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Granular activated carbon (GAC) is a highly porous carbonaceous, crystalline, granulated material with a tremendous surface area‐to‐volume ratio (Figure A3.7). It is made by heating a carbon‐based material to very high temperatures and then opening up its crystalline pore structure with steam or acid. Carbon sources include bones, coal, nut shells, peat, petroleum residues, sawdust, sugarcane pulp, wastewater sludge, and wood. Lignite coal‐based carbons have a pore size ideally suited for water treatment. GAC physically adsorbs heavy metals (e.g. copper) and dissolved substances (e.g. medications, organic carbon). In aquarium systems, GAC filters are commonly used to deal with the gradual yellowing of water due to the buildup of humic and fulvic acids (Spotte 1991). GAC filters adsorb these organic materials and improve water clarity.

GAC also removes oxidants such as ozone and chlorine by oxidation on its outer surfaces (not within its pore structure).

GAC can be part of the main‐stream filtration, used on a side‐stream, or plumbed as an auxiliary device which allows it to be turned on and off to manage water clarity or for short‐term removal of a target compound (e.g. praziquantel). Pulse‐treatment is often preferred, as there are concerns that critical organics and nutrients can be depleted by its overuse. It is most efficient when placed in‐line after mechanical and biological filtration.

GAC filters can easily perform chemical, mechanical, and biological water treatment but are exhausted once the binding sites are filled. Bacteria readily utilize the GAC as a growth surface, which also decreases its efficiency over time. GAC filters must be replaced when exhausted. This may be carried out when there is increased yellowing of the water or when post‐filtration levels of target substances (e.g. chlorine) are outside of acceptable ranges.

Figure A3.7 Granular activated carbon.

Source: Image courtesy of Catherine Hadfield, Seattle Aquarium.