The old influent sewer to the City of Chattanooga's Moccasin Bend Wastewater Treatment Plant included a 1,000-foot segment of 84Ó sewer in a tunnel under Stringer's Ridge. This segment hydraulically limited the existing gravity sewer system such that flows of more than 140 mgd overflowed to the Tennessee River.
The innovative treatment scheme begins at the IRPS. Due to the limited capacity of the influent gravity sewer and the need to expand the existing in-plant pump station for the increasing flows, the decision was made to construct the IRPS on the opposite side of Stringer's Ridge from the treatment plant. (See Figure 1.) The pump station and force main act as a relief pump station, delivering all flows of more than 140 mgd to a new wet weather treatment train at the treatment plant.
Construction of a parallel interceptor in tunnel and an in-plant pump station to handle the increased flows was projected to cost more than $7,000,000. By using the relief pump station concept, more than $3,000,000 was saved.
Some of the innovative features and design considerations of the IRPS include
- Tie-ins to operating 84-inch interceptor sewer,
- Owner procurement of major equipment,
- Unique wet weather screening,
- Remote operation using plant SCADA system,
- Odor control and station ventilation, and
- Simplicity of design.
Tie-ins
The pump station is constructed on the east side of Stringer's Ridge, parallel to an existing segment of 84-inch interceptor sewer. The station is designed so that flow from the existing interceptor sewer is routed through the facility on a continuous basis during normal flow conditions. During high flow conditions, when the existing interceptor sewer through the tunnel is surcharged, the overflow flows through innovative horizontal screens into the wetwell of the pump station. These excess flows are then pumped to the treatment plant's wet weather treatment train.Two major junction chambers were constructed over and around the existing interceptor sewer to provide for this flow-through operation. The upstream junction chamber includes a 126-inch x 144-inch electrically operating aluminum slide gate to divert the flow. The IRPS contains three additional large electrically operated sluice gates for flow control.
Owner Procurement
To control costs and to ensure timely delivery of major equipment items, separate owner procurement of special major equipment items was undertaken before final design completion of the IRPS.Bids on unique horizontal wet weather screening equipment were requested from two potential suppliers of fine bar screens. The pump station design also called for installation of five (with space for two future) submersible wastewater pumps. Several manufacturers could meet the specifications, but final selection would dictate electrical requirements, depth of operation and structural openings.
Procuring these items before completing the final design made it possible to provide bidders with definitive installation requirements, delivery schedules and electrical and structural details. In addition, the City of Chattanooga saved money by minimizing general contractor "mark-up" on this equipment. The total contract cost of the horizontal screens and submersible pumps was $420,000, well below the budget of $800,000 established during conceptual design.
Wet Weather Screening
Two mechanical, fine bar screens with 1ò4-inch (4mm) openings are used to screen the wet weather overflow into the pump station wetwell. The screens are all stainless steel construction, powered by hydraulic drives and sized to handle a flow of 60 mgd each. The design of the pump station provides for discharge of the collected screenings back to the raw wastewater channel where they are directed downstream to the main plant bar screens. Thus, a remote screenings handling facility is not required at the IRPS.Remote Operation
The IRPS is designed as an unmanned facility. Operation is by a programmable logic controller (PLC) that receives information from level monitors, pump status indicators, bar screen controllers and other operating parameters. All operations are transmitted across fiber optic cable to the wastewater treatment plant SCADA system. The plant operator can either monitor the pump station operation in the automatic mode as run by the PLC or exercise manual supervisory control through an operator console located at the treatment plant.Odor Control and Station Ventilation
An activated carbon odor control system is provided to preclude the release of odorous gases from the IRPS. The pump station is operated at a slight negative air pressure by directing all exhaust through a 10-foot diameter, fiberglass reinforced plastic dual bed carbon absorber. The odor control unit is designed for a peak hydrogen sulfide concentration of 22 ppm and VOC concentration of 0.2 ppm, with removal efficiencies of 99 percent and 95 percent, respectively.Simplicity of Design
The goal of the project was to provide a functioning, maintainable, environmentally sensitive pumping station at the least cost to the City of Chattanooga. Regular design meetings were held with the City of Chattanooga supervisors, operators and maintenance staff. An adequate sump pump and washdown system were provided so the wetwell can be completely dewatered and washed down after operating in the relief mode. All miscellaneous metals were specified as aluminum or stainless steel to minimize the need for painting. The site pavement extends beyond the fence line to minimize grass cutting.Split face concrete masonry units and a standing seam metal roof require minimal maintenance yet reflect the park-type setting of the surrounding area. An expensive pump removal system was not installed because the City's current maintenance fleet includes a vehicle with sufficient span and lift to remove the submersible pumps.
