- Another method of purifying highly polluted water - which is not employed however for furnishing water for domestic consumption - is the use of the septic tank. The septic tank is a device employed for purifying sewage so as to prevent the contamination of the streams below a city by the sewage of the city. The theory of the septic tank is to bring the sewage into contact with highly active bacterial organisms. These organisms have the property of attacking the organic matter in the sewage and oxidizing it and thus purifying the water. There is of course left in the effluent water the products of oxidation, for instance the effluent contains considerable quantities of nitric acid, or nitrates, due to the action of nitrifying organisms. The cellulose and starchy matters in the polluted water are also attacked by organisms which have the power of destroying them by fermentation or oxidation and converting them into harmless forms. The septic tank is composed usually of alternating layers of charcoal and coarse sand or gravel. The materials must be coarse enough to permit the purifying organism to find a nidus in the interstices, otherwise the septic tank would serve merely as a mechanical filter. This would operate to defeat its purpose, namely, to destroy the organic matter of all kinds which the sewage contains. This does not contemplate the destruction of particles of any noticeable size. For this reason the sewage which is to be purified must first be strained so that only the finer particles of its suspended matter remain therein. The sewage of Baltimore is now purified in a manner substantially as just described.