The waste material from a home makes its way through the waste piping system to a septic tank. This is an underground tank that is chambered and baffled in such a manner that it retains the organic material in the tank to give the bacteria a chance to break them down. As the bacteria break down the organic material, the undigested material sinks to the bottom of the tank in what is called a "sludge" layer. The undigested fats and soaps float to the top of the tank in what is called a "scum" layer. It is this "sludge" and "scum" that we want cleaned out of the tank periodically.
The inlet pipe of the septic tank is slightly higher than the outlet pipe and the tank is supposed to always be full. A half-full or empty tank usually indicates a leak in the tank and this is unacceptable. The theory behind the septic tank is to retain organic material in the tank and allow only cleaner septic effluent to enter the drainage field.
From the tank, the liquid is usually sent to a "distribution box". This is a concrete box, underground, approximately 18 inches square. It has one inlet pipe from the tank and may have several outlet pipes, depending upon how many different drainage trenches are involved in the system. The purpose of the distribution box (D-box) is to slow down the velocity of the effluent coming out of the tank and level it off so it gets distributed equally through all the drainage trenches.
Once again, the theory behind this process is to have relatively clean liquid flowing into the drainage field. It is the drainage field that is expensive to replace and we want to protect it from organic material, which can clog the underground soils and prevent the liquid from dissipating into them.
"In-sink Garbage Disposal" units and private on-site sewage disposal systems do not mix well together. A garbage disposal introduces more organic material into the tank than the bacteria can handle. This allows undigested material to enter the drainage field. A system can be designed to accommodate a garbage disposal, however, most are not.
Flushable items, such as sanitary napkins or diapers should never be introduced into a private on-site sewage disposal system. Nothing that can be thrown into the garbage should ever be sent into a private system.
Cooking fats and oils should be collected in a can and thrown into the garbage. These items do not digest well in a private system and they tend to clog the plumbing pipes as well.
Solvents such as paint thinners, kerosene, turpentine, etc. should never be introduced into the system because they tend to kill bacteria, thereby reducing the ability to digest organic material.
Photographic chemicals also have a debilitating effect, not only on the system, but they deteriorate the plumbing pipes as well.
The use of laundry detergents and bleach are fine, as long as they are used in moderation.