A septic system consists of a watertight container that functions as a detention tank for sewage sludge and a disposal field for the absorption of the liquid wastes. A septic tank is usually made of concrete but might be made of steel or fiberglass. Raw sewage from the house is discharged into the septic tank through the house drain line. (See FIG. 13-4.) After the sewage settles, the solids are decomposed by bacterial action and are converted into a liquid and a sludge that accumulates at the bottom of the tank. Several types of gases are by-products of the decomposition process, the most common of which is methane, an odorless and highly inflammable gas. The gases generated in the septic tank usually flow back through the house drain and are discharged harmlessly to the atmosphere at the roof-mounted vent stack.
When the level of the liquid (effluent) in the septic tank rises to the outlet port, the effluent flows through the outlet pipe to a drainage field. The drainage field, also called the leaching field, consists of a series of perforated pipes set into a bed of gravel. As the effluent flows through these pipes, it trickles through the perforations and is absorbed into the ground. The rate at which the ground absorbs the effluent (percolation rate) determines the size of the leaching field. When the topography changes abruptly or the area available for a leaching field is too small for adequate absorption, a seepage pit is used. It is basically a covered pit with an open jointed or perforated lining through which the effluent will seep or leach into the surrounding soil.
To determine the size of the leaching area needed for a house, percolation tests are taken in the area of the proposed sewage-disposal system. The rate of water absorption will depend upon, among other things, the type of soil and the level of the water table in that area. If the percolation tests are taken during a drought or when the water table is low (the water table rises and drops during the year), the concluding data might result in designing an undersized leaching field that will cause premature failure of the septic system. I know of a village just northwest of New York City where the septic systems in hundreds of homes failed within eight years of installation, some of them within two years. A properly designed and maintained septic system should last between twenty and thirty years; indeed, fifty or more years is not uncommon.
Failure of the septic system will usually show up in the area of the leaching field and not necessarily over the septic tank, although there might be indications in both areas. Look for wet spots or a lush growth of grass. Both can be accompanied by an objectionable odor. When the ground in the seepage field becomes saturated and can no longer absorb the effluent, the liquid will build up and accumulate on the surface. The effluent contains nitrogen and other compounds that are natural fertilizers. When the effluent surfaces, it causes vegetation in the area, particularly grass, to thrive and have a lush green color. However, a healthy-looking patch of green grass over the leaching field is not necessarily an indication of a septic failure. During dry weather, when grass is apt to grow very slowly and turn brownish, the ground over the leaching field (depending on the depth of the field) often contains sufficient moisture to promote the growth of the grass and maintain the green color.
For the leaching field to function properly, there must be sufficient voids in the soil so that the effluent will be absorbed by the ground. Over the years, the voids can be filled with suspended solids, reducing the rate at which the effluent is absorbed into the soil to the point where the effluent surfaces. When this occurs, it is necessary to install a new leaching field. Aside from the cost, a serious problem can arise concerning how to handle the effluent when there is no more room on the property for a new leaching field or seepage pit. If the house has a septic system, find out if there is sufficient room for expansion of the leaching field should it be necessary at a later date.