The treatment for removing dissolved iron salts usually involves (1) oxidation by air, chlorine, or ozone followed by filtration; (2) chemical precipitation followed by filtration; or (3) ion exchange. The capacity of the treatment plant, the pH of the water, and the presence of other contaminants determine which process is the most economical. Iron is usually removed more readily than iron and manganese together. The removal of dissolved iron chelated to organic compounds is usually accomplished by coagulation followed by settling and filtration.
Oxidation is accomplished most economically by aeration. Aeration also purges carbon dioxide from water, which keeps iron dissolved as ferrous carbonate. Iron oxides may be removed by settling, filtration often is necessary. If the iron is loosely bound to organic matter, the aeration process is slow and must be accelerated by iron oxide or manganese dioxide catalysts deposited on sand, crushed stone, or coke. Chlorine and ozone effectively oxidize iron at low pH in the presence of a high organic content.
Chemical precipitation by lime is usually effective if the iron is present as ferric humates. Above a pH of 9.6, most iron is removed as ferric hydroxide. Treatment is followed by coagulation, settling, and filtration. Ion exchange effectively removes ferrous and manganous salts using sodium zeolite. Air (oxygen) must be excluded in this operation to prevent oxidation to iron and manganese oxides, which can form precipitates and plug the ion exchange column. This process also removes other salts in the water and decreases hardness.
Aeration is the most economical iron removal method in large-capacity municipal treatment plants. Chemical precipitation is frequently used in beverage and food processing plants. Ozonation can selectively remove iron and manganese and preserve the mineral taste of water.
Lead is used mainly in various solid forms, as pure metal and in several compounds. Major uses are storage batteries, bearings, solder, waste pipelines, radiation shielding, sound and vibration insulation, cable covering, ammunition, printer's type, surface protection, and weights and ballasts.
Wastewaters containing lead originate from only a few of the processes that produce lead-containing products, such as plating, textile dyeing and printing, photography, and storage battery manufacturing and recycling.
Lead is a toxic, heavy metal limited to 0.05 mg/l by USPHS Drinking Water Standards, and to 0.10 mg/l by other standards. Discharge standards in sewer use ordinances usually limit lead to 0.5 mg/l.
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