Proper treatment of household wastewater is critical in preserving the health of the public and the environment. In unsewered regions, on-site domestic sewage treatment systems (OSDTS) are used to treat household wastewater (sewage).

Types of domestic systems include greywater diversion or treatment, incinerating toilets, composting toilets, and septic tanks. However the most common type of OSDTS is the aerated wastewater treatment system.

Aerated wastewater treatment system

Aerated wastewater treatment system contain a series of chambers where the sewage is treated in stages. The sewage enters the first chamber where the larger solids settle to the bottom forming a sludge layer. The top layer or “clarified” sewage flows into a second chamber where it is mixed with air and bacteria for further treatment. Once all the solid and organic matter has been removed, the treated sewage (effluent) is generally disinfected before being discharged through an irrigation system. Material that is not completely broken down by the bacteria in the system will build up over time and must be pumped out.

The Queensland Plumbing and Wastewater Code recommend that the sewage from an aerated wastewater treatment system meet the following criteria:

Parameter Acceptable level
Biochemical Oxygen Demand (BOD5)  20mg/L This is the amount of dissolved oxygen that is required for bacteria to break down organic material.
When liquid waste with a high BOD5 concentration is discharged into waterways, oxygen in the water is consumed to break down the waste.  This reduces the amount of oxygen available to other aquatic life, often resulting in organisms being killed.
A high BOD5 value indicates that the system is not working effectively.
Suspended Solids  30mg/L This is a measure of the amount of solid material present in the sewage. This material may be organic (fats, paper, etc) or inorganic (sand, plastic, etc), and can indicate either ineffective treatment or the entry of unsuitable material into the system.
Excess suspended solids in AWTS effluent can block the disposal system and may result in overflows and negative environmental impacts.
A high value may indicate that the sludge level in the bottom of the tank is getting high and may need to be removed.
Thermotolerant coliforms
(where disinfection is required)
 200cfu/100ml Thermotolerant coliforms (also known as faecal coliforms) are groups of bacteria that are commonly found in faeces and can be indicators of faecal contamination of waterways and water supplies.
The absence of thermotolerant bacteria from an effluent should not be taken as an indication that the liquid is safe to handle.  Potentially infectious viruses and protozoa may still be present, and contact with the effluent should always be avoided.
A value greater than 200cfu/100mL indicates that the disinfection step in the treatment process is not effectively reducing the population of potentially dangerous bacteria.

Brisbane City Council requires property owners to have their aerated wastewater treatment system tested annually.

We can assist owners to meet this requirement by:

If you prefer to collect your own sample, we can provide you with the appropriate sample containers, a sterile bottle for the Thermotolerant coliform test and a non-sterile plastic bottle for the BOD5 and suspended solids tests. Sample containers are free and can be collected from our laboratory.

When taking the sample it is very important to follow the instructions provided, especially for the Thermotolerant coliform test. We recommend that the samples are chilled after collection and delivered to our laboratory on the same day. You will need to supply the Aerobic Sewage Treatment Plant Service Report (DSTS 3) for us to complete along with an aerated wastewater treatment system Sample Submission Form.

Please contact us if you would like more information.