ANSI AWWA B605-18 pdf free download – Reactivation of Granular Activated Carbon
During reactivation, GAC is typically exposed to the following conditions: drying, desorption/devolatization, pyrolysis, and oxidation. The reactivation process requires high-temperature conditions. Reactivation is a form of regeneration. Regeneration can also include low-temperature processes that may not be as effective as reactivation. Varying conditions within a water treatment plant, such as process upsets, length of GAC service, or widely fluctuating influent water quality, will affect how efficiently a specific spent GAC may be reactivated. Because of this, characterizing a representative sample of the spent GAC and determining its reactivation characteristics (possibly by laboratory reactivation testing) are the most reliable means for projecting how the GAC will behave in a commercial reactivation system. For generic classification purposes, lightly loaded GACs are those that have been subjected to low loadings of organics (e.g., contaminants, total organic carbon [TOC]) and inorganics (e.g., calcium, oxidized iron, manganese). At the other extreme are highly loaded GACs that have experienced high loadings of organics and inorganics. The relationship between loading and reactivation will vary by GAC type and water quality, but in general, lightly loaded GACs will recover a higher percentage of adsorptive capacity than heavily loaded GACs. Metals can accumulate even in lightly loaded GAC and remain present at detectable concentrations after reactivation. Some loss of GAC will occur during reactivation. Causes for this loss include transportation and handling and reactivation losses. Therefore, makeup GAC is added to the reactivated GAC to bring the total GAC volume back to the original level. The makeup GAC should be virgin GAC or other reactivated GAC from the same potable water facility, exclusively. Use of virgin GAC as makeup avoids the potential liability of introducing leachable inorganic material from reactivated GAC from sources other than the purchaser’s own water.
Adsorptive Capacity. To accurately assess the effectiveness of reactivated GAC, test work should be completed using water from the particular plant in question. If the GAC sees significant variations in the water quality, every effort should be made to test each type of water quality, ensuring (or understanding) the effectiveness of the GAC in treating the varying water qualities. Evaluating the removal of a specific challenge compound, such as geosmin or 2-methylisoborneol, may be included in the test work. It is strongly recommended that users of reactivated GAC follow this approach, as it will reflect the actual conditions under which the GAC will be used. Various surrogate tests have been developed that give an indication of a GAC’s performance under specific conditions. The tests use a specific adsorbate at a high concentration to reduce the amount of time required to run the test. These tests are of limited versatility and are not necessarily indicative of a GAC’s performance for a given application. Examples of these tests are the iodine number, tannin value, and acetoxime adsorption tests. Iodine number is indicative of the total surface area of a carbon. As stated in ASTM * D4607, iodine number is the relative indicator of porosity in activated carbon. It does not necessarily provide a measure of the carbon’s ability to adsorb another species. Also, it must be realized that any relationship between surface area and iodine number cannot be generalized. This relationship varies with changes in carbon raw material, processing conditions, and pore volume distribution. Acetoxime number is used as an index of GAC’s ability to remove some low-molecular- weight organic compounds, such as volatile organic chemicals from groundwater. The acetoxime test’s applicability for evaluating GAC for use in surface water applications has not been demonstrated. The same can be said for acetoxime number as for iodine number; the number cannot be generalized and should be reviewed and/or tested for each individual application.