How Rapid Growth in Fort Mill Necessitated Evaluation of New Treatment Technologies


  • David Wankmuller, Paul Mitchell, Zhengzheng Wiley, Jory Wahlen, Anthony D. Greiner, James N. Struve

The Fort Mill Water Pollution Control Plant (WPCP) is designed to treat a permitted capacity of 3.0 mgd based on maximum week flow. Based on the design peaking factors, the resulting annual average permitted flow is 1.9 mgd. Currently the annual average flow is 1.0 mgd but due to the proximity of the service area to the City of Charlotte, annual average flows are projected to almost triple by the year 2024 and increase six-fold by 2044. In addition, the nitrogen loading to the treatment plant has almost doubled over the past ten years (the influent Total Kjeldahl Nitrogen (TKN) concentration increased by 80%), effectively reducing treatment capacity.

As a result, the Town of Fort Mill initiated a master planning effort to evaluate the existing plant and determine the upgrades necessary to increase capacity both near- and long- term. This presentation will discuss the findings of these evaluations.

The existing liquid train includes influent screw pumps, continuous rake screens, aerated grit tanks, intermediate pump station, anoxic tanks, aerobic tanks, secondary clarifiers, and chlorine disinfection. The existing solids train includes gravity WAS thickening and sand drying beds. At the time of the evaluation, the facility was only operating the aerobic tanks, but has the ability to operate an anoxic-oxic process.

The treatment process was modeled and the model calibrated using samples and data collected throughout the process over several days. The model was utilized to evaluate the current process and potential upgrades. The evaluation identified several near-term upgrades as well as prepared a flexible plan for long-term upgrades. Several processes were evaluated for both the liquids and solids treatment trains including; conventional activated sludge, MBR, BioMag, IFAS, GBT/BFP, sludge drying beds, and solar drying beds. Other options included pumping all or a portion of the sewage or the sludge to another utility.

Near-term solutions were developed to address existing aeration and solids handling challenges as well as improve plant electrical reliability. Since the service area is expected to see tremendous growth in such a short time period, a long term, phased approach was developed. The long-term approach looks at four different alternatives including: Conventional biological treatment 2) MBRs and 3) BioMag and 4) Flow diversion to another facility. The phasing allows for maximizing use of existing infrastructure while still allowing for flexibility for a variety of different innovative long-term alternatives including membrane bioreactors (MBRs), BioMag, and conventional treatment technologies.

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