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Understanding Your Onsite Wastewater System

The purpose of this description is to provide a basic understanding of more complex criteria.

An onsite wastewater system usually consists of 3 components:

    The treatment tank(s)
    The distribution system
    The absorption system

Step 1.) The treatment tank(s)

The treatment tank is a buried, watertight holding receptacle designed to receive the wastewater from the home. Treatment tanks are classified as either septic or aerobic. Wastewater from the home flows through the inlet baffle into the tank. This could be a single tank, dual chamber tank or a configuration of a series of tanks. Heavier solids sink to the bottom forming a sludge layer, lighter substances such as grease and oil, float to the top and form a scum layer. The liquid between the sludge layer and the scum layer leaves the tank by means of the outlet baffle and is piped to the distribution systems. Contacting a septic tank pumper to pump the tank thereby removing the sludge and scum layers, should be done at regular intervals, approximately every 2 to 3 years depending on usage.

Step 2.) The distribution system

Once the liquid leaves the treatment tank it is called effluent, and still contains disease causing bacteria and other pollutants. The distribution system is underground; its main function is to move effluent from the treatment tank to the absorption area, it does this by means of either gravity or pump.

    Gravity System In all gravity systems a means of ensuring that the effluent is distributed equally to each trench in the absorption area is needed. In most cases an underground distribution box (D-Box) is used. A D-Box has one inlet port and several outlet ports connected to laterals leading to the absorption area.
    Pump System. A pump system uses an underground holding tank that contains a pump, pump control floats and a high water alarm float; and is used to collect the effluent and then pump it to the absorption area. Control floats turn the pump on when the effluent reaches a set level in the tank, pumping a specific volume of effluent per dose, usually around 100 gallons, into the absorption area. If the effluent were to rise beyond the set level, called the head, then the high water alarm float would activate an alarm to alert the occupants of an issue within the tank.

Step 3.) The Absorption Systems

This underground component of an onsite wastewater treatment system is very important because it can be quite expensive and difficult to repair or replace. Treatment of the effluent occurs naturally as it moves through the absorption area and into the soil. There are several types of absorption systems, we will discuss the most common for our area: trenches, sand mounds, and cesspools. The only suitable landscaping material for over an absorption area is grass.

    Trench System or Bed System. The effluent reaches the trench or bed system by means of laterals and is distributed evenly through long narrow rectangle shaped trenches or a large rectangular shaped bed. The trench/bed is partially filled with aggregate, usually gravel. On top of the aggregate is a perforated pipe which distributes the effluent evenly throughout the absorption area. The perforated pipe is covered aggregate then soil backfill.
      Elevated Sand Mound. A sand mound is usually a bed type distribution system built on a manmade sand plateau. The sand provides a level surface for the aggregate, usually gravel, to be placed and is a natural filtering material. When the effluent reaches the sand mound it goes through a network of small diameter pipes. The effluent is pumped through the pipes in controlled doses to insure uniform distribution throughout the bed. The effluent leaves the pipes through small diameter holes in the pipes and sinks through the gravel and into the sand then into the natural soil.
      Cesspool or seepage pit. A cesspool receives all the homes wastewater acting as the tank, the distribution system and the absorption area. A cesspool is an underground pit built of concrete block. The solids are retained in the cesspool while the liquids are absorbed through the walls into the soil. A seepage pit is also an underground pit built of concrete block; but a seepage pit is preceded by a treatment tank where the solids are retained. The effluent enters the seepage pit where the liquids are absorbed through the walls into the soil.