Many homes in long-established suburban communities, as well as most houses located in rural areas, still depend on their own private waste-disposal systems to get rid of sewage. The most widely used -- and accepted -- method is a septic tank combined with a drainage field. Another commonly used type of disposal installation is a cesspool instead of a septic tank.

A cesspool differs from a septic tank in that it is really just a cement-block-lined seepage pit into which raw sewage flows as it comes from the drain pipes inside the house. Solid waste settles on the bottom, while the liquid waste seeps out through the many openings in the wall of the pit, as well as through the porous soil at the bottom of the pit.

As a rule, the solid waste retained at the bottom of the pit remains there, although part of it may gradually decompose, mix with the liquid waste and seep out into the surrounding ground.

Because they allow raw sewage to go directly into the soil, cesspools are not approved by local codes in many communities. They should never be located less than 150 feet away from any well or other natural source of water. They are only effective (as a leaching pit) when built in places where the soil is quite sandy or porous, and where there is not an excessively high underground water table. They should never be located closer than 20 feet away from the house foundation.

Septic tanks do a much better job of disposing of sewage, safely, hence they are approved in most communities where there is no municipal sewage system. As shown in the accompanying drawing the septic tank is a prefabricated, watertight concrete tank with an inlet too at one end. As sewage enters, the solid material settles to the bottom while the liquid floats to the top. Unlike a casspool, a septic tank does treat sewage before allowing it to seep out into the soil, because the solids are acted upon by natural bacteria that in time liquify them and leave only a small amount of treated sludge on the bottom.

A septic tank must be watertight to keep ground water from seeping in, so it can be located as close to the house as desired. As the tank fills, the liquid waste flows out through the discharge tee and then into the drainage field. This drainage field consists of a series of interconnected perforated pipes buried underground in a pattern that will most effectively disperse the waste in the soil. In most installations there is one pipe that goes from the septic tank to a distribution box, then the perforated pipes radiate out from this in various directions. The purpose of the distribution box is to spread the liquid waste equally to all pipes in the drainage field.

In a well-designed disposal system a special grease trap is installed in the main waste line going from the house to the septic tank (or cesspool), its purpose being to trap grease before it can enter the septic tank or cesspool. Grease in the system will cut down on its efficiency and necessitate more frequent cleaning out. In a cesspool, grease not only increases the need for frequent pumping out, it also clogs the openings in the wall, saturates the soil around the pit and shortens the life of that cesspool.

As a rule, when a cesspool starts to clog and fill up -- often indicated by a backing up of sewer lines inside the house, or by a foul odor in the ground directly above or near the pit -- pumping out will give only temporary relief, since the cesspool will soon fill up again. This is because the openings in the pool wall, as well as the soil around it, are probably clogged and no longer porous, so waste will not seep away as quickly as it did originally.

Some contractors pour acid into the cesspool (as a means of etching or eating away the grease) after it is pumped out, but most experts feel that this helps for only a short while, if at all. The only really permanent cure for a clogged cesspool is to dig a new one some distance away, then connect this with the original one to take care of the overflow. Better yet, install a septic tank and drainage field that will last almost indefinitely if properly maintained.

Because septic tanks accumulate sludge on the bottom, they must be periodically pumped out by a professional contractor who has the needed equipment. If the tank is the right size, pumping out should be required about once every two or three years, but this depends on the amount of flow the system gets each day, and on how careful occupants are about discarding unnecessary waste through the drain pipes (the more garbage you throw down the drain, the quicker the tank will fill up).

If a grease trap has been installed as mentioned above, emptying it regularly before grease can overflow into the main tank will help lengthen the time between pump-outs (and possible future clogging of the drainage field).

The homeowner who is not certain of how fast sludge is accumulating should check the level inside the tank about once a year. Septic tanks have a removable cover so the level of the sludge can be determined by probing with a long pole. Wrap the bottom of this pole with a layer of thick cloth or old towels, then push it down to the bottom and draw it out. The color on the rags will indicate the depth of the sludge at the bottom.

As the drawing shows, there is a layer of scum or floating waste at the top, as well as a layer of sludge along the bottom. When the depth of the sludge layer, combined with the depth of the floating layer of waste, is more than about one-third the depth of all the liquid in the tank (generally when the sludge depth exceeds 12 inches to 15 inches), it is time for pumping out.

Another way to check on the condition of the tank is to take the inspection cover off the distribution box and look inside. Since only liquid waste should be leaving the septic tank and flowing out into the drainage field, any sign of solid waste entering the distribution box is an indication of a septic tank that needs pumping out. If solid waste is allowed to flow into the drainage field the pipes will eventually clog and an expensive replacement job will be needed.

The best time of the year to have a septic tank pumped out is in the spring, because warm weather will hasten bacterial action, and incoming sewage will be treated more quickly if there is no sludge to interfere with the action. Also, warm weather makes it more likely that an over-full tank will give off obnoxious odors in the area near where it is buried.

Although various chemicals and special yeasts or other products have been thought to be helpful in reducing the accumulation of solids inside a septic tank, most experts agree that these products are of little or no real use: The only effective way to reduce solids is to have them pumped.