Conditions That Impact On Performance

Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered many organic chemicals are in a more adsorbable form. The adsorption process is also influenced by the length of time that the AC is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of AC in the filter and reducing the flow rate of water through the filter.

AC filters can be a breeding ground for microorganisms. The organic chemicals that are adsorbed to the AC are a source of food for various types of bacteria. Pathogenic bacteria are those that cause human diseases such as typhoid, cholera, and dysentery. AC filtration should only be used on water that has been tested and found to be bacteria free or effectively treated for pathogenic bacteria. Other types of non-pathogenic bacteria that do not cause diseases have been regularly found in AC filters. There are times when high amounts of bacteria (non-pathogenic) are found in water filtered through an AC unit. Research shows little risk to healthy people that consume high amounts of non-pathogenic bacteria. We regularly take in millions of bacteria every day from other sources. However, there is some concern for certain segments of the population, such as the very young or old and people weakened by illness. Some types of non-pathogenic bacteria can cause illness in those whose natural defenses are weak. Flushing out bacteria that have built up in the filter can be accomplished by backflushing the AC filter prior to use. Water filtered after the initial flushing will have much lower levels of bacteria and ingestion of a high concentration of bacteria will have been avoided. Some compounds of silver have been used as disinfectants, especially in European operations. Silver has been added to certain AC filters as a solution to the bacteria problem. Unfortunately, product testing has not shown silver impregnated AC to be much more effective in controlling bacteria than normal AC filters. The areas that require definition when specifying and szing a carbon adsorption system include:

Processing conditions:

• Concentration of adsórbate

• Temperature of liquid stream

• Flow rates and operating frequency

• Pressure drop in system Characteristics of the adsórbate:

• Relative molecular mass

• Solubility of the adsórbate

• Concentration relative to solubility limits

• Temperature of solution Selection of adsorbent for optimum efficiency:

• Specific adsorption isotherm

• Selection of optimum activity level

• Cost sensitivity analysis

• Consideration of thermal reactivation

We will now touch upon some of these factors. First, let's look at what we mean by system isotherm. Freundlich liquid phase isotherm studies can be used to establish the adsorptive capacity of activated carbon over a range of different concentrations. Under standard conditions, the adsorptive capacity of activated carbon increases as the concentration increases, until we reach a point of maximum saturation capacity. An example of an isotherm for phenol is shown in Figure 8.

10

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láradtloraafiytefle , Solubility 4000mg 1

-

Phenol 1 Sdu bitty 83000mg

Don centrad on 0nj f )

Don centrad on 0nj f )

Figure 8. Shows effect of solubility.

Equilibrium zone

Equilibrium zone |

High Imeä r flow rale Low linear flow rite

Length of column

Figure 10. MTZ plot showing equilibrium zones.

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed.

To best understand adsorptive solvent recovery we have to consider some fundamentals of adsorption and desorption. In a very general sense, adsorption is the term for the enrichment of gaseous or dissolved substances (the adsórbate) on the boundary surface of a solid (the adsorbent). On their surfaces adsorbents have what we call active centers where the binding forces between the individual atoms of the solid structure are not completely saturated. At these active centers an adsorption of foreign molecules takes place.

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsórbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsórbate concentration. Figure 11 shows the adsorption isotherms for several common solvents.

Remember that this technology is versatile, and is applied equally well to solvent recovery and pollution control applications in gas as well as liquid systems. Let's now focus attention on the applications in water treatment.

Concentration (g/cu.m)

Figure 11. Adsorption isotherms of commons solvents in vapor state.

Concentration (g/cu.m)

Figure 11. Adsorption isotherms of commons solvents in vapor state.

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