Septic systems work by separating solids from liquids in the septic tank and allowing the liquids to pass into the drain field where the soil purifies the water. Some of the solid materials in the septic tank break down by bacterial action, but others require pumping for removal.
The wastewater lines in a home join into the main line that dumps into the septic tank. The tank has baffles on each end that prevent the materials that float on the surface from backing up into the main line or passing through to the drain field, while still allowing liquid to pass freely through the tank. Each time a toilet flushes in the home, the water entering the tank forces the same volume of effluent into the drain field. The tank is usually 1,000 gallons or larger, providing a volume that allows time for bacterial digestion of solid wastes.
Water entering the drain field travels through the lateral lines where perforations in the pipe discharge it into a layer of washed gravel in the trench. The water then percolates through the soil. The soil serves as the final step in sewage treatment, purifying the water, so it enters ground or surface water without causing pollution.