Monday 21 May 2012

Pre-design drainage issues with Commercial rainwater harvesting systems

Often pre-design or design errors make RWH difficult

In our technical department, they often complain that the drainage doesn't work when it comes to the rainwater harvesting system.  They bemoan the fact that a fall or any additional access chambers haven't been drawn on the drawings.  The also complain that the rainwater harvesting system has been ordered too late and much of the drainage has been already installed, making the rainwater harvesting very difficult to function.

An order arrives

As soon as Freerain Ltd accepts an order for a commercial rainwater harvesting system, we request a copy of the latest drainage drawings to check for the following:
  • Water is running downhill (we've seen many drawings showing it going up!)
  • A pre-tank filter has been drawn with appropriate falls
  • The storage tank location has been marked
  • Only traditional roof areas are being drainage and not open gullies, drains etc
Below is a generic drawing for what a typical underground rainwater harvesting system may look like on a drainage drawing. In this case no invert level has been specified and you can see the inlet invert is 0.  Of course on site that this is anywhere from 500 to 3,000mm down.

Problems

As mentioned above there can be a number of issues and problems with getting a rainwater harvesting system to work within a commercial application.
  • Drainage built with a fall for the rainwater harvesting system
  • Fall in the pipework not accounted for
  • Not all the intended roof is sent to the tank
  • Open gullies and non-roof areas are drained into the tank
 Much of the above is really because of two reasons.  Firstly, the rainwater harvesting system is ordered/considered too late and secondly, the consultant for the drainage hasn't appreciated the requirements of the rainwater harvesting system.

Summary

Rainwater harvesting for commercial buildings and projects should be relatively easy to install, but all too often not all parties in the planning stages come together.  Therefore, on many occasions the late consideration or the lack of planning leads to costly work-arounds on site and on a few occasions this has led to already built drainage being ripped up and re-laid as required to incorporate the rainwater harvesting system.

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