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Rainwater harvesting: how to avoid sharks

Posted by Barry Nutley on 28 May 2010 at 9:31 am

A recent report by Which?, suggested that there are many "solar sharks" in the industry. We have been asked if there is a similar problem in the rainwater harvesting industry? And what to look out for?

Firstly, let me say that I don't believe that there is the same sort of problem in the industry, but here are, probably, the two biggest things to consider when considering whether or not to invest in rainwater harvesting:

Size:  How big should the tank be? There are several schools of thought here, and several calculators to help you correctly size the tank for your application. While some will look at your usage, and calculate a tank suitable for this, the best way is to calculate your potential rainfall capture. It doesn't matter how much water you need for your garden, toilets etc. If you can't collect it, you can't use it! It may sound obvious, but many over look this.

A simple calculator can be found here. As you can see, it suggests that you should size a tank capable of holding 5% of your annual potential collection. The reason for this is that we rarely go without rain for more than 18 days (ie 5% of the year).

From a cost view point, not only is a larger tank more expensive (although not usually that much more in comparison), but perhaps more importantly, it costs more to install. 

Installation: The other consideration (for an underground tank) would be how it is set in the ground:

Unless the area you wish to bury your tank in suffers from a high water table, there is not usually a need to set it in concrete. A suitable sand based surround should be fine. If your installer recommends setting it into concrete, ask why. Not only is it more expensive, but it's not environmentally friendly.

Other potential issues include pump/filter size/performance etc. But these tend to be very client/site specific. So I suggest you always ask your installer the reason for any suggestions he/she makes.  

Photo by muisam-ck

About the author: Barry Nutley is co-founder and director of Viridis Energie Consultants

If you have a question about anything in the above blog, please ask it in the comments section below.

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