Impact of septic tank desludge frequency on CWMS

Who this is for

What's being delivered

Risk matrix to help councils tailor pump-out frequencies based on catchment conditions and system performance.

About the project

This project investigated whether desludging frequencies could be safely extended.

Septic tank pump outs are costly, with councils spending approximately 10% of their CWMS operating budgets on septic tank desludging. For example, the City of Onkaparinga allocates around $300,000 annually to the program.

Current pump-out frequencies are largely based on theoretical sludge accumulation rates developed for on-site wastewater systems, rather than evidence from CWMS networks. CWMS networks differ from on-site systems because they're accessible and can be monitored and flushed. So, they may be less sensitive to solids leaving septic tanks.

Household water use has also changed significantly which may impact how the septic tank operates and the amount of solids that reach the network.

Project outcomes

The project found that septic tanks connected to CWMS are likely being pumped more often than necessary when councils apply desludging schedules intended for on-site disposal systems.

Results showed that with pump-outs every 4-5 years, BOD and suspended solids generally stayed within CWMS design limits at both the individual tank and catchment scale. Levels were low and changed very little between the 4 and 5-year intervals (and in the limited longer-interval data available).

This suggests that desludging periods could potentially be extended substantially before any negative effects on CWMS performance would be expected. The main constraint was the lack of data for tanks pumped at intervals beyond five years.

Visit continuous improvement on the City of Onkaparinga website to view next steps.

Risk matrix

To support implementation, a risk matrix was created to help councils tailor pump-out frequencies based on catchment conditions and system performance.

This enables councils to make evidence-based decisions that may lengthen desludging intervals. This would lower servicing costs and improve CWMS operational efficiency, while maintaining environmental and public health protections.

View the Septic Tank Desludging Risk Matrix (PDF 163.3 KB).

Project funding

This project was funded through the Community Wastewater Innovation Fund.

This scheme allocates funding to projects to support innovation within the CWMS sector. Successful applicants are decided through an annual grant round.