Maintaining a septic system | Redland City Council

Maintaining a septic system

Septic and sullage systems

Septic system

A septic tank (or black water) is a system that uses useful bacteria to break down human waste into a liquid (known as effluent). The effluent is discharged from the tank into a transpiration trench system.

A transpiration trench lets the effluent absorb into the soil. The grass and plants use the moisture and nutrients, and the wind and sun assist with evaporation.

Sullage system

A sullage system (or greywater) disposes of the domestic waste from kitchens (via a grease trap), baths, showers, basins and laundries.

A grease trap is a small tank, usually installed close to the kitchen, to take greywater from the kitchen sink and prevent grease entering trenches. You should clean the traps every three months and call the plumber if the trap is blocked.

Note: Most older houses on the Southern Moreton Bay Islands have separate septic and sullage systems. All new houses must have combined septic and sullage systems, known as all-purpose septic tanks or household sewerage treatment plants.

How a septic tank works

A septic tank pre-treats human waste before it enters the trench.

Three main functions happen in the tank:

  1. Solids are removed - As waste enters the tank, the flow is reduced and larger solids sink to the bottom (sludge) or rise to the surface (scum).
  2. Waste is biologically treated - Solids and liquid waste in the tank decompose by bacterial and natural processes.
  3. Sludge and scum is stored - Sludge and scum will decompose but some solid material will remain. Tanks need space to store sludge and scum between cleanings.

Caring for a septic tank system

Below are some tips for maintaining your septic tank system

  • Use only cleaners suitable for septic tank systems.
  • Be aware that, if any family members are taking antibiotics, this may affect the system’s bacteria.
  • Divert stormwater away from the disposal area.
  • Repair all leaky toilets immediately.
  • Have an annual tank check.
  • Check the depth of solids and scum - when they take up too much space, the wastewater can’t settle and too many solid particles flow into the trenches and become clogged.
  • Don’t use disinfectants to clean the toilet as they will destroy the useful system bacteria.
  • Don’t place any material other than toilet paper into the toilet.
  • Don’t dispose of cat litter, sanitary napkins, paper towels or tissues in the toilet as they are slow to decompose and will quickly fill up the tank.
  • Don’t dump grease or fats down the kitchen sink as they solidify and contribute to blockages.

Common problems and fixes


The most common problem is when there is a pH imbalance in the tank and it becomes acidic. This causes bad odours in the system. This usually happens when the wrong things are placed in the system.


To restore the tank’s pH and improve the odour:

  1. Mix 0.5kg (500g) of hydrated lime with water in a bucket.
  2. Flush the mixture down the toilet 2-3 times a day for up to 3-4 days or until a maximum of 5kg of lime is used.
  3. Mix any lime residue in the bucket with more water.
  4. Flush the mixture down the toilet.

You shouldn’t do this process immediately before pumping the tank due to solids releasing into trenches and lime affecting the soil.

You can also use enzyme pellets to break down fatty materials in the tank’s trenches. The pellets are available at hardware stores. Get the tank checked annually.

Cleaning and pumping out the tank

Tanks need to be checked annually. How often they need to be cleaned depends on the tank’s capacity, the flow of wastewater and the volume of solids in the wastewater.

If more people live in the house than the system is designed for, the tank may be too small and will need more regular pumping.

Some older systems may need the sludge to be removed. This process is recommended every two to three years.