Table 9172 Summary Of Influence Of Contaminant Properties On The Absorbability Of Organics

Parameter

Influence on Absorbability

Molecular weight

Solubility

Structure

Substituent group

Temperature Properties of carbon

Other

High molecular-weight compounds adsorb better than low molecular-weight compounds.

Low-solubility compounds are adsorbed better than high-solubility compounds.

Nonpolar compounds adsorb better than polar compounds.

Branched chains are usually more adsorbable than straight chains.

Large molecules are more adsorbable than small molecules.

Hydroxyl generally reduces absorbability.

Amino generally reduces absorbability.

Carbonyl effect varies according to host molecule.

Double bonds effect varies.

Halogens effect varies.

Sulfonic usually decreases absorbability.

Nitro often increases absorbability.

Adsorptive capacity decreases when the water temperature increases.

Adsorption is directly proportional to the surface area of the carbon used.

Virgin carbon has more adsorptive capacity than regenerated carbon.

Iron and manganese (if present at significant levels in the water) can precipitate onto the carbon, clog its pores, and cause rapid head loss.

Biological growth on the surface of the carbon can enhance the removal efficiency and increase the carbon service life. If the growth is excessive, however, it can clog the carbon bed.

Excessive amounts of suspended solids (above 50 ppm) or oil and grease (above 10 ppm) can affect the efficiency of the carbon.

Source: U.S. Environmental Protection Agency, 1985, Handbook, remedial action at waste disposal sites, EPA/625/6-85/006 (Washington, D.C.: U.S. EPA).

The goal of the design is to find the optimum contact time which provides the lowest carbon usage rate. Typical design parameters for carbon adsorption are shown in Table 9.17.3.

The contact time and carbon usage rate for a compound are usually determined through laboratory testing. A common test method is the bed depth service time (BDST) analysis, also called the dynamic column test study (Adams and Eckenfelder 1974). In this test method, three to four columns are connected in series as shown in Figure 9.17.10. Each column is filled with an amount of carbon which provides superficial contact times of fifteen to sixty minutes per column. Effluent from each column is ana lyzed for the chemicals of concern, and the effluent-to-influent concentration ratio is plotted against the volume of water treated by each column. Figure 9.17.11 shows an example of a dynamic test where four columns are used and each column represents fifteen minutes of contact time Tc. The curves obtained are called breakthrough curves since they represent the amount of contaminated water that has passed through the carbon bed before the maximum allowable concentration appears in the effluent.

Once the breakthrough curves are determined, the carbon usage rates can be calculated as:

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