Hagerstown WWTP – Sludge Storage & Digester Conversion Improvements

Key Accomplishments

Johnston Construction Company

• JCC retained and coordinated GHD, Inc. as its Design Engineer for the engineering portions of the Design-Build scope and participated in initial, 30%, 60% and 90% design workshops with Synagro and the City of Hagerstown.
• Provided final design drawings and specifications along with as-built documentation, equipment information, project scheduling and operations and maintenance documentation as part of the completed Design-Build system.
• Converted the existing digester into an approximately 243,000-gallon sludge storage tank, supporting a two-tank system with approximately 486,000 gallons of total sludge-storage capacity.
• Performed inspection and evaluation of the existing tank to determine the work necessary for conversion to long-term sludge-storage service.
• Cleaned and rehabilitated the existing tank and repaired concrete cracking and spalling where required by the approved Design-Build scope.
• Removed and disposed of the existing tank cover and provided a new aluminum dome-type cover complete with access provisions, ventilation, odor-extraction connections and associated accessories.
• Installed a new sludge aeration and mixing system consisting of two 75 HP positive-displacement blowers, each designed for approximately 1,150 SCFM at 8.3 psig, complete with sound-attenuating enclosures and associated accessories.
• Installed a new stainless-steel coarse-bubble diffuser system within the converted sludge holding tank to provide continuous mixing and aerobic conditions.
• The aeration system incorporated approximately 52 coarse-bubble diffusers, together with manifolds, headers, drop piping, supports and associated structural components.
• Installed associated 8-inch ductile-iron process-air piping, butterfly valves, fittings, supports, wall penetrations, insulation and connection components serving the blower and diffuser systems.
• Core-drilled existing concrete tank walls and installed corrosion-resistant penetration systems incorporating Type 316 stainless-steel components and mechanical link seals.
• Water-blasted and recoated the existing ductile-iron sludge feed and withdrawal piping and replaced associated piping supports as part of the tank conversion.

Electrical Services Division

• Electrical Services Division, LLC served as the Trade Subcontractor supporting Johnston Construction Company’s Design-Build conversion of Secondary Digester No. 2 into an additional sludge holding tank at the Hagerstown Wastewater Treatment Plant.
• Performed the electrical power, control and instrumentation work associated with the new sludge-storage aeration system, including two new 75 HP positive-displacement blowers and their VFD control systems.
• Demolished the existing Panel DGA and installed a new 600A, 480/277V, NEMA 12 power panel, including a 600A main breaker with adjustable trip and dedicated breakers serving the two new blower systems.
• Installed an aluminum equipment backboard in the blower area to support the two VFD blower control panels and associated safety disconnect switches.
• Furnished and installed conduit, conductors and electrical connections to provide 480V power from the new distribution panel to both VFD blower control panels and the two 75 HP blowers.
• Furnished and installed two 100A, 3-pole, NEMA 12 safety switches serving the new blower installations.
• Extended 120V control and auxiliary power to equipment including the blower-enclosure exhaust fans and the sludge-storage level transmitter.
• Installed control wiring between the blower control panels and field devices including differential-pressure switches, high-discharge-pressure switches, high-discharge-temperature switches and motor-winding-temperature switches for both blower systems.
• Provided conduit, wiring and connections from the blower control panels to the plant’s existing PLC for alarm and operating-status interfaces.
• Installed instrumentation wiring between the tank-mounted level sensor and level transmitter for continuous sludge-storage level measurement.
• Provided the 4–20 mA level signal interface from the new level transmitter to the existing PLC, supporting integration of tank-level monitoring into the plant control environment.
• Supported PLC/SCADA integration by providing the field conduit, wiring and signal connections required for blower alarms and tank-level monitoring. PLC and HMI software configuration remained outside ESD's subcontract scope.
• Removed existing conduit and wiring associated with the obsolete mechanical level meter as part of the conversion to the new electronic level-monitoring system.
• Coordinated electrical work with the new mechanical process equipment, blower systems, instrumentation and existing Digester Control Building infrastructure.