It is convenient to divide sewage purification into two processes, the preliminary process and the final, or finishing process, and, while the preliminary process, in itself, never accomplishes purification, yet it is of considerable value in facilitating and increasing the rate and efficiency of that purification. The most common preliminary treatment is sedimentation, by which the larger solids in suspension are allowed to settle in a tank or tanks so that the filter beds later used are relieved from the accumulation of those deposits. Under the name of septic tank such a receptacle for suspended solids has been exploited as a complete method of purification, and many underground tanks have been constructed in various parts of the country which have, at the time of their installation, been considered competent to furnish all the necessary purification. When it is remembered that less than one-half of the organic matter in sewage is in suspension and that the best results in any sort of a tank succeed in depositing only one-half of those suspended solids, it can readily be seen that a tank, whether called septic or settling, cannot be a complete method of treatment. In reality, such a tank does little more than take out from the sewage the greasy material and a certain proportion of the suspended matter. Whatever part of this is organic matter may, by a particular arrangement of the tank, be considerably reduced in quantity, so that the intervals of cleaning can be extended, but in every tank the removal of the deposits is necessary, and subsequent treatment is required if adequate purification is accomplished. The final, or finishing, process may be carried out according to any one of several methods. It may be done by discharging the tank effluent into a system of agricultural drains laid just below the surface of the ground, called sub-surface irrigation. It may be done by removing the top soil from a bed of sand placed by nature, and needing little except suitable surface distribution to insure the most efficient purification. For a small plant, instead of a sand filter, for which the sand is found naturally in a suitable location, an artificial filter may be built by preparing an enclosure and carting in sand for filling. Where no sand is available, or where its use would be uneconomical, broken stone may be used to ensure final treatment. With stone, on account of its large voids, the enclosure must either be water-tight, and the outlet pipe must be provided with a valve or other device so that the sewage under treatment may be held in the enclosure or tank long enough to deposit the solids in suspension and to be acted on by the bacteria concerned. This method is known as the contact bed treatment. Or, finally, the desired results may be obtained by spraying the sewage onto a deep layer of broken stone, the method being called the sprinkling filter treatment.