Composting toilets (dry and wet)

Composting toilets (dry and wet)


A composting unit used in conjunction with a composting toilet

Composting toilets use microorganisms to decompose human waste mixed with wood shavings, lawn clippings and other organic matter into humus. There are two types of composting toilets - dry and wet.

Dry composting toilets collect urine and faeces in a sealed chamber beneath the toilet pedestal, where microorganisms decompose the mixture of human waste and extra organic matter. About three-quarters of the material is converted to carbon dioxide and water vapour. Air drawn through the compost pile removes these gases and assists the microorganisms to break down the material.

The remaining compost moves slowly down a sloping floor by gravity as more material is added to the pile. It then moves under a dividing baffle into a humus chamber. After a period of time that varies with usage (from every 2-3 months to once a year) this material is suitable to remove. Excess liquids drain to a small absorption trench and may be treated with greywater.

Composting toilet diagram

A composting toilet

Dry composting toilets collect and treat only toilet waste (blackwater) to a primary standard (a similar standard as septic tanks). Greywater from the bathroom and laundry needs to be treated separately, either by a separate greywater treatment system or by a septic tank or aerated wastewater treatment system.

Wet composting systems can be used in conjunction with flushing toilets. Worms and bacteria work to break down the solids in a similar manner to biological filter systems. Compost is removed from the base of the pile though a separate chute, and the wastewater that collects at the base of the tank is either pumped or flows by gravity to a land application area. Wet composting systems treat all the wastewater from the house and don't need separate greywater management.

Common problems with composting toilets are:

  • odours from the compost pile caused by ineffective composting or not enough ventilation
  • clogging in compost chamber caused by not regularly removing humus
  • the wrong system in wrong location. Wet composting toilets are best located away from a dwelling, while dry composting toilets are more suitable under houses.

Effluent (treated wastewater) from dry composting systems and toilets may be released onto your property through:

  • absorption trenches and beds
  • evapotranspiration absorption beds.


Tuesday 25 June
1,349,194 ML
2,581,850 ML
9,317 ML
1,331 ML
-7,278 ML
Tuesday 25 June