Collecting rainwater for domestic use is nothing new; it’s been done since Roman times and is still used today in modern, sustainable homes. Jae Lester from RainWater Harvesting looks at how technology has changed, though the principle remains unchanged

In the UK we have the problem of either too little or too much water. Obviously preventing 30% leakage of the mains supply will help enormously but there are other things that can be done to support this. Sometimes we need to look to the past to find solutions for the future. 

WHAT IS A RAINWATER HARVESTING SYSTEM?

Water is collected off the house roof via standard guttering and downpipes. It is filtered and stored in an underground tank. When required it is then pumped back into the home to be used for flushing toilets, washing machines and outside use which greatly reduces the demand for mains supply. The question in our quest for sustainability is should we use mains water for non-drinking purposes? There is no need to flush toilets with processed drinking quality water bought from miles away.

Installing a system is best suited to a new build or major renovation as there will be a rainwater feed to toilets and other non-potable appliances. Indoor taps, basins, showers and baths will remain on the main supply.

The harvesting tank is buried underground – there is no light and only a small temperature variance to keep the water cool and fresh. 

Tank size calculators are available on the web. Sizes are calculated on location, collectable roof area, number of people/toilets, and application for the water. Systems are designed to require minimum maintenance. There is a filter which allows water to flow into the tank but leaves and debris are directed into a soak away. Filters are designed for easy access so once or twice a year it is worth removing and flushing with clean water. 

Within the tank, there is a pump which activates when a drop in pressure is identified. 

WHAT HAPPENS IF THE RAINWATER IN THE HARVESTING TANK BECOMES LOW?

Tank sizes are calculated traditionally on a 21 day supply, as it was assumed that it would rain within that timescale. However should the water run low we still require the toilets 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.

There are two main types. In a direct feed system the pump is directly connected to the appliance and 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 system the water is pumped up into a header tank and appliances are serviced from there. The latter method only requires the pump to be operated when the header tank needs replenishing. Running costs can be as low as 1p per person per day. Systems are designed to prevent any cross contamination and it is important to consider WRAS (Water Regulation Advisory Service) approved systems. These have undergone extensive testing and are accepted by all water providers as compliant if installed in accordance with the manufacturer’s specification.

TOO MUCH RAIN

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 collected within the tank first, preventing localised flooding.

Emerging systems allow a volume of the tank to be used for non-potable applications, but are designed with extra capacity specifically for 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 soakaway at a controlled rate so reducing the risk of downstream flooding. This can be as low as 0.05 litres per second per plot, and so will not overwhelm the network.

It is the overwhelming drainage system that needs to be addressed which is how harvesting rainwater as a SuDS solution would be highly beneficial. Returning water to the storm drain at a controlled rate is an efficient way to collect for reprocessing which may become more important with increased demand. The extra capacity is calculated based on all ‘worst case’ scenarios and any other criteria that planning requires by Micro Drainage. 

INSTALLATION

A groundworker is required to site the tank under the manufacturer’s instructions. Some tanks are ‘shallow dig’ so only require a depth of 1 – 1.5 m making it easy to install in areas where there is a relatively high water table and ease of calculation of levels. 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.  

A plumber will then install the pipework to the appliances and connect both the rainwater and mains supply, and an electrician will be required to connect the pump and sensors to activate the flow of water.

Modern systems are designed to make installation relatively simple and therefore should not add significant cost to a build project. Complete systems with mains backup cost in the region of £2000-£3500 ex VAT. This cost can be offset with the assistance of a planning application for water usage and drainage strategy. A combined system can be an economic way of achieving a SuDS plan where discharge from site can be actively calculated. 

In conclusion, rainwater harvesting comes at a small material cost for long term sustainability and as a first line defence against flooding.

Jae Lester is director at RainWater Harvesting