Index >> Waste Water Microbiology >> Chemical Precipitation

Chemical Precipitation - Certain soluble inorganic materials can be removed by the addition of suitable reagents to convert the soluble impurities into insoluble precipitates. These precipitates can be flocculated and removed by sedimentation.

The extent of removal depends on the solubility of the product and this is usually conirol1ed by factors such as pH and temperature. Chemical precipitation can be used in industrial 'wastewater treatment. For example, to remove toxic metals from effluents. Such effluents may contain considerable amounts of hexavalent chromium which is harmful to biological systems.

The removal of chromium can be accomplished by the addition of .ferrous sulphate and lime. Ferrous iron first reduces hexavalent chromium to trivalent chromium by simultaneous oxidation of ferrous iron to ferric. The resulting forms can be precipitated as hydroxides by lime.

Chemical analyses for individual ions are given in terms of corresponding ions. Total hardness of a sample is always calculated from both Mg++ and Ca++ or from anyone of them as the case may be. For example, in the above sample only with Mg++, the total hardness is 98.5 mg/l (as CaCO3).

Some surface waters and many ground waters may have a hardness of several hundred mg/l. Hardness above 300 mg/l is usually considered as undesirable. For potable water supplies water with upto 75 mg/l hardness is considered as soft. The method of precipitation softening adapted depends on the form of hardness present.