SITE PREPARATION
Tank sizes are traditionally calculated on a 21-day supply, however should the water run low, toilets still need to flush
collected within the tank first preventing localised flooding. New emerging systems allow a volume of the tank to be used for non-potable applications, but have an extra capacity specifically when there is excess rain. This extra volume is calculated specific to the site and meets the criteria for severe rain events. Water is released back into the storm drain or soak away at a controlled rate so reducing the risk of downstream flooding. This can be as low as 0.05 l/sec per plot, and so will not overwhelm the network.
INSTALLATION
activates when a drop in pressure is identified. For example, when a toilet is flushed there will be no water in the cistern so the pressure will be low and water will flow to refill.
NOT ENOUGH RAINFALL? Tank sizes are traditionally calculated on a 21-day supply, as it’s assumed it will rain within that timescale. However, should the water run low, the toilets still need to flush. Therefore if the water is going to be used within the home and not just for the garden, it is important to specify a mains back up system in case of severe drought conditions. There are two main types of systems –
direct and gravity fed. In a direct feed system, the pump is directly connected to
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the appliance and so runs every time a toilet is flushed. If rainwater becomes critically low a small amount of mains water would be introduced into the underground tank. In a gravity-fed system, the water is pumped up into a header tank located in the loft space and appliances are serviced from there. The latter method only requires the pump to be operated when the header tank needs replenishing. This can make running costs as low as 1p per person per day.
TOO MUCH RAINFALL
The advantage of collecting rainwater is that it is a relatively simple process and it also has a further benefit regarding Sustainable Urban Drainage (SuDS). During heavy rainstorms, water is
In a new build, there are a number of trades to complete the build, and a small amount of time by some of these will be required to install the rainwater system. Apart from the electrical element however, most of the work can be project managed by any competent person. A groundworker is required to dig the hole and site the tank under the manufacturer’s instructions. Some tanks are ‘shallow dig’ so only require a depth of 1-1.5 metres, making it easy to install in areas where there is a relatively high water table and ease of calculation of levels. The laying and connection of pipework to and from the house. There must be a ’fall’ from the downpipe so the rainwater flows towards the harvesting tank. It is very important that the piping from the roof to the tank is ‘closed’ and there are no open gulleys. This will help keep the water as clean as possible. A plumber is required to install the pipework to the appliances and connect both the rainwater and mains supply, and an electrician is needed to connect the pump and sensors to activate the flow of water. Modern systems are designed to make installation relatively simple however, and therefore should not add significant cost to a building project.
Jae Lester is a director at RainWater Harvesting
issue 02 2021
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