In addition to the increased production of sewage due to the growth of population, the per capita production of waste water is growing: in many cities it may amount to 600 litres per day per person. At the same time, its content of organic and mineral pollutants is also large and may amount to 10 litres of wet sludge per person daily, or about 50 kg of dry solids per person per year. Domestic and municipal sewage contains decomposable organic matter that exerts a demand on the oxygen resources of the receiving waters. This biochemical oxygen demand (BOD) is a measure of the weight (per unit volume of water or waste water) of dissolved oxygen consumed in the biological processes that degrade organic matter; it is determined by means of a standard test procedure. BOD values range from approximately 1mg/litre (for natural waters) to 300-500mg/litre (for untreated domestic sewage). The organic matter consists primarily of carbohydrates, proteins from animal matter and miscellaneous fats and oils. The specific classes of organic compounds found in sewage include amino-acids, fatty acids, soaps, esters, anionic detergents, amino-sugars, amines, amides, and many others. Much of the impurity in municipal wastes is material capable of settling, which may be deposited at the bottom of receiving waters to form deep layers of organic sludge. Dissolved salts in the form of ions such as sodium, potassium, calcium, manganese, ammonium, chloride, nitrate, nitrite, bicarbonate, sulphate and phosphate are the main inorganic constituents of sewage and other waste-waters. There are also significant traces of heavy metals, such as mercury, synthetic chemical and drugs. Domestic and municipal sewage invariably contains a variety of micro-organisms, some of which may be pathogenic. Although most human intestinal pathogens do not survive for extended periods outside the body of the host, there is evidence that they may remain sufficiently viable in different types of aquatic environment to be able to infect man.