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Septic Systems and How They Work

Septic systems are made up of three parts:

  • The drains which transfer wastewater from your house to the system
  • The septic tank and any additional treatment tanks where bacterial action breaks down the wastewater
  • The disposal area which takes treated water and disperses it within your property.

Types of septic systems

The most common types of septic systems are:

Primary systems
These are made up of a single septic tank:

  • Solids settle to the bottom forming a sludge layer
  • Fats and greases collect at the top
  • Bacteria naturally breakdown the wastewater before it goes to the effluent disposal area

Secondary systems
These systems consist of a large tank with several treatment compartments:

  • The first is like a primary systems septic tank
  • The second mixes air with the wastewater to assist bacteria to break down solids
  • The third allows for settling of solids
  • Some systems have a fourth step which disinfects through chlorination or UV saturation

Wastewater from secondary systems is treated to be safer than wastewater from primary systems.

Effluent Disposal

All systems have some form of effluent disposal area which disperses the treated wastewater across your property – almost always underground. The most common types are:

  • Absorption and transpiration trenches
  • Sub surface irrigation
  • Mounds and wick trenches

System Specific Factsheets (Environmental Health Professionals Australia)