Inflow and Infiltration
Inflow
Inflow is stormwater from rain entering the wastewater system. This can be due to:
- Cross connections between the stormwater and wastewater networks, where a stormwater pipe is mistakenly connected to the wastewater network or vice versa.
- Discharge of roof water from a roof to a wastewater gully trap.
- Low gully traps allowing stormwater runoff from a driveway or other paved areas to flood our wastewater network.
Infiltration
Infiltration is groundwater entering the wastewater system through joints or breaks in private drainage. High levels of infiltration indicate that the pipes are in poor condition and that they need to be replaced. These types of problems are generally found in older parts of the city.
Effects
High rates of inflow and/or infiltration cause problems in the wastewater network. Wastewater pipes have a maximum capacity based on the diameter of the pipe and the gradient (slope) at which the pipe has been laid. Stormwater entering our wastewater network during rain events can quickly cause the flow capacity of our pipes to be exceeded, resulting in wastewater overflows at low points in the network including discharges to streams and beaches, which can cause pollution.
The property owner is responsible for fixing or replacing their private drainage.
How to Test for Inflow and Infiltration
If you want to subdivide your property you will need to ask a registered drainlayer to test your private drainage for inflow and infiltration problems. The tests to be carried out will depend on the age of the pipes and the pipe material, as outlined in the following table. Your registered drainlayer can advise you on this.
|
Pipe Material |
Pipe Age |
Tests Required |
Action Required |
|
Non PVC |
Unknown or greater than 45 years |
Visual inspection |
Replace all existing private drainage |
|
Non PVC |
Less than 45 years |
Visual inspection and a low pressure water test |
Replace all existing private drainage based on inspection and test results |
|
PVC |
Unknown or greater than 15 years |
Visual inspection and a low pressure water test |
Remedial works to be undertaken |
|
PVC |
Less than 15 years |
Visual inspection |
Remedial works to be undertaken |
Notes.
1. Visual inspections will consider low or non-compliant gully traps and suspected cross connections between the stormwater and wastewater drainage. Smoke tests may also be requested to confirm cross connections as required.
2. The low pressure water test will test the private drainage pipe from the gully trap to the lowest inspection bend, before the connection to the public wastewater pipes.
3. If the water test cannot be carried out Manukau Water Limited may require a CCTV of the private drainage.
Please note that a building consent is required to repair or replace private drainage pipes.
Why does Manukau Water Limited want to eliminate inflow and infiltration? Inflow and infiltration can reduce the effectiveness of the public wastewater system and lead to pollution of the environment. This can put public health at risk.Inflow and infiltration can also lead to:
- Pump stations handling large volumes of unnecessary water
- Hydraulic overloads greatly reducing system efficiency
- Increased operating expenses due to the processing of ground water and storm water that do not require treatment
- Unnecessary equipment wear
- Increased collection system maintenance and cleaning
Note: There are other acts and regulations that may impact on the requirements for inflow and infiltration prevention and control of cross-connections.
Who pays to fix inflow and infiltration? Your contract with Manukau Water Limited clearly states you must not allow inflow or infiltration into our network. If you do not prevent inflow or infiltration then please note that our contract allows us to restrict or disconnect, where permitted by law, the supply of water and/or wastewater services to your property if you allow potential contaminants to enter our water or wastewater networks, or where you connect storm water systems to our wastewater network. We could also rectify the problem and recover the cost of doing so from you, and/or we could notify Manukau City Council of your property’s non-compliance
If you believe there is inflow or infiltration occurring in the Manukau Water Limited network then please contact us:
Phone: 09 265 4488
Email:manukauwater@manukauwater.co.nz