A septic tank is a large, underground, watertight container, typically about 9 feet long, 4-5 feet wide and 5 feet tall that is connected to the home's sewer line. While typically designed with a 1,000-gallon liquid capacity, the size of the tank is legally determined by the number of bedrooms in the home. (Septic tanks come under the legal supervision of counties in Montana.) Septic tanks may be rectangular or cylindrical and may be made of concrete, fiberglass or polyethylene.

Raw waste water from the bathroom, kitchen and laundry room flows into the tank where the solids separate from the liquid. Light solids, such as soap suds and fat, float to the top and form a scum layer. This layer remains on top and gradually thickens until you have the tank cleaned. The liquid waste goes into the drainfield, while the heavier solids settle to the bottom of the tank where they are gradually decomposed by bacteria. But some non-decomposed solids remain, forming a sludge layer that eventually must be pumped out.

Septic tanks may have one or two compartments. Two-compartment tanks do a better job of settling solids and are required in some areas for new installations. Tees or baffles at the tank's inlet pipe slow the incoming wastes and reduce disturbance of the settled sludge. A tee or baffle at the outlet keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments.