The Difference Between a One & Two Stage Septic Tank

Septic Tank Fundamentals

• When wastewater leaves a house equipped with a septic tank, it is collected in a large (hundreds or thousands of gallons) tank designed to hold it for an extended period of time. Gradually, solid waste settles out of the water, and is digested at the bottom of the tank by anaerobic bacteria (bacteria that don't require oxygen). Treated water with much of the solid waste removed flows out of the tank and into a series of pipes that pass it slowly into the surrounding soil, allowing other natural processes to finish the process of filtering the water back into the natural environment with a minimum of contaminants or pollutants.

Disadvantages of One-Stage Tanks

• A single stage tank has two primary disadvantages that limit its environmental friendliness and utility. Firstly, because there is no division between water and solid waste other than settling action, each time new water is added to the tank it stirs up the existing material, reducing efficiency. Secondly, because sludge at the bottom and waste-bearing liquid (called "effluent") are in the same tank, water that is removed continues to have large quantities of undesirable solid waste in it---new solid waste is constantly being added and mixed in.

Imhoff Tanks

• One way of solving the disadvantages of a single-stage tank is with an Imhoff design, where the solid waste settles not into the bottom of a single tank but into a second compartment altogether. This ensures that digestion of the sludge can continue uninterrupted, and means that the water removed from the septic system has a lower quantity of solid waste. Imhoff tanks are more efficient than traditional septic systems, and have no moving parts or other mechanical components that might fail, although their two-stage system means that they must be somewhat taller than single-stage septic tanks.

Aeration Systems

• Another multistage system intended to address the failings of basic septic tanks uses oxygen-breathing (aerobic) organisms instead of anaerobic ones. In these systems, wastewater flows into a tank where air is constantly filtered through the sludge, promoting its digestion by aerobic bacteria. Advanced aeration-based systems include a final stage where the effluent liquid is purified, either chemically or by the use of an antibacterial agent like ultraviolet light. These two-stage systems are more complex than one-stage septic tanks or Imhoff tanks, but provide a more environmentally friendly treatment of wastewater.