Most domestic septic tanks (see Fig. 2) are designed to hold between 500 and 1000 gal of liquid sewage, the actual size of the tank depending mainly on the number of bedrooms or occupants in a house, as described below. This sewage consists of both solid and liquid waste matter. When the sewage is first discharged into the septic tank, that portion of the solid wastes that is capable of disintegrating into small particles in the liquid will do so. Those particles that are heavier than the liquid will slowly settle to the bottom of the tank and become part of a layer of sludge lying there. The lighter particles will rise slowly to the top of the liquid and become part of a layer of scum floating there.
Living in the sludge are vast numbers of anaerobic bacteria, which are bacteria that can live and reproduce in the absence of oxygen. (The layer of scum is very effective at excluding atmospheric oxygen from the sewage.) The bacteria feed upon the fresh sewage, and, as a consequence, the sewage is broken down into chemically simpler substances, which include mainly water plus a number of gases such as methane (also called swamp gas or sewer gas), carbon dioxide, carbon monoxide, hydrogen, hydrogen sulfide, and sulfur dioxide, plus smaller quantities of other gases. These gases, with the exception of the first four mentioned above, are responsible for the very pungent, putrid stench we associate with decomposing organic matter.
These gases bubble their way up through the liquid and find their way out of the septic tank and into the atmosphere via the house sewer, main soil stack, and main vent in the house, the end of which projects above the roof (for a description see PLUMBING). Most of the sewage is decomposed within 24 hr; very little of it actually remains in the tank as solid sludge.
Anyone who has studied elementary biology will recognize the similarity in the way that sewage is broken down by anaerobic bacteria in a septic tank with the digestive process that takes place in the intestines of all living creatures and also with the decomposition of dead organic matter that occurs throughout nature. In a septic tank the end result of this process of digestion, or decomposition, is a relatively small amount of indigestible solid matter that remains behind in the tank as sludge and scum plus a relatively large quantity of effluent, a milky white, malodorous, and potentially toxic fluid.
This effluent must now be converted into a clear, odorless, nontoxic liquid. This is accomplished by discharging the effluent into the soil under conditions that will allow the minute particles that remain suspended in the effluent to be filtered out. At the same time, atmospheric oxygen that has made its way into the soil will combine with the anaerobic bacteria in the effluent. The chemical combination of this oxygen with the bacteria (that is, the oxidation of the bacteria) will destroy them by converting them into harmless carbon and nitrogen compounds.
In most domestic septic tank systems, the discharged effluent usually enters a system of drain tiles, which is the absorption system (see Fig. 3), with adjacent tiles having slight gaps between them. These gaps allow the effluent to seep out of the dispersal system and leach into the soil.
This kind of absorption system works most effectively when the effluent can spread itself throughout as large a volume of the soil as possible. For this reason, the tiles must be laid dead level, or with only a very slight slope, so that the effluent can run through the entire system and not concentrate in one location. As the effluent seeps out between the gaps, it soaks deeply into the soil, and the solid matter suspended in the effluent is filtered from it by the particles of soil.
Oxygen is rarely found in soil at depths of more than 5 ft. If oxidation of the anaerobic bacteria is to occur, therefore, the absorption system must be installed fairly close to the surface of the earth, which means that the septic tank and all the interconnecting sewer lines must also be located fairly close to the surface of the earth.
In addition, if both the air and the effluent are to make their way deep into the soil, there to meet, the soil must have a sandy or gravelly texture. The more granular the soil, the smaller the absorption system need be, because both the air and the effluent will be able to make their way through the soil with relative ease.
On the other hand, the denser and more clayey the soil, the smaller the voids between the particles and the less efficiently the absorption system will operate. To compensate for this inefficiency, the size of the system must be increased. As soils become increasingly heavy, however, a point is reached at which the soil is so impermeable that it is impossible for either air or effluent to make its way through the soil at all. When this is the case, it will be necessary to install what is called a sand filter system.
Sand Filter System
In a sand-filter system (see Fig. 4), the soil is removed to a depth of from 5 to 6 ft and layers of coarse sand and gravel totaling 4 1/2 to 5 ft in depth are laid down in its place. After this sand filter system has been installed, a network of distribution drain tiles is laid down on top of the bed of sand. The effluent will seep out between the tiles, as already described, and soak into the sand, where it will be both filtered and aerated.
This rejuvenated effluent must now be disposed of in some way; its removal is accomplished by means of a network of collection drain tiles that has been installed at the bottom of the sand filter, under the distribution tiles. The system of collection tiles will collect the filtered and oxidized effluent and lead it away to some nearby body of water, to a drainage ditch, or perhaps to a more porous layer of soil nearby. By this time the effluent will be quite harmless and odorless. Although a sand filter is much more expensive to install than the more usual kind of subsoil absorption system, it is exceptionally efficient, which means that the overall area required for the system can be much smaller.
Very often, when the absorption system must be buried rather deeply in the soil, whether it is a subsoil absorption or sand filter type, vent lines are installed that lead from the drain tiles to the surface of the earth. These vents allow air to circulate through the system, from where it can also make its way into the soil or sand filter.
It very often happens that a site is much too small to permit the installation of an absorption system of the required size, even though the soil may be permeable enough to allow the effluent to percolate through it. In this case, a seepage pit (or leaching cesspool, or dry well) must be built in place of an absorption system.
A seepage pit (see Fig. 5) is just that: a circular pit dug into the soil into which the effluent is drained. The pit must be large enough to hold all the effluent that is likely to be discharged from the septic tank in a 24-hr period, and it must be capable of allowing all this effluent to seep into the surrounding soil within the same 24-hr period.
A seepage pit is constructed of masonry units, either stone, brick, or concrete blocks, but, whatever the material, the individual masonry units must not be mortared together. Instead, the joints between the units are left open, for it is through these joints that the effluent will seep into the soil.
The soil must, of course, be porous enough to enable the effluent to leach into it at the required rate; otherwise the effluent will simply back up into the septic tank. A seepage pit cannot be dug too deeply into the soil, however, because oxygen must still be able to make its way through the soil to the effluent.
The construction of a seepage pit may also be complicated by the fact that permeable and impermeable layers of soil are mixed together, which will prevent the efficient drainage of the effluent into the soil. Nor can a seepage pit be dug if the groundwater level is too high because, for reasons of health, the effluent should never come in direct contact with ground water. The local health code usually requires that the bottom of a seepage pit be at least 2 to 4 ft above any groundwater.
If one seepage pit having the required capacity cannot be built on the site, then it will become necessary to build two or three, or more, seepage pits. The distance separating the pits must be sufficient to prevent their leaking into each other. Seepage pits must usually be spaced at least three times their diameter apart.
A seepage pit may also be constructed in tandem with a subsoil absorption system if the soil is not permeable enough to drain away all the effluent that is discharged into the drain age tiles. In this case, a system of collection drainage tiles is installed about 2 ft below the distribution drainage tiles and the partially cleansed effluent is allowed to drain from the collection system into the seepage pit, from whence it can make its way into the soil. In this type of duplex system, the seepage pit can, if necessary, be located a considerable distance from the subsoil disposal system.
Or a seepage pit can be built in tandem with a sand filter. In this case, the seepage pit acts more like a temporary storage tank than as a purification system.