Adsorption in Water Treatment
A GAC (granular activated carbon) filter after an intermediate ozonation has many purposes:
- To remove chemical compounds or ozonation by products by adsorption.
- To degrade such substances by biological activity on the surface of the GAC by bacteria.
- To destroy the residual ozone in the water fed to the GAC filter – this takes place in the top few centimeters of the GAC bed.
In a GAC filter different competitive processes take place simultaneously:
- fast adsorption;
- slow adsorption;
- biological effects; and
- biological effects enhanced by ozonation.
Between the fast and the slow adsorption there is competition. New GAC will first adsorb a lot of weakly adsorbed compounds, e.g. alcohols, ketones, aldehydes, acids, aliphatics and colloids. These compounds will then be displaced from the active carbon surface by more strongly adsorbed pollutants, e.g. aromatics, chlorinated, non-aromatics and high molecular weight hydrocarbons.
The displaced, weakly adsorbed material will be readsorbed further down in the filter. This phenomenon is called the “chromatographic effect.”
A look at adsorption
Three types of adsorption can be distinguished for a GAC filter:
- Exchange adsorption (electrical attraction of the solute by the adsorbent).
- Physical or ideal adsorption by weak van der Waals forces.
- Chemisorption or chemical adsorption (chemical reaction of the adsorbate with the carbon).
In water treatment application is the primary mechanism is the physical adsorption, which is reversible followed by the chemisorption, which is generally considered as irreversible.
A GAC filter offers an excellent surface for biological activity. The rough surface provides numerous good places for attachment. Such biological activity has become evident in full-scale plants, showing that the amount of organic carbon removed is far beyond that, which can be removed by adsorption alone.
Removing bacterial nutrients
It is well established that an ozone step will produce aldehydes and ketones by oxidation of the carbon double bonds. These products are nutrients for the bacteria, which are always present in a distribution system, If these nutrients are not removed during the treatment process, they will promote rapid and dramatic growth in the distribution system. This situation has to be avoided and is done by the introduction of an appropriate biological treatment step, e.g. GAC filter or slow sand filter. If such biological filters are present after the main ozonation, the organic compounds will be biodegraded on the surface of the filter media.
An additional measure to avoid a bacterial regrowth problem in the distribution network is the dosing of a disinfection agent at the end of the treatment plant, e.g. chlorine dioxide or chloramines.
With a new GAC filter, rapid adsorption is dominant and nearly all-organic material can be adsorbed. According to the adsorption capacity of the filter and with increasing running time, the rapid adsorption will decrease in favor of a slow adsorption. At the same time a biological activity is started and will additionally reduce the organic matter.
It is important to note that an additional role of ozone in this process is the dramatic extension of the life of the GAC filter. By creating a more biologically active bed, the subsequent life is extended many times over.
If a pre and/or main ozonation is introduced before GAC filtration, a further enhancement of the biological activity takes place. A further reduction of the oxidizable carbon will be achieved while a part of the byproducts will be absorbed on the GAC and/or consumed by bacteria.
Such a combination dramatically improves the final water quality.
