INTERFERENCE WITH ABSORPTION PROCESS

Total Organic Carbon
Total organic carbon (TOC) is the heterogeneous mixture of organic compounds including primarily humic substances as well as humic acid and fulvic acid. These compounds are adsorbable and reduce the capacity of the carbon to adsorb the compounds of interest. However, some of the substances which comprise TOC are non-adsorbable, resulting in the immediate appearance of TOC in the effluent (i.e. breakthrough).

Iron and Manganese
Iron and manganese are known to cause fouling problems in filtration systems. Both may be oxidized by dissolved O2 and precipitate within the GAC pores. Capacity loss is expected as pores become filled with oxidized minerals.
Accelerated degeneration of the carbon will also result if inorganic compounds are not removed from the carbon prior to regeneration. During successive GAC regeneration cycles, an increasing ash content may cause an accelerated degradation of the GAC structure, as indicated by decreasing iodine and molasses numbers. These changes may be attributed to the oxidation of the carbon structure which may be catalyzed by inorganic compounds such as Fe2O3, CaO, and NaO.

Calcium Carbonate
Lime softened water or water supersaturated with calcium carbonate may result in the deposition of calcium carbonate on the GAC particle which could cement the filter grains together, cause an increase in grain size and a deterioration in water quality, or reduce the adsorptive efficiency of the GAC.

Turbidity and Coagulants
In some cases where highly turbid water is applied directly to GAC, breakthrough may occur quickly. This has been attributed by some to coating of the GAC particles by the solids, which acts as a barrier against adsorption. It is also possible that other factors in these cases, such as competitive adsorption from background TOC, may have contributed to observed effects in these cases.

Biological Growth
Biological growth may occur within the filter, and, over time, may plug the filter resulting in increased operating pressures and requiring more frequent backwashing. However, microbial activity may extend the bed life for biodegradable compounds. Degradation of organic substances by microbial growth on carbon may contribute to improved removal of compounds including TOC and benzene.

Desorption
Adsorption is a dynamic and reversible process, and, as a result, desorption of contaminants may also occur. Desorption may result from:
1. Competition from more strongly adsorbing species.
2. Reduced influent concentrations, causing materials to be desorbed as a new equilibrium is established.
3. Destratification or mixing of the GAC, again causing carbon originally exposed to higher contaminant concentrations to establish a new equilibrium under a lower contaminant concentration.
4. Changes in water quality, such as pH.
5. Increases in temperature.
Desorption also has the effect of leaving additional capacity for periodic high concentrations of contaminants of incoming species.