The spectrum of species to be removed in case of water treatment is very broad and depends not only on the final quality requirements but also on the feed quality of water which would vary from one source to another. Activated carbon is a very effective adsorbent for removal of impurities from water because it is inherently very stable in water.
The attraction between dissolved contaminants and the carbon could result because the contaminant itself is hydrophobic — that is, it naturally separates from water. This means the carbon surface is more attractive to the molecule. Alternatively, the molecule could simply exhibit a natural attraction toward carbon. Sometimes both scenarios are present at once. Hydrophobicity is an important factor in activated carbon filtration, as the majority of organic pollutants in our water exhibit this quality. While they do not bind with the highly polar water molecule, they do bind with carbon, which is non-polar. This is why activated carbon is uniquely capable of removing otherwise difficult volatile organic chemicals, byproducts of disinfection, herbicides and pesticides.
Generally for water treatment applications,
- Activated carbons with a larger pore size in comparison with those used for adsorption from a gas phase are used. However, the composition of various pore sizes strictly depends on the target contaminant removal. For instance, activated carbon is manufactured to have a high percentage of micropore volume for dechlorination and removal of micropollutants. Whereas, activated carbon with high percentage of large micropore and small mesopore volumes is preferred for potable and waste water applications. And, activated carbon with a high percentage of large mesopores and macropores is preferred for decolorization and other waste water purification.
- The most widely used filtration units in water treatment applications use fixed-bed GAC filters. However, the treatment of liquids with high flow rates and/or high concentrations of contaminants requires a frequent replacement of carbon adsorbent. In such cases pulsed or moving bed systems are installed. In this technological solution the purified water enters the filtration column from the bottom and flows upward. At the same time, GAC is dosed downward from the top and it is removed continuously or in pulses at the bottom of the filtration column.
- In the majority of the adsorption systems employing PAC, the adsorbent is added to the mixed liquid to be purified directly or as a previously prepared suspension (batch contact unit operation). After reaching the equilibrium, the carbon is removed from the suspension by sedimentation or filtration, followed by its discharge. In order to avoid the technological problems such as undesired decantation, the appropriate amount of PAC must be used. The most widely used filters for PAC separation units are plate and candle filters.