George Maseeh and Brian Leto
Granular Activated Carbon (GAC): A Viable Alternative for Stage 2 Compliance
ISSUE:
The advent of new DBP regulations have utility managers weighing their options — is GAC the right approach for you?
Background
With the promulgation of the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 D/DBP Rule), water utilities are now faced with more stringent requirements concerning the concentrations of total trihalomethanes (TTHMs) and five haloacetic acids (HAA5) in their distribution systems. One approach to lowering the levels of these contaminants is to remove organic precursors using granular activated carbon (GAC). This mode of treatment is gaining popularity as utilities become aware of the potential unintended consequences associated with converting secondary disinfection from free chlorine to chloramines.
How GAC Can Benefit Your System
GAC treatment is also highly effective for removing volatile and synthetic organic chemicals from ground and surface waters and is currently regarded as a viable treatment barrier against emerging contaminants such as endocrine disruptors and pharmaceutically-active compounds. Furthermore, GAC treatment removes taste and odor causing compounds such as 2-methylisoborneol (MIB) and geosmin and can thus improve (and protect) the aesthetic qualities of a water supply.
A well-designed GAC system will not only remove the organic precursors that form DBPs but will also protect against tastes, odors, and emerging contaminants.
Options for Implementing GAC
GAC applications for surface water treatment include post-filter contactors and filter-adsorbers. Post-filter contactors are concrete basins or steel vessels filled with GAC that follow the filtration process (e.g. granular media filters or membranes). Filter-adsorbers involve replacing some or all of the media in conventional filters with GAC. A variety of factors influence whether one or both of these applications are suitable for a particular treatment plant. For both approaches, Malcolm Pirnie has conducted pilot testing, conceptual evaluations, and detailed designs for applications ranging from less than one million gallons per day (mgd) to 175 mgd.
Comprehensive Approach
An effective GAC implementation strategy should address several factors:
- establishing contaminant removal goals
- balancing contact time and carbon change-out frequency
- considering partial treatment (treating less than 100% of the flow)
- considering seasonal GAC use during high DBP formation periods
- determining the influence of GAC type on cost and performance
Malcolm Pirnie’s GAC solutions provide the highest level of performance at the lowest cost. Our experience can help you deal with the issues of implementing a GAC process in a new or existing facility, including:
- incorporating the GAC facility within the hydraulic profile
- designing the facility to accomodate GAC loading and unloading
- management of recycle flows from the GAC process
- process optimization once operations commence
We invite you to contact one of our GAC experts:
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