Reducing stormwater in the sewer

Reducing Stormwater in the sewer image_Banner_600x400.png

The $5 million Reducing Stormwater in the Sewer project is looking at improvements to the stormwater and sewerage networks. 

Mullumbimby is a priority area for this project. 

Inflow and infiltration have been a recurring problem in the town since the 1960s, when the sewerage network was built.

Aims of the project

  • To reduce stormwater entering the sewerage system.
  • An improved sewerage and stormwater network to better accommodate the area’s growing population and extreme weather.
  • Undertake flow monitoring of the sewerage network to better understand how it operates during rainfall events.
  • Inspection of Council’s sewer and stormwater mains and residential sideline connections to assist in identifying problem area.
  • Undertake rectification works and visual assessment (CCTV) and smoke testing.

What is inflow? 

Stormwater that enters directly into the sewer system.

Causes of inflow

Runoff from hard surfaces, such as roofs and driveways, which enters the sewer system through incorrectly installed downpipes, drains or low-lying gully traps that overflow when it rains.

A gully trap prevents smells and overflows from entering buildings. They are usually located outside your house, generally in the area outside your kitchen, and should be above ground level to ensure that stormwater does not enter the sewer system.

What is infiltration?

Groundwater, or stormwater that seeps into the ground, then incorrectly enters the sewer system through defects in sewer pipes or joints.

Causes of Infiltration 

Cracked sewer pipes that have deteriorated from:

  • age
  • being placed under pressure from local development and heavy traffic
  • damage from tree roots
  • poor installation or maintenance.

Groundwater can then infiltrate through the cracks in the sewer pipes.

Infiltration usually occurs where the groundwater is higher than the sewer pipe or where soil above the sewer system becomes saturated from rain events.

Problems caused by inflow and infiltration

During heavy rain, excess stormwater that incorrectly enters the sewerage system can cause it to back up and overflow sewage into homes and the environment.

When stormwater or groundwater incorrectly enters the sewer system it can cause problems and increase costs for the community through:

  • Property damage: Flooding on private residences and businesses can occur as sewage pipes become overloaded with stormwater and groundwater and this does not drain away fast enough;
  • Environmental damage: The health of our waterways and its users can be compromised by overflow from the sewerage system when it enters our rivers and creeks;
  • Infrastructure damage: Roads and surfaces deteriorate with surface flooding and poor drainage, increasing maintenance costs for Council and ratepayers; and
  • Electricity and treatment costs at the sewage treatment plant increase to process the extra water in the sewerage system.
  • The efficiency of the sewerage network will be improved as the identification of defected pipes will prompt targeted repairs. 
  • Costs of damage to personal property and homes will be reduced as efficient plumbing systems and sideline connections will reduce the risk of stormwater overflow and localised flooding.
  • The risk of wastewater overflow into residential properties and street frontages will be reduced. 
  • Electricity and maintenance costs of sewage pumping stations will reduce.
  • Risk of pollution to local rivers and creeks will reduce and the biodiversity and health of the waterways and its users will be protected. 
  • Street aesthetics and stormwater drainage in Mullumbimby will be improved. 
  • Reducing water waste and using water more efficiently will help protect Mullumbimby’s water source for the future.