<This is the conclusion of a two-part article that began in the March issue of Water Quality Products.
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.
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.