmer arms, rebalancing of the skimmer mechanisms, and replacement of the bottom scraper squeegees with stainless-steel components. Torque-overload protection and alarm functionality were incorporated into the replacement drive systems, with electrical equipment required to maintain the appropriate Class I, Division 1 rating.
• A major project challenge was performing the rehabilitation while keeping the active wastewater treatment plant operational. Only one thickener could be removed from service at a time, and the specifications limited each outage to 45 calendar days after draining. All major equipment and materials had to be onsite before an outage began. Once rehabilitation of the first thickener was complete, it had to demonstrate at least 48 hours of successful operation before work could begin on the second unit.