Comparing Carbons for Disinfection Byproduct Control
- Maggie Pierce, Sara Gibson, Mark Bishop, David Briley
Activated carbon use is increasing as water systems face Stage 2 Disinfectants and Disinfection Byproduct Rule compliance. Granular activated carbon (GAC) and powdered activated carbon (PAC) are two alternatives for removing organic precursors that differ in practical application and operating philosophy. Considerations in choosing the carbon type should include a careful evaluation of finished water quality objectives, operational impacts, costs, sustainability and other non-economic factors.
Implementation of GAC necessitates a higher capital cost due to a large footprint and specialized equipment; however, PAC typically requires high doses (20 to 60 mg/L) for dissolved organic carbon (DOC) removal, resulting in larger O&M costs. The use of PAC can be challenging due to the uncertainties regarding dose control while ensuring DBP compliance as source water quality changes. An additional PAC consideration is residuals management, particularly during peak organic loading conditions. For water systems using chloramines or other compliance strategies, PAC could be an effective secondary tool for disinfection byproduct (DBP) control during raw water quality excursions.
Post-filter GAC adsorption is a more efficient use of carbon as it takes advantage of DOC removal in upstream processes. The use of GAC is a more sustainable process since carbon is regenerated and reused, in comparison to the single use and disposal of PAC. Additional considerations for GAC operation include monitoring for DOC breakthrough, periodic backwashing, and washwater management.
The full costs of activated carbon applications for DBP compliance are becoming more fully understood as experience in the water industry is gained. As use of activated carbon increases in other industries, such as automotive, pharmaceutical, and energy (i.e. fracking), the cost and availability to water treatment applications will be affected. Applications of activated carbon may be evaluated and optimized based on historical raw water quality and water quality goals. A unique process modeling approach has been developed to provide insight into water quality objectives, risks, and associated costs of compliance with PAC or GAC usage.
This paper will present considerations for the decision to use activated carbon for DBP control. Example costs for a range of plant capacities, and opportunities to optimize capital and operating costs with modeling of GAC and PAC performance will be presented. Case studies will be discussed that include PAC and GAC assessments and/or implementation for water systems throughout the region.
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