Treatment of Dilute Metal Effluents in an Electrolytic Precipitator
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18-10-2010, 03:55 PM
A.W. BRYSON and K.A. DARDIS
Excessive concentrations of metals in industrial effluents may adversely affect the performance of sewage purification works. Although there is existing technology for treating these effluents, it has not found wide application due to costly equipment and chemicals and the absence of sufficient space on most plants. In this paper the feasibility of removing these metals from dilute solutions by electrolytic precipitation using a particulate electrode is investigated.
Based on laboratory investigations a test plant was constructed which was compact, did not require the addition of chemicals and could be operated by unskilled personnel. The plant was installed at an electroplating works on the Witwatersrand and was extremely successful in treating wash waters containing copper, nickel chromium and zinc.
The harmful effects of heavy metals on human and animal life have been known for a very long time: for instance, lead poisoning was probably first recognised by Hippocrates (370 BC) and mercurialism was described by Pliny (AD 29) in his writings on the diseases of slaves (Hunter, 1975). It is only in recent years, however, that public concern, spurred by such incidents as outbreaks since the early 1950s of minamata disease (the deformation of the foetus in the womb due to mercury absorbed by the mother), has led to the introduction of strict legislative standards governing exposure to heavy metals and the discharge of effluents containing these metals.
Such effluents are produced by a broad spectrum of sources which include the metal refining, metal plating, tanning, chlor-alkali, anodising and automotive industries. The wastes contain a wide range of metal ions including arsenic, antimony, cadmium, chromium, copper, lead, manganese, mercury, silver, selenium and zinc. Electroplating effluents can also contain additional quantities of extremely toxic cyanide compounds. The magnitude of the problem may be gauged by the fact that it is estimated that at least 10 000 tons of copper are discarded in electroplating rinse waters in the U.K. annually (Tseug and Mahmood, 1977) and in the United States the value of nickel and chromium lost in plating wastes, has been conservatively estimated at 50 — 60 million dollars per year (Cherry et al., 1975). In 1973 losses of chromium and nickel in the Johannesburg municipal area alone were estimated to cost a quarter of a million rands (Funke and Coombs, 1973).
Toxic effects of metals have been observed at very low levels (Funke, 1973; Nemerow, 1971) and can pose problems during treatment in conventional sewage works where, above certain threshold concentrations, they can disasterously inhibit the bacterial action of the sludge digestion process. Even when the metal concentration of the raw sewage is low enough for efficient removal, the metals could still be concentrated to levels in excess of 1 000 mg/1 in the sludge discarded by the works and would make the sludge unacceptable as a nitrogenous fertiliser for crops or grazing (Coker and Davis, 1978).
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