How Does a Septic System Work?
System Description:

A septic tank uses natural processes to treat and dispose of the wastewater generated in your home.  It typically consists of a septic tank and a drainfield, or a soil absorption field.  The septic tank provides the first step in treatment.  As the wastewater flows into the tank, the heavier solids settle to the bottom to form a sludge layer, and the lighter solids, greases, and oils float to the top to form a scum layer.  The liquid wastewater (effluent) that flows from the tank flows into gravel-filled trenches in a typical drainfield where it is distributed via performed pipes and then treated by the natural soil system. 
System Operation:

The septic tank provides some biological treatment of the sludge and scum layers that accumulate there.  The majority of treatment occurs in the drainfield where the effluent enters the soil and is treated as it percolates to the groundwater.  The soil acts as a biological and physical filter to remove harmful substances, including disease-causing bacteria and viruses, toxic organics and other undesirable wastewater constituents remaining in the effluent.

Outlet filters or baffles are located in the tank and are designed to prevent the sludge and scum from flowing into the  drainfield.  If the tank is not pumped-out on a regular basis to remove the accumulated solids, the tank will fill with sludge and the solids will be washed-out into the drainfield, or clog the outlet filter.  If solids reach the drainfield or clog the outlet filter, they will quickly clog the soil (pot-holes will appear) and eventually lead to system failure.

Why Maintain Your System?

There are three main reasons why you should maintain your septic system.

First, poor maintenance results in failed systems requiring repairs at a minimum and sometimes a system replacement.  Repairs or replacement costs can be thousands of dollars, whereas a periodic inspection and pumping costs only about $100 - $200.

Second, untreated sewage contains disease-causing bacteria and viruses, as well as unhealthy amounts of nitrate and other chemicals.  Failed septic systems can allow untreated sewage to seep into wells, groundwater, and surface waterbodies, where people get their drinking water and recreate.

Third, contamination of waterbodies by failed septic systems pollutes water supplies, closes shellfish beds and recreational areas, and creates offensive odors.  Quality of life, recreational opportunities, livelihoods and tourism decline, and with them go the property values and economic vitality of the area.