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Post-filter GAC Adsorption

Advanced treatment technologies to deliver high-quality drinking water

  • 36"


  • 4

    GAC Adsorption Units

  • Lima, Ohio

    Lima, Ohio

Helping our client navigate and achieve regulatory compliance

Stantec worked closely with the City of Lima officials to select post-filter, granular activated carbon (GAC) adsorption to upgrade its existing surface water treatment plant (WTP). The addition of GAC enhances finished water quality at the plant and allows the city to consistently meet its water quality goals by addressing periodic taste-and-odor (T&O) issues, preparing the city for continued regulatory compliance and enhancing disinfection efficiency.

The city opted to upgrade its 30-mgd conventional surface water treatment plant primarily due to concerns about meeting the total organic carbon (TOC) concentrations mandated by both the Stage 2 Disinfectant and Disinfection Byproduct Rule and the Long-Term 2 Enhanced Surface Water Treatment Rule. The existing plant employed two-stage treatment—coagulation and softening followed by recarbonation, rapid gravity filtration, polyphosphate addition, and chlorine for disinfection. The addition of GAC adsorption units manages seasonal T&O issues that result from the city’s need to use several large source-water reservoirs, used year-round as a water source of supply, and remove additional TOC, further reducing DBP levels and providing an additional barrier against microbials.

Stantec conducted bench-scale and desk-top studies to determine the best treatment technology for the plant, selecting GAC over intermediate-ozonation combined with biologically-active filtration (BAF) for its effectiveness for DBP precursor removal.

Our team constructed four new GAC adsorption units between the dual-media filters and clearwells at the existing WTP. We enhanced disinfection efficiency by modifying the flow pattern through the existing clearwell system through design of a new low-head pump station. The addition of baffles to some existing clearwells and piping modifications to alter the flow of filtered water through the city’s existing clearwell system resulted in disinfection efficiency—increasing the effective volume factor by minimizing short circuiting and dead spaces. A new 36-inch pipeline connection from the GAC to existing clearwells allows for the efficient and simultaneous use of new and existing clearwells. This modified clearwell arrangement permits individual clearwells to be removed from service in the summer for routine maintenance and mitigates periodic T&O issues that were complicated by the existing clearwell system flow pattern.

At a Glance

  • City of Lima

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