Replacement and Upgrade of Two Large Municipal Wastewater Pump Stations

I  Berkeley County, South Carolina  I

Berkeley County Water and Sanitation (BCWS) in South Carolina operates two regional pump stations (PS001, PS002) that together handle 98% of the municipal wastewater upstream of the Lower Berkeley Wastewater Treatment Plant (LBWWTP). The force mains from the two pumping stations, which are interconnected 1.5 miles from the headworks of LBWWTP, represent a complex pumping system handling combined peak flows in excess of 42 million gallons per day (MGD).

When PS001 began experiencing frequent pump failures that necessitated its replacement, rather than a simple change in pumping equipment, the Weston & Sampson Team was selected to study and evaluate the cause of the pump failures and make recommendations for improvements to modernize the entire station.

We began by studying the complex hydraulics of PS001 to develop potential modifications, analyzed hydrogen sulfide (H2S) gas odor emissions, and conducted a comprehensive evaluation of future service area growth. Our subsequent design incorporated several innovative modifications to the existing wetwell and a robust and redundant electrical system to allow the wetwell halves to operate independently for maintenance without the suspension of service. We designed the station with complete redundancy, improving operation and maintenance (O&M) capabilities, eliminating station shutdowns, and increasing electrical efficiency to reduce power costs and provide optimum performance throughout its anticipated lifespan. Our combination of pumps to handle average daily flow and wet weather events reduced annual energy use by 20-30% and lowered O&M costs.

Berkeley County is coastal and can be heavily impacted by winds and rains from hurricanes and tropical storms. Since PS001 is upstream of the Charleston Water System water treatment plant intake, a sanitary sewer overflow would represent a large health risk to the general population of the city. Our robust and redundant design of the electrical system, along with the installation of a sufficiently sized generator, not only help with O&M but also improve climate resiliency and greatly mitigate the potential risk of an overflow. The study and design of PS001, from identifying critical weaknesses and previous design flaws to implementing modern and innovative improvements, has become the new standard for BCWS regional pump stations. The end result of these efforts is an efficient, modern, and maintainable pump station, thoughtfully designed for ease of operation and flexibility within the collection system. Ms. Ashley Yeh, Director of Engineering for BCWS, stated “the study and design of PS001 has become a model for our regional stations, and many of the design features have been replicated to great effect at other large stations.”

PS002 was constructed in the early 90s and conveys 40% of the flow to LBWWTP and required complete replacement, as its service area is rapidly growing and expected to surpass PS001 in volume. With the success of the upgrades to PS001, BCWS entrusted Weston & Sampson with full in-house design and construction services for the replacement of PS002. We performed extensive modeling to confirm the hydraulic interaction between PS001 and PS002. Our influent chamber design allows for entrained air to be released prior to moving to the pump intakes, which helps with pumping efficiency and reduces the likelihood of cavitation and corresponding pump damage. Similar to PS001, we also designed a motor control center (MCC) that is able to take half of the electrical system offline for maintenance and inspection while the other half remains in service. The MCC also boasts an HVAC system that places positive pressure inside the building to prevent H2S gas from intruding into the building.

The upgrades to PS001 and the full replacement of the PS001 and PS002 force mains provided a very complex challenge due to the interconnectivity of the system. Simply taking such a critical component of the county’s wastewater infrastructure out of commission for months or more at a time was not feasible. Making the whole system work through various phases of project design and construction required many levels of detailed study and planning and forward-thinking engineering. The American Council of Engineering Companies (ACEC) of South Carolina recognized the rehabilitation of PS001 with a 2019 “Engineering Excellence Award” for its innovation and thoughtful design.

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