AC filters used for home water treatment contain either granular activated carbon (GAC) or powdered block carbon. Although both are effective, block AC filters appear to be more effective in removing chlorine, taste and halogenated organic compounds.
The amount of AC in a filter is one of the most important features since it affects the amount and rate of pollutant removal. More carbon in a cartridge means more capacity for chemical removal, lengthening a cartridge lifetime. Particle size will also affect the rate of removal; smaller AC particles generally show higher adsorption rates.
Rust, scale, sand or other sediments can block an AC filter. Foam or cotton filters (often called sediment or fiber filters) placed between the cartridge and incoming water solve the problem.
An AC filter must be deep enough so pollutants will adsorb to the AC in the time it takes the water to move through the filter. The filter depth required depends on the flow rate of water through the filter. The slower the flow rate, the better the removal. The poor performance of some end-of-faucet devices is probably due to improper filter depth.
Physical and chemical characteristics of the water will also affect performance. Acidity and temperature matter. Greater acidity and lower water temperatures tend to improve the performance of AC filters.