Water is, usually, free from oil, but small amounts of oil is likely to get mixed from the lubricating oils always used for lubrication of the pumps. Moreover, the steam generated is, generally, employed to drive engines and the exhaust steam on being condensed gives virtually distilled water, except that it contains oil droplets brought from the lubricating oil used in the engine. Vegetable or animal oil may get hydrolyzed yielding fatty acids, which may have corrosive effect in water treatment plant equipments, plumbing systems and all pipe lines. Moreover, the, fatty acids may combine with alkalinity, thereby producing soaps, which forms sticky coatings on the inner metallic surface of boiler. These coatings attracts other impurities, thereby forming thicker deposits. In order to remove oil front the exhaust steam, the latter is passed through “mechanical oil separator”, which is a chamber provided with a number of metal plates. As the oil droplets bearing exhaust steam passes over the plates, droplets of oil are retained on the surface of metal plates and oil droplets. Free-steam obtained, which is then condensed and condensate again used in boiler. For getting very high quantity water, sodium aluminate is added to the condensate, when aluminum hydroxide produced on its hydrolysis, entraps all oil droplets.
Sewage is the liquid wage which includes human and house hold waste waters, industrial weeks, ground wastes, sired washings and worm waters. Sewage besides about 99.9 percent water contains organic and inorganic matters in dissolved, suspension and colloidal state.
Fresh domestic sewage is, usually, grey-green to grey yellow in color, but is darkens rapidly with time due to its decomposition, Fresh domestic sewage has a rather sweet smell, but when stale, it has on offensive odor due to the evolution of hydrogen sulphide, ammonium sulphide, phosphate, etc.