The Process and Water Quality Specialists

Adsorption

GAC Fundamentals

Granular activated carbon (GAC) and powdered activated carbon (PAC) have been used in drinking water treatment for a variety of purposes.  Applications of GAC adsorption in drinking water include organic contaminant removal from groundwater, taste and odor control, and reduction of disinfection by-products and NOM. GAC adsorption involves transfer of contaminants from the liquid-phase to the solid-phase. When the capacity of the GAC for the contaminant is exhausted, the GAC is removed and replaced with fresh GAC. The spent GAC is regenerated and prepared for reuse.

Some utilities that prefer to continue using free chlorine for secondary disinfection in light of tougher DBP regulations have implemented GAC to reduce levels of natural organic matter, which is a precursor of DBP formation in the presence of free chlorine.  Other utilities have implemented either GAC or PAC for the reduction of taste-and-odor (T&O) compounds like MIB and Geosmin.  The advantage of PAC is that it can be implemented by adding PAC particles directly to the water for removal of MIB and Geosmin on a seasonal basis, with later settling out of the PAC.  This approach has a lower capital cost than GAC.  The disadvantage of PAC is that it is difficult for operators to control dosing and removal of the particles from the water and to know when a T&O event is occurring and dosing is needed.  GAC, in contrast, is typically operated as a fixed bed and is a proven technology that can be expensive depending on the frequency of carbon replacement that is required.  The frequency of carbon replacement depends on the adsorbability of the compounds present.

A simple way to look at the adsorbability of various compounds (single solute) is via the Freundlich equilibrium expression:

 q = K C1/n

The term q represents adsorbent phase concentration (mg/g); the term K represents the Freundlich adsorption capacity parameter (mg/g)/(L/µg)1/n; C is the liquid phase concentration (µg/L); and 1/n is the Freundlich adsorption intensity parameter (dimensionless).  A rough assessment of adsorbability can be made based on a tabulation of Freundlich adsorption parameters, specifically a higher K means a compound is more amenable to adsorption.  A more rigorous analysis is required in design evaluation given that the parameters depend on adsorbent type, temperature, concentration, etc. 

Multicomponent mixtures and the presence of natural organic matter add complexity.  Mathematical models are available to aid in the design of adsorption processes and Trussell Tech has an engineer on staff who is a co-author of a premier adsorption modeling package, AdDesignS™, available from Michigan Tech.