Carbon Adsorption

Activated carbon in the powdered and granular forms is used to remove phenolic tastes and odors from drinking water supplies. In wastewater treatment applications, where phenol content is considerably greater than in potable water applications and the flow is continuous, granular carbon systems are more economical.

Depending on the concentration of phenol and other organic compounds in the wastewater, activated carbon will adsorb from 10 to 25 lb of phenol per 100 lb of carbon. This capacity can be determined from isotherm and column test data. In general, phenol adsorption improves as the pH decreases.

Adsorption at high pH is poor, since phenolate salt forms and is difficult to adsorb. This is an advantage in applications where phenol recovery is worthwhile. The phenol is adsorbed at the low pH and reclaimed as sodium salt by chemical regeneration, using hot caustic. If the phe-nolate cannot be reused, regenerant disposal is a problem. Also, if quantities of other organic substances are present in the waste stream, they too will be adsorbed. These organic compounds may not be desorbed during caustic regeneration, which will decrease the phenol capacity of the carbon upon subsequent regeneration. If chemical regeneration does not sufficiently recover the phenol capacity of the carbon, thermal reactivation will be required.

Figure 8.2.13 is a flow diagram of a granular carbon system for phenol removal employing chemical regeneration and phenol recovery. Pretreatment consists of acidification to pH 4.2 to precipitate the suspended solids and clarify the overflow. The phenol content of the feedstream ranges from 400 to 2500 mg/l, and the effluent objective is less than 1 mg/l phenol (Gould and Taylor 1969).

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