Chromium discharge

Chrome tanning is the most important tanning method in leather production. The effluents from the tannery house contain a considerable amount of chromium. A limit to the chromium discharge is mandated by pollution regulations in almost every country. It is necessary to recycle chrome tanning materials from the effluents. The most common way of recovering the spent chromium salts is by precipitation (Thorstensen, 1993). The pH of the effluents may be raised to the precipitation point of the chromium salts, which precipitate as a hydrated chromium oxide. 

Modem manufacturing processes quench the roast by continuous discharge into the leach water held in tanks equipped with agitators. At this point the pH of the leach solution is adjusted to between 8 and 9 to precipitate aluminum and siHcon. The modem leaching operations are very rapid because no or htde lime is used. After separation of the ore residue and precipitated impurities using rotary vacuum filters, the cmde Hquid sodium chromate may need to be treated to remove vanadium, if present, in a separate operation. The ore residue and precipitants are either recycled or treated to reduce hexavalent chromium to Cr(III) before disposal. 

Environmental Concerns. Dyes, because they are intensely colored, present special problems in effluent discharge even a very small amount is noticeable. However, the effect is more aesthetically displeasing rather than ha2ardous, eg, red dyes discharged into rivers and oceans. Of more concern is the discharge of toxic heavy metals such as mercury and chromium. 

Chromium (total) 0.1 0.1 Some people who use water containing chromium well in excess of the MCL over many years could experience allergic dermatitis Discharge from steel and pulp mills erosion of natural deposits... 

Mass Balance Estimates. Based on National Pollutant Discharge Elimination System monitoring reports, the total daily discharge of trace elements into the main stem Willamette River is of the order of 100 pounds per day. Seventy-five percent of the total is zinc with the bulk of the remainder due to chromium and copper. Table 6 identifies industrial and natural sources of trace elements into the Willamette basin. The table indicates that an average of 97 percent of all trace element loading to the basin is natural in origin. The natural component is due to weathering of soil and rocks in the basin and this... 

Applicable local, state, and federal environmental laws require that the waste generated by the nickel-chromium plating process be pretreated to provide a discharge acceptable to the public wastewater treatment system. 

Chromium can exist as either trivalent or hexavalent compounds in raw wastewater streams. The chromium that passes through the POTW is discharged to ambient surface water. Chromium is toxic to aquatic organisms at levels observed in POTW effluents15 ... 

All four wastewater treatment systems introduced in this chapter are technically feasible for treating nickel-chromium plating wastewater in order to meet the maximum permissible concentrations shown in Table 6.1 for industrial wastewater discharge into a municipal sewerage system15 or Table 6.2 for discharge to surface waters.21... 

Generally, water is used in this plant to cool, leach, filter wash, scrub, heat, and washdown. The unreacted ore is slurred and sent, along with chromium and other impurities originally present in the ore, to the treatment plant. The boiler blowdown, which is sometimes contaminated with chromium escaping from the process area, adds to the volume of wastewater coming from the plant. The non-contact cooling water from the plant contains dissolved sulfate, chloride, and chromate thus it is sent to a wastewater treatment plant. The scrubber water may be used to slurry the ore or discharged. 

Cadmium, barium, and chromium Other potential pollutants based on raw materials are zirconium, asbestos, and iron Note Process water use and discharge in this category are limited... 

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