Wastewater removing chromium

New York City, which drew some of its water supply from a point three and a half miles to the south, installed seven shallow test wells near Liberty Aircraft in 1945 and initially found no contamination. When the war ended, further investigations began, with dozens of new monitoring wells installed. In 1948, chromium was detected in three of the city test wells near Liberty, and a plume of contamination was traced for a mile from the plant. At Grumman, chromium had been found in public supply wells more than 100 feet deep. By 1949, all of Long Island s aircraft plants had installed treatment systems to remove chromium from their wastewater.31... 

Frank WL, McMullen MD. (1996). Removing chromium from groundwater and process wastewater. The National Environmental Journal March-April 36-39. 

AN, 0-Carboxy methyl chitosan/cellulose acetate blend nano filtration membrane was prepared in acetone solvent. It had been tested to separate chromium and copper fiom effluent treatment. The highest rejection was observed to be 83.40% and 72.60%, respectively (Alka et al., 2010). A chitosan/cellulose acetate/polyethylene glycol ultra filtration membrane was prepared with DMF as solvent. It was focused to be efficient in removing chromium from artificial and tannery effluent wastewater. The highest rejection rate was responding (Sudha et al., 2008).Cross-linked chitosan/polyvinyl alcohol blend beads were prepared and studied for the adsorption capacity of Cd from wastewater. The maximum adsorption of Cd(II) ions was foimd to be 73.75% at pH 6 (Kumar et al., 2009). 

In removing excess free chlorine from municipal or industrial water and from wastewater, sodium sulfite competes with bisulfite or sulfur dioxide. Other commercial appHcations of sodium sulfite in wastewater treatment include the reduction of hexavalent chromium to the less toxic Cr " salts as well as the precipitation of silver and mercury. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

Strong reducing agents such as sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate are used in the iron and steel finishing sites to reduce hexavalent chromium to the triva-lent form, which allows the metal to be removed from solution by chemical precipitation.21 23 Metal-containing wastewaters may also be treated by chemical precipitation or ion-exchange. 

Treatment for the removal of chromium and nickel from electroplating wastewater involves neutralization, hexavalent chromium reduction, pH adjustment, hydroxide precipitation, and final solid-liquid separation.15 37 48... 

Ferrous sulfide acts as a reducing agent at pH 8 to 9 for reduction of hexavalent chromium and then precipitates the trivalent chromium as a hydroxide in one step without pH adjustment.5162 So, the hexavalent chromium in the nickel-chromium plating wastewater does not have to be isolated and pretreated by reduction to the trivalent form. The new process is applicable for removal of all heavy metals. All heavy metals other than chromium are removed as insoluble metal sulfides, M(II)S. 

Concentrations of hexavalent chromium from metal finishing raw wastes are shown in Table 9.8. Hexavalent chromium enters wastewater as a result of many unit operations and can be very concentrated. Because of its high toxicity, it requires separate treatment so that it can be efficiently removed from wastewater. 

Some innovating treatment technologies may be introduced in the treatment of wastewater generated in the aluminum fluoride industry to make its effluent safer. The ion exchange process can be applied to the clarified solution to remove copper and chromium. At a very low concentration, these two pollutants can be removed by xanthate precipitation.24 A combination of lime and ferric sulfate coagulation will effectively reduce arsenic concentration in the wastewater. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Prominent among the heavy metals found in the wastewater generated in the copper sulfate industry are copper, arsenic, cadmium, nickel, antimony, lead, chromium, and zinc (Table 22.11). They are traced to the copper and acids sources used as raw materials. These pollutants are generally removed by precipitation, clarification, gravity separation, centrifugation, and filtration. Alkaline precipitation at pH values between 7 and 10 can eradicate copper, nickel, cadmium, and zinc in the wastewater, while the quantity of arsenic can be reduced through the same process at a higher pH value. 

Hexavalent chromium and metals such as zinc and nickel that are present as impurities in the chromites ore are predominant pollutants associated with the sodium dichromate plant. They are generally removed through alkaline precipitation, clarification, filtration, and settling processes. The wastewater is treated with sodium sulfide to reduce hexavalent chromium to trivalent chromium,... 

Owing to the low limits for concentrations of chromium the proposed processes for wastewater treatment concentrate on the removal, for example, by flocculation and precipitation, but as a result chromium-containing sludge/precipitate or concentrates are obtained that need further treatment. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Chromate, dichromate, permanganate, chlorate and hypochlorite and other oxidants are readily reduced hy hydrazine for example, removal of chromate from wastewater may he achieved fuUy hy converting water-soluhle chromate to insoluble precipitate of chromium hydroxide, Cr(OH)3 ... 

Other Important Considerations with Membranes Oxidizers such as sodium hypochlorite (i.e., CIO2), bromine, iodine, and ozone, which are typically used in the disinfection of wastewater, are not well tolerated by thin-hlm membranes. Such disinfectants can thus influence the efficacy of membranes in removing contaminants such as PPCPs. Furthermore, membranes can become fouled by microorganisms that can metabolize the membrane material. Thus, microbial counts of >100 cells/mL can be problematic. Likewise, dead-cell debris can also cause fouling. Membranes can also be fouled by heavy metals such as chromium. Thus, if heavy metals are deemed a problem, they should be precipitated from the wastewater prior to the filtration with membranes. 

In 1996, costs for SLM technology treatment of a wastewater system typical of that found in the nickel-plating industry were compared to those for reverse osmosis treatment of the same amount of wastewater. It was assumed that wastewaters would be processed at a feed rate of 30/gal min, 15 hr/day, and 20 days/month. It was also assumed that a total of 175 lb of nickel sulfate, 45 lb of nickel chloride, and 225 lb of chromium salts would be removed. 

Electrochemical treatment has been used for many years in the mining and utility industries and is a proven technology for removing hexavalent chromium from wastewater. 

Environmental Research and Development, Inc., offers the neutral process, which reduces hex-avalent chromium using sulfide catalyzed by ferrous iron, while precipitating heavy metals at pH ranges from 7.4 to 8.4. The vendor has combined this technology with cross-flow microfiltration to remove heavy metals from contaminated groundwater and wastewater without the need for large clarifiers. The technology has been used at U.S. Department of Defense (DOD) sites and is commercially available. 

Gao, P., Chen, X.M, Shen, F. and Chen, G. (2005) Removal of chromium (VI) from wastewater by combined electrocoagulation-electro flotation without a filter. Separ. Purif. Technol. 43, 117-123. 

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