Can granular activated carbon (GAC) filtration systems trap nitrate and then download it at high enough concentrations to be of concern?

Yes, it is possible. Research confirms that GAC filter systems can take up and hold (adsorb) nitrate during treatment processes to remove certain organic chemicals. Then at some unknown frequency, the carbon releases the nitrate into the treated water. This has proved to be a problem with some California water systems using GAC filtration to remove dibromochloropropane (DBCP) from ground water supplies. Modification to the GAC systems may be necessary to prevent this storage and release of nitrate.

Granular activated carbon (GAC) filtration is effective in removing organic contaminants from water. Since, organic chemicals are implicated in producing taste, odor and color problems, GAC filtration is used to improve water aesthetically. When water is passed through a GAC filter, the carbon particles attract and remove contaminants, like hydrogen sulphide, heavy metals (lead, mercury and copper), chlorine and organic compounds. GAC filters use a cartridge packed with granules of activated carbon. While some filters contain bacteriostatic materials (which prevent the growth of bacteria), these filters can still eventually start trapping bacteria, allowing it to grow on the filter. For this reason, it is essential to flush the filter daily with cold, treated, potable water to remove any bacterial residue. Activated carbon is expensive.

A disadvantage with activated carbon is the high emissions of sulphur dioxide generated from the heating process in manufacturing carbon from coal. The media can become a breeding ground for micro organisms. One of the drawbacks of a GAC filter is its tendency to “channel,” where water creates distinct paths through the media. This greatly reduces the available contact area, which shortens the effective life of the filter. It also means that additional pre-filtering becomes necessary because the carbon is not at a uniform pore size for this purpose.

Several years ago, this laboratory reported (Water Engineering & Management, March 1994) that nitrification in the piping from basins to laboratory taps was responsible for changing the quality of basin water. This suggested that the laboratory tap water was not appropriate for checking certain parameters of basin waters. Nitrification in our plant occurred frequently with ammoniated waters in the piping from the primary basins having low levels of monochloramine (1.0­p;1.5 mg/L), but was rarely detected in the sand filters or in the piping from the secondary basins where ammoniated waters flow with high chloramines levels (3.5­p;4.5 mg/L). It is possible that nitrification or some other bacterial metabolic activities were occurring in the GAC filters. This conclusion was based on the following facts: complete absence of disinfectant, constant presence of heterotrophic bacteria (104­p;105 cells/ml) in the filter effluent, and presence of the free and combined ammonia in the filter influent. Nitrification in the GAC filters was detected within two months of installation. This was shown by an increase in nitrate and nitrite levels, and a simultaneous reduction of total ammonia in the filter effluent. The presence of these anions results not only in the reduction of chloramines in the distribution system, but it also has the potential to lead to adverse health effects such as methemoglobinemia.

Therefore, strict monitoring for these anions or controlling their formation is necessary whenever nitrification is suspected in GAC filters. Nitrite and nitrate are both regulated by primary maximum contaminant levels of 1 and 10 mg/L as N respectively, sampled at the source or entry to the distribution system. During monitoring of the catalytic activity of GAC, it was found that ammonia determination plays an important role in understanding the nitrification process in the GAC filters.

Thus, studies with ammonia specification (free, combined, and total ammonia) before and after filtration made it possible to investigate relationships between GAC aging (acclimatization), ammonia removal, and nitrite formation. The filter influent carries ammoniated water, both free and combined ammonia (monochloramine), that are potential substrates for nitrifying bacteria. This is true unless the concentration is very low. The raw water contains ammonia at a level that is less than 0.1 mg/L as an NH3-N.

Occurrences of nitrification in GAC filters are mostly associated with high concentrations of the total ammonia in the filter influent (>1.5 mg/L). In some cases of feeding ammonia, free and combined forms dissipate and cannot be traced to either nitrate/nitrite or free ammonia after filtration. It would be interesting to pursue the fate of the lost ammonia in GAC filters. It may follow that at the inductive period, some activated carbon is converted to activated carbon oxides changing monochloramine to nitrogen gas. (This information was provided by a GAC supplier.) It is also well known that when nitrifying bacteria, ammonia is converted into N2O under low dissolved oxygen. This type of metabolic pathway shift could be occurring in the GAC media. Therefore, ammonia dissipation could be continuing in effluent water without a nitrate and nitrite release.