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Rainwater harvesting - a simple guide -

Full article as published by Plumbing and Heating and Air Movement News

RAINWATER HARVESTING

With increasing concerns about the UK’s water supplies, many homeowners are receptive to ideas that will help to reduce their water consumption, but how can you maximise the benefits of a rainwater harvesting system? Marcus Bicknell, Marketing & Technical Director at RainWaterHarvesting.co.uk addresses some of the key issues.

When is the best and most cost-effective time to install a rainwater harvesting solution?

It’s cheaper and easier to install a rainwater harvesting system in new builds or major renovation projects. However, installation isn’t complicated and can be undertaken by a builder or plumber without specialist knowledge.

In a new build, depending upon the size of the tank and the type of mains back up accessory chosen, the cost of equipment for a three to four bedroom house typically ranges between £2,000 and £3,000. In a retrofit, however, installation costs would be another £1,000–2,000. The reason for this is that in a new build situation, the digger and plumber would be on site already.

In a retrofit situation, digging the hole and fitting separate rainwater pipes to appliances may involve additional manpower or equipment. What are the most popular rainwater harvesting kits and why? Underground tanks are mostly used in the domestic market. Such tanks are marginally more expensive than above ground models, but are preferable because the stored rainwater remains fresh and the tank is out of sight and out of the way.

What size of tank would my customer need?
This depends on several factors. The most important of which are:
(1) Roof area (how much rain you can potentially collect).
(2) Rainfall (much higher in Scotland than south-east England).
(3) What the rainwater will be used for (number of WCs and washing machines, large garden, car-washing, etc).
(4) The drought protection period desired (e.g. two or three weeks of rainwater during a dry spell).

However, in terms of cost effectiveness, I recommend that you install as large a tank as possible. In relation to the difference in price between the tank sizes, the installation cost will not vary as much, so it’s always worthwhile putting in a larger tank. Rainwater Harvesting tanks are huge!

How can I get one into a garden?
In comparison to the 200 litre water butts that people are used to having in their gardens, a 2,700 litre rainwater harvesting tank is going to be a big surprise. However, delivery and installation is not as daunting as it sounds. Some tanks are manufactured in two halves which can then be put together on site. They can, therefore, be installed in even small back gardens.

Where should I position the rainwater harvesting tank?
The tank should not be located closer than 2 or 3 metres from the building, and at the most suitable place for capturing rainwater from the whole roof. Guttering and downpipes may have to be altered. Our Carat tank, for example, comes with a turret which is extendable. This allows the tank to be buried deeper, depending on the land layout. This is particularly useful if you require the tank to lie under a hardstanding where vehicles will pass. A concrete base is recommended and the tank should gradually be filled while back-filling with gravel. The tank can be installed where there is a high water table if concreted up halfway to the rim.

What do I need to know about filters and pumps?
Filters are either installed in the downpipe, or in the tank entrance. In the case of the Carat range, the self-cleaning filter is in the turret. To ensure that the filter self cleans efficiently, the pipe bringing rainwater in should be at a sufficient angle so that the water pressure pushes debris out through the siphon. The pump normally sits in the bottom of the tank, and is activated on demand. A floating pipe intake ensures the cleanest water enters the pump.

Do I need a soakaway and why?
It’s very important to provide an adequate soakaway for filtered debris and also for excess rainwater once the tank is full. The
pipe from the siphon to the soakaway should be angled to allow a good flow.

How important is it to have a mains back-up?
A mains back-up facility is essential when using rainwater to supply WCs and the washing machine. There are currently three options:
(1) A float switch in the tank connected to a solenoid valve, with a tundish to provide the required airbreak so rainwater can’t enter the mains system. This is the cheapest solution. If the level drops too low, the float switch activates the valve which allows just enough mains water into the tank to feed the appliances.
(2) A rain manager has the pump, valve and a small tank in a unit usually placed in the utility room or garage. This system allows the small tank to fill with mains water when the main rainwater tank is empty and this small tank feeds the appliances. This allows easy access to the pump, but the pump will be activated every time a toilet is flushed and this can be noisy.
(3) At RainWaterHarvesting.co.uk we have developed the Rain Director smart header tank and controller. This device not only provides a fail-safe mains backup when rainwater runs out, but also has other advantages: fewer pump cycles (saving eight times the normal pump energy use), the avoidance of discoloured water, continued water supply during power cuts, and automatic and user-controlled functions such as holiday and flush modes.

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