How a septic system works??
There are two basic parts to a septic system, the tank and the drainage system.
The Septic tank's basic objective is to separate solids from liquids, retaining the solids in the tank, and passing on as clear as possible a liquid to the drainage system. The heavier material sinks to the bottom and is called sludge. The lighter material floats on the surface of the liquid and is called the scum layer. The liquid in the middle, although relatively clear, contains soluble or suspended organic matter. It is this liquid that passes, though the outlet baffle, to the drainage system. The drainage system may be either a drainfield, or drywell.
The tank holds the liquid sewage so that there will be time for small particles of organic matter to be digested or separate before being carried out of the tank into the drainage system where they can clog.
There are three things that affect the "digestion/settling time.
If you have a 1,000 gallon tank and pass through 1,000 gallons of water in one day and life were perfect (i.e. no short circuiting), the average digestion/settling time would be 24 hrs. So a larger volume tank provides increased time for liquid treatment, thus a clearer effluent.
Needless to say, conservative water use increases the time available for liquid layer treatment, thus improved effluent quality. Additionally, all effluent contains some solids. For every gallon of clean water added, a gallon of effluent with solids is drained. Conservation is very important.
Baffles in the tank keep floating solids from bypassing the system and floating right out the line. They also reduce "short circuiting or channeling" where the waste follows a channel through the tank, thus reducing the time in the tank.
Older tanks were single compartment tanks, but starting in July 1976, two compartment tanks were installed. With two compartment tanks, everything that escapes the first compartment (and would be clogging your drainage system) gets a second chance to settle out, and more time for treatment.
The processes in the tank are anerobic, that is without oxygen, while the processes in the drainage system are aerobic. Bacteria in the tank (supplied by you) release enzymes which break down the waste for their digestion. There is enough bacteria in a normally functioning tank to do the required job of waste breakdown/digestion. Thus, additives are not required unless chemicals/excess bleach are used that kill the bacteria colony.
Avoid flushing things that do not break down such as femminine sanitary, plastic or rubber. Also avoid using garbage disposals since the waste does not break down sufficiently and fills the tank with damaging solids.
Drainfields are the most common form of drainage for newer systems. A system consists of a distribution box and drainlines. The distribution box takes the effluent from the tank and by virtue of being installed level, evenly distrbutes the effluent flow to each drain line. Most commonly, waste is distributed via gravity, but in some instances, (i.e. tank deeper than drainfield) a pump is used.
Some systems require a dose to the drainfield (larger volume, less frequent), thus a 2nd tank is provided where either a syphon is installed (gravity feed) or a pump (operated by floats) distributes the effluent to the drainfield actively by pumping.
Drywells are very common for older, (60's and older) systems. Drywells are currently installed only by exception. Drywell constuction varies widely (especially the older ones) but is generally a 10 ft deep by 6-7 ft diameter, loosely stacked concrete block structure surrounded with drain rock and topped with a concrete lid.