What Does Carbon Really Cost? Case Studies for Implementing GAC at Existing WTPs
- Maggie Pierce, David Briley - Hazen and Sawyer
Granular activated carbon (GAC) adsorption is an effective process for DBP control, and its use is increasing as water systems grapple with Stage 2 Disinfectants/Disinfection ByProducts Rule compliance. GAC is effective not only for DBP precursors, but also taste and odor compounds and emerging contaminants. These ancillary benefits make GAC a desirable choice when free chlorine is used for secondary disinfection. GAC can be regenerated once the adsorption sites have been exhausted, making it a more sustainable process that other options for DBP control. There are numerous considerations that should be addressed to ensure successful implementation of a GAC adsorption process at an existing WTP. This paper will focus on several case studies to present considerations, challenges, and opportunities for implementing GAC adsorption process for DBP control.
Design considerations such as empty-bed contact time, carbon selection, and GAC change-out frequency should be evaluated thoroughly. Impacts to the existing treatment facility such as chemical systems, hydraulic profile, and disinfection must be analyzed for compatibility with carbon treatment. At an existing WTP, the new GAC facility must be integrated into the existing processes. The WTP hydraulic profile must be evaluated thoroughly. Often, intermediate pumping is required due to the significant headloss through a GAC process. However, considerations for pump control for flow management through the WTP are critical. Facility footprint and space constraints can also be a significant challenge. Case studies will be presented on siting GAC facilities on sites with limited footprint and impacts to facility design as well as tanker truck access for carbon change outs.
Carbon selection is essential and requires pilot testing and/or bench scale testing to assess alternate carbon types, alternate manufacturers, empty bed contact time (EBCT), performance (DBP precursor removal), and bed life.
The capital and operating costs of GAC adsorption for DBP compliance will be presented for a range of WTP capacities. A unique process modeling approach has been developed to provide insight into water quality objectives, risks, and associated costs of compliance with GAC usage. There are also unique approaches to utilize a partial treatment strategy by blending GAC effluent with filtered water to optimize capital and operating costs, GAC bed life, performance, and reliability.
This paper will present considerations for implementation of GAC for DBP control at existing WTPs. Several case studies will be presented for a range of plant capacities. Design considerations for retrofitting existing WTPs with GAC adsorption will be presented as well as challenges in designing GAC facilities on site with limited available footprint. Opportunities to optimize capital and operating costs with modeling of GAC performance will be presented.
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