Septic System Types

All on-lot wastewater treatment systems consist of three basic components. They are:
  1. The treatment tank
  2. The distribution system
  3. The absorption system
The treatment tank is a watertight vessel which is buried in the ground and receives all waste from the house. Treatment tanks may be designed to function anaerobically (without oxygen) or aerobically (with oxygen). Most treatment tanks are anaerobic. The treatment tank is designed to retain all solids from the house. The solid material that is heavier than water will settle to the bottom of the tank and is called sludge. The solid material that is lighter than water will float to the top of the tank and is called scum. Between the layers of sludge and scum is relatively clear liquid called effluent, which is allowed to pass to the distribution system. Baffles at the inlet and outlet pipes prevent solids from leaving the treatment tank. Naturally-occurring bacteria will digest approximately 50% of the solids in the treatment tank. The remaining solids must be removed by periodically pumping the treatment tank. Multiple treatment tanks or multi-compartment treatment tanks have been used in systems since 1998. These systems have proven to be more effective at separating solids, and therefore producing a cleaner effluent, than single tank systems.

The distribution system is a series of pipes that carries the effluent from the treatment tank to the absorption system. The effluent flows by gravity to a distribution box or to a tank with a lift or dosing pump, or to a siphon chamber.

The absorption system is very important because it is the most expensive component of an on-lot wastewater treatment system. It is also the most difficult of the components to repair, should problems occur. Many different types of absorption systems are currently in use, including:
  1. subsurface seepage pits
  2. subsurface seepage trenches
  3. subsurface seepage beds
  4. at-grade beds
  5. elevated sand mounds
  6. drip irrigation
  7. spray irrigation

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