How to …Harvest rainwater

INTRODUCTION

Rainwater runoff becomes storm-water in an urban context and is not classified as potable water by Local Authorities. Rainwater harvested from roof-tops in suburban environments has the same status, as it has not been processed through the municipal water purification system.
Harvested, untreated rainwater is therefore only useful for cleaning purposes, the filling of toilet cisterns and for garden or crop irrigation, as opposed to drawing water from the municipal supply system. Note that the direct connection of harvested water systems to the municipal supply system, such as a toilet flush valve, is illegal as contamination of the municipal supply by untreated water may occur via these connections.

Needless to say, all harvested water would need to be treated or boiled prior to use for human consumption. Rainwater harvesting can play a major role in conserving the available water reserves. Do not, however, confuse rainwater harvesting with the collection and re-use of grey water, which is the collection of water from baths, basins and showers. Although these systems run parallel, grey water re-use systems have the advantage of operating continuously, as opposed to only being rejuvenated while it is raining.

Rainwater harvesting systems may be as basic as connecting a lay-flat hose to a down-pipe and directing it to a garden bed or swimming pool. Another idea is as simple as putting a drum under a downpipe. More complex systems collect rainwater from more than one gutter downpipe and direct overflow to secondary containers. Other systems collect run-off water at a low point on the property and then utilise a pump to “lift” the water to the point of use.

Choice of tank and sizing

The system described in this editorial is the use of a water storage tank that is connected to the gutter downpipe system. This is known as a “dry system” as the water stops flowing after the rain has stopped falling.

The first question that one faces is what size of tank to install. One also has to consider how long the collected volume of water will last in terms of what its intended use is. One may say the larger the storage capacity the better, however budget and space constraints will ultimately determine the storage capacity that one can install.
Two or more smaller tanks placed close to the gutter downpipe is a practical solution from the point of the ease of installation and efficiency of collection, however larger roof areas such as commercial buildings or sheds can justify larger tanks. These are often interconnected in order to collect the overflow from an adjacent tank.
The size of the roof area as well as the amount of rainfall will determine the amount of water collected. One square metre of roof area, with 1 millimetre of rainfall, will collect 1 litre of water. Therefore a 100 square metres of roof area will deliver 100 litres of water for 1 mm of rainfall.

Tanks availability

Vertical roto-moulded plastic tanks are available from any agricultural co-op or hardware store these days and in sizes from 100 to 2500 litres, with industrial tanks available up to 20 000 litres. Tanks made from corrugated iron and other materials are also available, however the range of plastic tanks is the most convenient option for the DIY homeowner.

One must however take into account where the tank will be placed before buying it. “Slim-line” tanks take up less footprint area but are a lot taller than their tubby brothers, which may complicate the pipe system needed to divert water from the gutter to the tank. Ideally, tanks should be placed higher than the intended destination of the water in order to create a static head to achieve a reasonable gravity flow-rate, however should this not be possible, a small pump may be fitted to the tank. One should also plan to fit a strainer or a collector system to the inlet in order to keep leaves and other organic matter from accumulating in the tank.

This organic matter degrades overtime into a sludge which may lead to the clogging of outlet valves and pipe systems connected to the tank. The sludge will need to be removed from time to time, dependent on the efficiency of the inlet strainer.

Many tanks have three threaded outlets, each with its own purpose. The upper outlet is the overflow, which should be led to a suitable point such as a storm-water channel or preferably to another storage tank. The next outlet is about 500 mm above the base of the tank. This is usually where the outlet valve is fitted, as the water abstracted at this point in the tank is free from sludge and debris that will accumulate in the bottom of the tank and is high enough to enable a bucket to be filled from the valve. The lowest outlet should also have a valve fitted and is there to act as a sludge removal point when opened or to completely drain the tank, if required.