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Dunedin City Council – Kaunihera-a-rohe o Otepoti
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How a mudtank works

Dunedin has three types of mudtanks: a siphoned outlet pipe, a straight outlet pipe and a bubble-up. Below are some information about the three different designs, followed by a brief description of the mudtank.

How a mudtank works

A mudtank has three main components.

  1. Inlet (grate)
    • The inlet is made up of one or more steel grates set in the kerb. The grates allow water to flow into the tank, but prevent larger debris and vegetation from getting in. The size and number of grates will depend on the catchment area of each tank.
  2. Tank
    • The tank is the area that collects the water and any silt or small debris that passes through the grate.
    • The silt and debris are separated from the water as it sinks to the bottom of the tank, and the water discharges through the outlet pipe.
    • Maintenance of these tanks is important to avoid build-up of silt and any potential blockages.
  3. Outlet pipe
    • The outlet pipe takes the water from the tank to the stormwater system.
    • Most of our mudtanks contain a siphoned outlet (image 1 below). In a siphoned outlet, the half-bend pipe design prevents hydrocarbons (fuels, oils, etc) from entering the stormwater network as they float on top of the water in the tank.

Dunedin has three types of mudtanks: a siphoned outlet pipe, a straight outlet pipe and a bubble-up. Below are some images that illustrate the three different designs, followed by a brief description of the mudtank.

mudtank diagram

Mudtank – Siphoned outlet pipe

A standard tank has the outlet pipe set at least 300mm above the floor of the tank. The siphoned design means the water sits above the outlet to prevent hydrocarbons (oil, etc) from entering the stormwater network. The height of the outlet pipe varies between mudtanks – the higher the outlet  pipe, the higher the water level.

mudtank diagram straight outlet

Mudtank – Straight outlet pipe

A mudtank with a straight outlet pipe works similarly to the siphoned design, but the straight outlet means the water level is at the same height as the outlet pipe.

mudtank diagram bubble up

Bubble-up mudtank – No outlet pipe

A bubble-up mudtank looks like a regular tank from the surface but functions quite differently. A bubble-up tank has an inlet pipe but no outlet pipe; instead the water is piped into the tank and ‘bubbles up’ from the tank and down the kerb into the next tank or catchment structure. The  water level in a bubble-up tank is at the top of the grate, which can give the impression it is blocked or malfunctioning. We identify bubble-up tanks with a blue triangle.

Bubble-up tanks are typically used to transfer water across an intersection or road below ground, or as an alternative to a stormwater system where there is no piped stormwater network present.

Mudtank issues

If a mudtank is blocked or causing surface flooding, it is most likely one of two things:

  1. The inlet (grate) is blocked with litter or debris on top.
  2. The outlet pipe is blocked with silt or debris that is restricting the flow.

If you see a blocked mudtank, be sure to let us know on 03 477 4000.

Mudtank maintenance

In order to keep mud tanks operating effectively, the DCC undertakes a regular cleaning regime. Currently we are working to ensure that all tanks are cleaned before the end of May 2017. In addition, there is a regular street cleaning programme which helps to prevent debris and silt from blocking the  tanks.

Doing your part

Make sure no litter or vegetation from your property ends up in the kerb or mudtank. As mudtanks are generally situated at low points, any litter or debris on the road or kerb will eventually end up here when it rains. This can cause the tanks to block.

Please do not pour paints, oils, cements, etc onto the road or kerb as these will contaminate the water and can set in the tank or pipe causing blockages.

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