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Granular Activated Carbon (GAC)

Granular Activated Carbon (GAC) is made of organic materials high in carbon such as coconut shells, nutshells, wood, or coal.  GAC filters have a tremendous surface area resulting from its highly porous structure. Its effectiveness depends on the flow rate of the water and the contact time.  GAC allows water to pass through it while adsorbing other organic materials and certain dissolved chemicals, such as:
  • Chlorine and other halogens
  • Volatile organic compounds (VOCs) which include: 
    • Trihalomethanes (THMs) (by-product of chlorination)
    • Many organic solvents such as benzene, ethylbenzene, toluene and xylene (BTEX) 
  • Many organic chemical pesticides and herbicides
  • Some harmful algal bloom (HABs) chemicals such as Microcystin toxin

GAC is not effective for removing minerals and salts such as arsenic or nitrates.  GAC does not remove microorganisms.

Types of GAC filter systems:

  • Whole–house (POE)
  • Point-of-Use (POU)

GAC filters can be installed at the point-of-use (POU) or as a whole house treatment system depending on the contaminant being removed. For example GAC systems are installed to reduce undesirable taste and odors although it may not considered necessary for health reasons.  The GAC filters will also remove other chemicals like tannic acids, residual chlorine and chlorine by-products (THMs).Granular Activated Carbon Treatment System

Whole-house GAC systems installed to treat released VOC or pesticide contaminants must be sized by an experienced professional water treatment dealer based on the level of contamination. Water treatment dealers generally rely on the GAC manufacturers to help determine the appropriate type and amount of GAC to use. The system should consist of at least two large tanks filled with GAC to a depth of at least two feet. The type of GAC used is based on cost and the desired chemicals needing to be removed. The three primary types of GAC are coal-based, coconut hull-based, and wood based. 

A whole-house GAC system is installed in series of at least two tanks and monitored for chemical breakthrough at the end of the first tank. When breakthrough occurs, a tank with new GAC is always installed at the end of the run (closest to consumption) and the older GAC tank re-routed as the first treatment tank. 

GAC Filter Replacement

Prior to installing GAC filters, it is important to know the average water use and the type and concentration of the contaminants.  This will help determine how often the filters will need replaced.  Whether using a POU or POE GAC system the activated carbon only last a certain period of time.  Spent carbon can release previously removed chemicals back into the water.

The GAC provides a nutrient base for microbiological growth and can become breeding grounds for bacteria, including pathogenic bacteria.  If the media is not replaced in a timely manner, the GAC will cease to work and also could release toxins that had previously been removed back into the water.  Steps should be taken to remove any pathogenic bacteria ahead of GAC filtration.

When using GAC, the water should be checked on a regular basis to determine chemical and microbiological breakthroughs.


Page Updated 05/25/2018