In situations where the soil contains considerable moisture or is even saturated, the field may be improved by partially encircling it with a tile line laid to serve as a drain. Such a line should be on the high side and have surface outlets for removing the water from the soil. It should not be laid so close to a disposal tile line that it will drain the sewage effluent from the disposal field onto the surface of the ground. [Illustration: Slope of disposal tile 2 to 4 inches per 100 feet. Slope of underdrain tile not less than above. Plug upper end of underdrain tile lines, lower end to discharge into rock-filled seepage pit or into other approved outlet. Figure 10.--Filter trench with underdrains.] When the tile field is underlain by stratified rock or where under-drainage is necessary, advice should be sought from the public health authorities, as regulations in some States may not permit the use of certain methods. CARE AND MAINTENANCE OF SEPTIC TANKS A septic tank when first used does not need starters, such as yeast, to promote bacterial action. A good septic tank normally requires no maintenance other than a yearly inspection and an occasional cleaning. Frequency of cleaning depends on the capacity of the tank and the quantity and composition of the sewage. Tanks of the size recommended in this bulletin may require cleaning at intervals of 3 to 5 years. The tank should be cleaned when 18 to 20 inches of sludge and scum has accumulated. If a drain has not been provided, sludge may be removed by bailing or by pumping with a sludge or bilge pump. It is not necessary to remove the entire liquid contents. Burial in a shallow pit or trench with at least 18 to 24 inches of earth cover at a point remote from water sources is the most practical method for disposing of these wastes. A septic tank is intended to handle sewage only. Coffee grounds and ground garbage may be included if there is an ample supply of water for flushing and the tank is cleaned more frequently than would otherwise be done. The size of the tank should be increased at least 25 percent if these materials are included in the sewage. =_Do not use matches or an open flame to inspect a septic tank, as the gasses produced by decomposing sewage may explode and cause serious injury._= EFFECT OF DRAIN SOLVENTS AND OTHER MATERIALS Soap, drain solvents, and other mild cleaning or disinfecting solutions used for normal household purposes cause no trouble in the tank. Constant use in large quantities, however, and disinfected wastes from the sickroom may prove harmful. Wastes from milk rooms, strong chemicals used in sterilizing equipment or in photographic work, and the wastes from filters or water softeners not only reduce bacterial action but also cause abnormally rapid accumulations of sludge and clogging of the tile lines. PROTECTION AGAINST FREEZING