On-site wastewater management systems (OWMS) are available in various designs, each using multiple chambers and mechanisms for the treatment of wastewater to produce site-specific effluent (treated wastewater) tailored to the quality required for its local disposal or final use. The 2 main types of systems are:
- primary simple systems which are comprised of only septic tank/s with no secondary treatment for sub-surface disposal only
- secondary complex systems which are comprised of settling chamber with secondary treatment/s (UV, chlorine, mechanical filters, and so on) for surface irrigation or sub-surface disposal only.
An OWMS collects, settles and treats wastewater to remove scum, organic matter, nutrients, pathogens, and grease. Solids are separated from wastewater and settle at the bottom of the tank to be decomposed by bacteria or pumped out by desludging.
The water that remains after solid waste is separated from wastewater is called effluent. Effluent must be disposed of or have an end-use. The quality of effluent varies depending on the level of treatment it has received, and its quality determines how it can be disposed of or used.
To create effluent, wastewater is first passed through a settling chamber where heavier solids settle and form a semi-liquid sludge mass at the bottom of the tank. Solids that float, such as oils and greases, rise to the top.
Separated from the solid wastes is the effluent water left in the middle. The bacteria present in the waste will naturally decompose solids in the tank, but over time the solids (sludge) will settle at the bottom of the tank and need to be periodically removed by emptying (desludging) the tank. A system will fail to work safely if the sludge accumulates
Primary systems – simple (septic tank/s only, no secondary treatment)
Simple systems (otherwise known as primary systems) rely on flotation and settlement to separate solids from liquids for the disposal of the clarified but still contaminated effluent water (treated to primary quality) from septic tanks and composting toilets.
Wastewater treated to primary quality is only suitable for disposal and release below ground via soil absorption trenches, mounds and evapotranspiration beds or trenches.
It is important for schools to know what quality of effluent their system is designed to produce, as stated in their permit, to ensure the correct disposal or end use occurs.
Secondary systems – complex (solid separation with secondary treatment)
Complex systems rely on solid separation combined with a secondary treatment system (STS) to clean and treat the effluent so that it can be used for irrigation (treated to secondary quality).
A secondary system typically consists of multiple chambers or mechanical components that provide additional aeration, filtration and disinfection. Filters are used to break down and remove solid contaminants, while chlorine or ultraviolet light (UV) is used to disinfect effluent.
These systems produce higher quality treated effluent that can be recycled for surface irrigation purposes. Where approved, wastewater treated to secondary quality can also be dispersed to land via subsurface irrigation.
It is important for schools to know what quality of effluent their system is designed to produce to ensure the correct disposal or end use occurs.
Schools are not permitted to use greywater for internal re-use (toilet flushing, washing machine). External re-use of greywater for watering gardens via dedicated purple taps and hoses is also not permitted at any school premise using OWMS.
Common treatment methods
Primary and secondary OWMS use various treatment methods and combinations of treatment systems designed to treat and manage wastewater. These include:
- biofiltration – uses sand filters, contact filters or trickling filters to ensure that any additional organic matter/sediment is removed from the wastewater
- aeration – a biological process where organic matter is decomposed by bacteria using oxygen saturation by introducing air to wastewater. Typically, the aeration process can last for a few hours and is very effective
- oxidation ponds – typically used in warmer climates, this method utilises constructed ponds/lagoons, allowing wastewater to pass through for a set period before being retained for 2 to 3 weeks
- disinfection – uses ultraviolet light (UV) or chlorine.
Reviewed 09 January 2023