The Septic Tank

The main purpose of the septic tank is to temporarily retain and store wastewater to allow solids, and fats, oils, and greases (FOG) to separate before the wastewater flows to the drainfield. The tank is generally designed to provide a 2-day retention time for wastewater settling to occur. Heavy solids sink to the bottom forming a layer of ‘sludge’, while light solids and FOG form a floating ‘scum layer’.

The minimum tank size for a single-family home is typically 1,000 gallons. Various types of tanks are approved for use in Rhode Island – tanks can vary in size, configuration, and material. Tanks that were installed prior to current RI state standards (adopted in 1970) could vary from steel drums to concrete cylinders. Tanks are most commonly constructed from concrete, though fiberglass tanks are regularly used on lots with limited access to heavy equipment.

The interior of tanks may also vary, generally containing either one or two compartments. Single-compartment tanks have a divider at the outlet end to keep floating FOG from leaving the tank and entering the drainfield. The tank shown here is a typical two-compartment tank, split by a divider called a baffle; the first compartment contains 2/3 of the tank volume, the second contains 1/3. Two compartment tanks are typically larger than single-compartment tanks, with a capacity of 1,500 gallons. Wastewater flows from the first to the second compartment through a gap in the baffle at a height that should correspond with the ‘clear zone,’ or the most clarified water in the tank. Additional separation of solids and FOG occurs in the second compartment. The two-compartment configuration, combined with a larger storage volume for settling and storing waste, ensures that the greatest possible amount of separation of solids and FOG takes place before the wastewater leaves the tank to be distributed to the drainfield. Because of this additional protection for the drainfield, some communities such as the town of Charlestown RI, require a 1,500 gallon, two-compartment tank for new construction.