PAC is costly and is discarded after one use whereas GAC, although about two to three times the cost of PAC, can be reactivated after exhaustion and reused. Since the introduction of more rigorous standards for drinking water quality the use of GAC has become the predominant process for the removal of organic matter including micropollutants. GAC adsorbers are commonly installed downstream of rapid gravity filters used for turbidity removal.

Activated carbon can be made from wood, coal, coconut shells or peat. (If you want to find high quality activated carbon manufacturers, please click here) The material is first carbonized by heating and then is ‘activated’ by heating to a high temperature whilst providing it with oxygen in the form of a stream of air or steam. Sometimes chemical activation by phosphoric acid is used. It is then ground to a granular or powdered form. It is a relatively pure form of carbon with a fine capillary structure which gives it a very high surface area per unit of volume. The adsorption capacity of GAC is described by various parameters including Iodine Number and BET surface area.

GAC adsorbers are of conventional rapid gravity or pressure filter design and the basic design parameters are the EBCT and bed depth or hydraulic loading (m3/h.m2). Bed depths up to 2.5 m for rapid gravity filters and 3 m for pressure filters are used.GAC characteristics vary according to the base material used. For example, the adsorptive capacity for the pesticide atrazine varies in the order wood>coconut shell>peat>coal. However, coal-based GAC finds wide use for most water treatment applications as it has a distribution of both mesopores (2–50 nm diameter) and micropores (up to 2 nm diameter), a structure suitable for medium to large (colour, taste and odour) and small organic molecules micropollutants, respectively. Pilot plant work or Rapid Small Scale Column tests should be used to optimize the GAC type and other design parameters such as adsorption capacity (by Freundlich adsorption isotherm) and to determine the life of carbon between reactivation. EBCT varies for different micropollutants and is usually in the range 5–30 minutes; for pesticides it is 15–30 minutes and for DBPs and VOCs it is about 10 minutes.