Norman Wastewater Treatment Plant, OK, USA

Due for completion in January 2005, the current phase of work at the Norman wastewater treatment plant represents the latest part of a major program of expansion and refurbishment which began in 1998. Ideal Construction were awarded the current contract by Norman Utilities Authority (NUA) in April 2003. Completion had originally been scheduled for August 2004 before being extended in October 2003.

The project involves a series of improvements to the Headworks primary treatment facility and the Westside lift station, chiefly consisting of upgrading solids removal, constructing two new vortex grit chambers, installing influent metering and providing flow splitting facilities. In addition, associated piping, electrical work and instrumentation will also be required.

The current project will add a further $5 million to the $12.5 million Phase 1 cost.

BACKGROUND

A combination of ageing plant, limited capacity, burgeoning population and increasingly stringent regulatory standards necessitated the program of improvement. The original work included the construction of three new aeration basins, a blower building, two new secondary clarifiers and a new sludge pump station, together with upgrades to the electrical system and process control. Phase 1 was completed in May 2000 and increased the plant's hydraulic capacity from 30,000m³/d to 45, 000m³/d. When fully completed, the upgrade program will have further extended this to a final capacity of nearly 100,000m³/d.

The Headworks facility was constructed in 1964 and the improvements required were originally intended to have been included in the earlier work, but funding constraints delayed its upgrade until the current round. The performance of the preliminary treatment will be enhanced, principally by minimizing the abrasive material carried to the downstream processes, which increases wear on the sludge pumps, shortens their life expectancy and increases the frequency of digester cleaning.

Built in 1972, the Westside lift station had reached the limits of its capacity, its pumps being unable to meet the demands of wastewater inflow from the Brookhaven Creek and Normandy Creek basins during times of extreme rainfall. As a result, it was recommended for replacement in the area's Wastewater Master Plan.

These improvements have been partially funded by grants from the Environmental Protection Agency (EPA) through the Oklahoma Water Resources Board (OWRB). In order to obtain this grant funding, the wastewater planning and associated environmental provisions required EPA/OWRB approval, which delayed the commencement of construction by 12 months but ultimately led to the provision of nearly $2.5 million towards the work.

Camp Dresser and McKee were awarded the contract to design the project and on 20 March 2003 bids were received from three contractors; after tender evaluation, NUA awarded the contract to Ideal. Construction began on 8 June 2003 and is now scheduled to be completed by 18 January 2005.

WASTEWATER PLANT IMPROVEMENTS

The work includes the installation of an influent splitter box for the four incoming interceptors, two self-cleaning mechanical screens and conveyors within the existing pump station building, and the construction of two vortex-type grit chambers for sand and gravel removal. An aerated grit removal system had been originally recommended but the alternative was selected for the final design on the basis of lower cost and similar operational performance. Influent flow metering will be fitted to the Westside lift station, along with three centrifugal pumps and two new self-cleaning mechanical screens with screenings conveyors.

Studies have determined that the hydraulic characteristics of the four influent sewers allow for gravity flow directly into the new headworks facility, which eliminates the need for the existing pump station. As a result, the pre-existing screw pumps are to be decommissioned as part of the project and the new influent junction box is to be constructed on the east side of the abandoned structure. Each influent line will be equipped with sluice gates, which will be open under normal circumstances but can be closed periodically to divert the flow from each line through a new 45cm (18in) bypass line to the equalization basins. The splitter box will measure this and divide the flow between the primary treatment trains.

The two new step screens to be installed in the screw pump station structure have been selected because they offer the highest open area of any bar screen (73%) and the lowest loss of head, which is a critical aspect in the overall hydraulic design of the project. In addition, their reliability, low power consumption, reduced maintenance requirement and long life significantly improve operational costs.

The new lift station design is a drywell / wetwell type with three vertical close-coupled centrifugal pumps, equipped with submersible motors and variable speed drives. Provision will be made for a future fourth pump. Two-step screens upstream will exclude debris from the wastewater prior to pumping and a transit-time flow meter on the influent line will monitor the Westside sewer inflow.

Two new submersible pumps, each with a nominal 5.6 million m³/d capacity, will be installed in the existing sump at the equalization basins to return water to the headworks and a new 0.4m force main is under construction to tie into the existing main.

KEY PLAYERS

Norman Utilities Authority own and operate the plant and the Federal Environmental Protection Agency provided funding through the Oklahoma Water Resources Board. Project design was by Camp Dresser and McKee, with Ideal Construction as the main contractor for the current project, BRB Contractors having been responsible for the first phase. Sanitaire provided the aeration system, Lamson the centrifugal blowers, Westech the secondary clarifiers and Fairbanks Morse the RAS/WAS pump station. Electrical and instrumental work was done by Oil Capital Electric and Bauman. Other contractors involved in the project included Midwest Demolition, Stowers, Tarver, American Fence, Omni Mechanical Services, Atlas Foundations, Hansen Masonry, Peterson and Southwestern Roofing.

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