Figure 13. Carbon-bed conliguraiions.
ticn. Carbon columns are operated in two basic modes: fixed-bed or moving/pulse modes. In the fixed-bed mode of operation, all of the activated carbon in the column is replaced with fresh or regenerated material when breakthrough of the contaminants occur. In the moving or pulsed mode of operation, only that portion of the carbon that has been exhausted Is removed and replaced.
Technical Evaluation —
A technical evaluation of the activated carbon adsorption processes for treatment of wastewaters containing metallic compounds Is presented here.
Waste Feed Considerations—The process of adsorption by activated carbon involves the partition of metals such as Cr, Pb, Cu, and As from waste streams and their penetration into the pores and surfaces of activated carbon. Adsorption is based on the principle of attractive forces [i.e., van der Waals) that exist between metal ions and activated carbon surfaces. In general, the carbon adsorption process requires that the concentration of metal ions 1n the wastewater streams be less than 1000 ppm (preferably less than 500 ppm). Contaminants such as calcium or magnesium in concentrations greater than 500 mg/L or oils and greases in concentrations as low as 10 mg/L reduce the effectiveness of the carbon adsorption process by clogging and coating the pores, and decrease the removal efficiency of metals present in the solution. In general, if suspended solids concentration in the feed stream is greater than 10 mg/L, pretreatment for solids removal will be required. Concentrations of organics in the wastewater greater than approximately 1,000 mg/L may result in excessive activated carbon regeneration.
Pretreatment Requirements--Contaminated wastewaters often require pretreatment to increase the metals removal efficiency by activated carbon. Major pretreatment requirements include equalizing the flew and concentration uniformity of contaminants by filtration and adjusting the pH and temperature. In general, if the flow is turbid or the concentration of the suspended material is greater than 10 mg/L, filtration of the solution should be performed in the system by using a multimedia pressure filter, membrane, or ultrafiltration,
Posttreatment Requirements—The treated water from carbon adsorption systems is generally suitable for discharge to surface waters and rarely requires reaeration to increase the level of dissolved oxygen or other treatment methods to remove particulates not eliminated by carbon. Other aqueous streams generated by activated carbon systems, such as backwash and carbon wash, can be recycled or directed into settling basins.
Activated carbon has a constant adsorption capacity for different metallic compounds, and it must be regenerated when it becomes saturated. This can be achieved by using a strong acid or base to dissolve and wash the metal particles from the pores and surfaces of the activated carbon. Depending on the type and concentration of metal ions 1n the washing solution, the metals may be recovered by electrowinning; however, no information is currently available with regard to any such recovery system being used in conjunction with carbon adsorption systems for metals removal. Other options for treatment of carbon include incineration, crushing, shredding, and incineration in cement kilns.
Performance Data-A few activated carbon systems are being used to remove metals from wastewaters; however, data pertaining to full-scale applications of these systems for metals removal are either incomplete or unpublished because of proprietary considerations. Host available data are related to experimental studies performed in laboratories. The following subsections present some available data on removal of chromium and mercury from wastewaters by activated carbon systems.
Chromium--Huanq et al. (1977) investigated the removal of Cr** as a function of pH and Or** concentration. The adsorbent used In this research was a coimercial activated carbon, Calgon Filtrasorb 400, in a continuous mixed batch system. Results indicate that removal of chromium from solution increased with increasing pH values ($ S to 7) and then decreased at pH values greater than 7. Almost no chromium removal occurred above a pH of 10. Results also showed that the rate of the chromium adsorption increased with the Cr*4 concentration, Hexavalent chromium was removed from the solution by reducing it to Cr form in the presence of activated carbon at pH values less than 6. In the absence of activated carbon, the Cr*4 remained in the hexavalent state.
Polaroid Corporation investigated removal of hexavalent chromium from an aqueous waste stream generated by a slide film production facility.5 Different treatment techniques were studied, including 1on exchange, electrochemical treatment, sodium metabisulfite reduction, ferrous sulfate reduction, and carbon adsorption. Results of the investigation indicated that activated carbon adsorption was the most technically and economically feasible technique for removing chromium from the waste solutions. Figure 14 is a schematic of the treatment system. Sulfuric acid is used to regenerate the saturated carbon. The acid is then pH-adjusted and filtered to remove precipitated chromium. During the pilot-scale study, it was found that pH adjustment and filtration of the solution significantly increased adsorption of chromium onto the activated carbon. Adjustment of the pH below 5 increased the adsorption capacity of the activated carbon by 5 or 6 times. At pH values lower than 2.5, the adsorption capacity decreased as a result of the reduction of the Cr** ions and the dominance of the cationic trivalent chromium ions in the solution.
Mercurv--Rosenzweiq (1975) reported the successful application of a full-scale activated carbon system for removal of organic mercury. In this system, suspended solids were first removed from pesticide manufacturing discharges by coagulation and flocculation with iron salts and polyelectro-lytes, and the aqueous streams were treated by passing them through a series of activated carbon columns. This process removes mercury at a rate of 0.05 kg per kg of carbon and the exhausted carbon is thermally regenerated.
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