Dechlorination activated carbon

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Water from the wastewater treatment plants of paper mills, power plants, etc., contains high chlorine residues in aqueous media, which causes environmental concern. Several methods have been used for dechlorination, including granular activated carbon, hydrogen peroxide, sodium thiosulfate, ammonia, sodium sulfite, and metabisulfite. In addition, ferrous sulfate hep-tahydrate has also been proposed for the removal of chlorine residues. 

Tieman TO, Wagel DJ, Vanness GF, et al. 1989a. Dechlorination of PCDD and PCDF sorbed on activated carbon using the KPEG reagent. Chemosphere 19 573-578. 

Chloroethanes have been shown to undergo dechlorination by an enzyme system that is similar to the hepatic microsomal mixed function oxidase system (Van Dyke and Wineman 1971). Dechlorination was inducible by phenobarbital and required oxygen and NADPH. However, dechlorination also required a factor from the cytosolic fraction of the liver homogenate for optimal dechlorinating activity. In terms of structural requirements, dechlorination was enhanced if the carbon atom containing the chlorine had only one hydrogen. In a microsomal incubation, 13.5% of... 

Effluents from sewage treatment plants are not allowed to contain residual chlorine in excess of tolerable values as determined by water quality standards. For example, in discharges to trout streams, the residual chlorine should not exceed 0.02 mg/L. Thus, chlorinated effluents should be dechlorinated. Sulfur dioxide, sodium sulfite, sodium metabisullite, and activated carbon have been used for dechlorination. Because sulfur dioxide, sodium sulfite, and sodium metabisulfite contain sulfur, we will call them sulfur dechlorinating agents. Dechlorination is an oxidation-reduction reaction. The chemical reactions involved in dechlorination are discussed next. 

The unit operation of carbon adsorption is discnssed in Part II of this book. The use of activated carbon in dechlorination wiU nse the same unit operation, except that in the present case, the carbon will be consnmed by chemical reaction. In the nnit operation as discussed in Part II, the operation is pnrely physical, and carbon can be regenerated, which is also the same case in dechlorination except that there is a large loss of carbon between regenerations. 

Example 17.18 A total flow of 25,000 m /d is to be dechlorinated after disinfection using chlorine. An activated carbon bed is to be used for dechlorination. If the total residual chlorine (TRC) is 0.5 mg/L, how many kilograms of activated carbon are lost from the bed per day ... 

As a result, MWP sorted from municipal solid wastes can be processed in a refinery, which has a dechlorination unit installed prior to the hydrocracking unit. Even though commercial catalysts showed satisfactory performance at high temperatures, neutral catalysts based on activated carbon can also be utilized for this purpose. 

In 1927, O.scar and Hudolf Adler of Czechoslovakia were granted a patent for the use of activated carbon for water dechlorination [4]. Their work seemed to rekindle interest in the application of carbon in water treatment in both Europe... 

Another consideration in breakpoint chlorination is dechlorination to remove the chlorine residual from the final effluents before it is discharged. Very often, dechlorination using sulfur dioxide or activated carbon may be needed when the breakpoint chlorination process is used. A new dechlorination technology has been introduced in another chapter of this handbook series (2). UV dechlorination is recommended by Wang (45). 

Activated carbon has been used in water and wastewater treatment facilities and industries for dechlorination (9). Free as well as combined chlorine from water can be removed by activated carbon. In water treatment plants carbon filters effectively remove dissolved organic matter in addition to removing chlorine. 

Chloramine reactions do not release COj gas or destroy the integrity of the carbon. However, chloramine reactions are much slower than chlorine reactions with activated carbon. Finally, although carbon can effectively remove chlorine from potable water, it is more expensive than other dechlorination methods (9,10). 

The chloramine removal efficiency of catalytic carbon is reported to be an order of magnitude greater than that of conventional activated carbons used for dechlorination. Various factors such as empty bed contact time (EBCT), influent chloramine concentration, particle size, and temperature influence treatment efficiency using catalytic carbon (11,12). In a study using water containing 2 mg/L influent chloramine concentration, an increase in EBCT from 10 to 30 s increased the volume of water treated to below 0.1 mg/L chloramine from 250 bed volumes to 11,000 bed volumes. In a different study, reducing the mesh size from 20 x 50 to 30 x 70 increased the bed volumes treated from 11,000 to 28,000 at a 30 s EBCT and 2 mg/L influent chloramine... 

The limitations to using catalytic carbon for potable water dechlorination include (i) inability to dechlorinate free chlorine (only chloramines are removed by catalytic carbon) and (ii) potential loss of carbon life due to fouling by organic compounds or oxidation by various compounds. Furthermore, catalytic carbons are generally more expensive than activated carbon as well as other dechlorination methods currently available. 

V. L. Snoeyink and M. T. Suidan, Dechlorination by activated carbon and other reducing agents. In Water and Wastewater Disinfection, J. D. Johnson, (ed.), Ann Arbor Science Publishers Inc., Ann Arbor, MI, 1975, pp. 339-358. 

Process modifications and alternatives Metabisulfite, bisulfite, or sulfite salts can also be used. Automatic or manually fed systems can also be used. If chlorine is used at the site, sulfur dioxide is preferred, since identical equipment can be used for the addition of both chemicals. Alternative dechlorination systems include activated carbon, and ponds (sunlight and aeration). UV is also an effective dechlorination technology (60,61). 

To eliminate residual free chlorine from hquid, granular activated carbon adsorption or chemical reduction (with reducing agents, such as sulfur dioxide, sodium bisulfite, and sodium metabisulfite) are the most common processes for dechlorination. Ultraviolet (UV) irradiation process is gaining wider acceptance as a dechlorination process (30,45,46, 60,61). 

Dechlorination can be performed with activated carbon. Chlorine or hypochlorite, OCU for example, react with the carbon surfece to give chloride, an acidification of the solution and oxidized groups on the carbon surface. These chemical functions may be decomposed into carbon monoxide or dioxide. A schematic overall reaction is written ... 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

As remarkable as the performances of these membranes are, aU aromatic PA membranes have one severe drawback their susceptibility to degradation by chlorine. The removal of chlorine prior to membrane separation by activated carbon, sodium sulphite (Na2S03), or sodium bisulphite (NaHSOa) addition is, therefore, mandatory. Dechlorination by NaHSOs in stoichiometric excess can, however, be ineffective in seawater feed because dissolved oxygen in seawater reacts with the chemical. Further, the absence of chlorine can lead to biofouling that is often irreversible. 

Free chlorine (CI2) is an oxidant. Polyamide RO and NF membranes are sensitive to chlorine. Hence, water must be dechlorinated by passing it through an activated carbon filter or by the addition of a reducing chemical such as sodium sulphite, sodium bisulphite, or sodium metabisulphite to feed water. However, it is necessary for CA membranes to protect them from bacterial attack. 

One of the uses of o-chloronitrobenzene is in the production of o-chloroaniline, which is obtained by catalytic reduction with sulfided palladium/active carbon catalysts. These catalysts enable dechlorination to be suppressed. 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

Secondary Dechlorination Free chlorine can be destroyed or reduced from OCl to Cl by catalysis, chemisorption on activated carbon, or addition of a chemical reducing agent. While in principle some sort of reduction could be used for the entire process of dechlorination, it would be impractical not to recover most of the chlorine as the element. The reduction or decomposition process therefore is used only as a backup measure. The process is also useful in wastewater treatment. 

Activated Carbon. As a special category of catalytic decomposition of species of free chlorine, we include chemisorption on activated carbon. Beds of granular activated carbon have long been used to dechlorinate water after its puriflcation [231-233]. Because of its tendency to form surface oxides, activated carbon has the ability to abstract the oxygen generated by hydrolysis of chlorine. As an example, the reaction of HOCl can be written as ... 

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