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<This is the conclusion of a two-part article that began in the March issue of Water Quality Products.
AC Working Status, Determining Remaining Adsorption Capacity
The AC user wants to obtain a high quality finished water and exhaust the AC to its maximum. Sending partially exhausted AC for regeneration is costly, but providing customers with pollutants in the water has many liabilities. Perhaps the best way for the AC user to obtain the goal of high quality water and complete use of the AC adsorption capacity is to use AC in a dual adsorber as illustrated below. This design provides a leading AC bed and a finishing AC bed; these beds can be reversed by simple valving.
Measurement of aqueous chemicals can be made at various points in this design such as the raw water, after the lead adsorber, after the finishing (polishing) adsorber, and in the distribution water. Multiple point measurements tend to assure the finished water quality by providing a total system evaluation. Using the leading bed until the chemical in its effluent is the same concentration as the influent provides, maximum use of the AC. When breakthrough in the leading bed occurs, it is replaced with fresh AC. The original finishing bed becomes the leading bed with the new fresh AC working as the finishing bed. Thus, dual bed adsorbers help the AC user to obtain good AC use and excellent finished water quality.
Often the AC users want to know how much longer they can use the AC before it is completely exhausted and needs replacing. There are many known cases where the AC was exhausted and still in use; this results in bad finished water quality. AC does such a good job polishing water that it can be assumed to last forever. It is not uncommon in potable water applications for the AC bed to last two to three years before it needs to be replaced. The AC user must remember that it will always need to be replaced eventually.
For example, 100 grams of AC can adsorb 40 grams of toluene. Thus, if you have water with 5 ppb of this toxic chemical with no competing adsorbate, the AC user will be able to treat 9 million liters with 100 grams of AC in a well-designed system. This is an over simplification, but it gives you a rough idea of the power of AC.
Every water application will have its own unique challenges. Competitive adsorption is almost always present. This phenomenon accounts for the observation that a compound can be observed in the finished water, but it is not seen in the raw water. The AC can concentrate an aqueous organic that later is pushed off the AC in a plug flow by a stronger adsorbing compound.
Activated Carbon Feasibility, Adsorbates on Spent AC
Chemical analysis of the raw water provides information about chemicals that must be removed to obtain the desired finished water quality. Techniques are available to determine chemicals in water at parts per billion (pbb) levels and lower. Removal of organics from water by AC is predictable and yields important information about the chemicals on the spent AC. Regenerators often ask AC users for a list of what is on the spent AC to help classify it as hazardous or non-hazardous and to tailor its regeneration. An AC that was used to treat hard water may contain high levels of calcium and magnesium. These metal salts are not removed by furnace temperatures, but pretreatment with acid washing will remove them and provide a better regenerated AC. Without this pretreatment, the AC user could observe a milky white effluent when the regenerated AC is put back in use; this is due to the calcium and magnesium salts being leached out of the AC by the raw water. This turbid effluent could last for several thousand bed volumes.
Basic Adsorption Rules
There are some basic adsorption rules that can provide the AC user knowledge about their spent AC. Low solubility and slightly soluble compounds (such as petroleum) are good candidates for AC adsorption, as are high molecular weight organics (mw> 75). Highly soluble compounds (such as some alcohols, aldehydes and ketones) have a strong affinity for water and are more difficult to adsorb. Specifically, the following classes of compounds are readily adsorbed on AC: petroleum hydrocarbons; aromatics (benzene, toluene, xylene); surfactants; chlorinated hydrocarbons (solvents, insecticides, herbicides, hydrocarbons); organic dyes; and organic acids. The capacity of a granular AC to adsorb organic compounds is related to molecular surface attraction, the total surface area available per weight of carbon, and the concentration of contaminants in the water stream. Higher concentrations are adsorbed better.
The basic instrument for evaluating AC use is the adsorption isotherm and minicolumn, which represents the empirical relationship between the amount of contaminant adsorbed per unit weight of carbon and its equilibrium water concentration. Liquid phase adsorption isotherms have been developed for most commercial AC for a variety of specific compounds. Computer modeling also is used to estimate carbon life in the presence of a mixture of compounds in water.
We hope this article makes the AC users think about their systems and helps future AC users to be successful. We are advancing to an understandable system that can be predicted, and we still have things to learn about AC.