So You Want to Store Water
Consider “What are you collecting water for?” The answer is not as straight forward as you may think. We use water for many things - drinking, washing clothes, bathing, cleaning (cars, floors, windows, or dishes), swimming pools, gardening, to name but some.
The type and quality of water collected can vary from house to house depending upon where you live, the way it is collected, and how it is stored. For instance, water collected from a rusting iron roof is likely to contain too many impurities to drink. An unsealed tank may become home to mosquito larvae or the final resting place of birds and small rodents.
If you want to collect water which is good enough to bathe in or even to drink, then you need to give careful consideration to all the materials the water comes into contact with and how it is stored. In most cases it is preferable to use mains water for drinking and use tank water for all other purposes. If you wish to drink your water but are unsure if it is safe, then it is recommended that you have it analysed.
How Much Water?
The amount you collect is depends on both rainfall, and roof area. For an average household with 100m2 of roof surface space and which is home to 4 adults, one 5,000 litre tank would supply around half their annual water needs. Assuming the tank is filled on average around 12 times per year this represents 60,000 litres of water. If you want enough water to meet all of your household needs then a similar sized household may need one 10,000 litre tank or two 5,000 litre tanks.
• To calculate how much you might collect - 1mm of rainwater on 1m2 of roof will produce 1 litre of water.
• You can collect water from sheds, greenhouses, garages, gazebos and other structures.
• Don’t drink water collected from roofs with lead-based components or flashings.
• If your roof area is small, why not consider adding a veranda?
• If in doubt about your water quality, get it tested by water analysts.
Water from roofs can also be collected into underground tanks. These can be constructed on site by digging out a pit and then lining it with concrete. Alternatively, pre-formed tanks can be used. The tank is sealed so that groundwater will not enter. Underground tanks are ideal where outdoor space is at a premium such as in city courtyard homes. They also have the benefit of keeping water cool in summer and unfrozen in winter. However, they can be difficult to inspect and repair should anything go wrong and are more expensive to install initially.
Types of Water Tank
Water tanks are the most expensive part of any water catchment system and there are many options in terms of construction materials.
Polyethylene (plastic) are relatively cheap, lightweight, can blend in with landscape, different colours and sizes available, and reinforced types suitable for underground. They can eventually degrade in sunlight, can’t be painted easily, dark colours absorb heat and need to be shaded, fittings may leak, and underground reinforced tanks are expensive
Concrete Tanks are strong, durable, good underground option, and very versatile if made on site. The are also permanent fixtures, needs to be plastered inside with suitable material if used for for drinking water, are labour intensive to make. They can raise pH of water from lime leaching out of concrete.
Galvanised Steel (zinc coated) are long lasting, many sizes available, may be painted. Eventually they corrode, iron may leach into water, not particularly attractive, old types may contain lead, brass fittings cause corrosion, zinc lining may leave unpleasant taste.
Ferro-cement (concrete, steel reinforcement bars and wire mesh). These are prefabricated tanks, cheaper and thinner than concrete, suitable for underground or aboveground use. They may contain toxic chemicals which can leach into water, may crack in heat, may need to be painted white to refract sunlight.
Fibreglass are relatively cheap, lightweight, easily repaired, many sizes available, come with fittings attached so less chance of leakages. Fibreglass tanks need food grade resin lining for storing drinking water.
The tank you choose will depend on your budget, aesthetic preferences and what’s available. Whatever type you choose, the tank must be opaque to prevent algae from growing inside it. You can disguise a tank in many ways. For instance, if your home is built out of stone, you could build a stone structure around you tank.
Keeping Your Water Clean
If you do collect and store your own water, then you must ensure the water is suitable for the purposes required. Although some water may be affected by air pollution, it is unlikely to be a significant problem. Water collected in industrialised areas may contain more pollutants than water collected in a pristine location. Similarly, water collected in an arid climate may contain more dust particles. In either case, levels are likely to be negligible but consult your local council if concerned.
However, dust, pollutants and other types of contaminant can come off a roof and into tanks. When collecting water from roofs it is important to ensure your roof and gutters are kept clean and free from fallen debris. Remove overhanging tree branches if necessary. Gutter guards can be installed to keep twigs and leaves out. The inlet pipe and the tank overflow must have a sieve guard to keep out animals and birds. Remember, your water comes into contact with everything on your roof!
If unsealed, water tanks can become contaminated by insects such as mosquitoes, spiders, or beetles. Vermin like mice and rats may also fall in into the tank, as may birds. All water tanks should be sealed and have a tightly fitting manhole cover for inspection purposes. Poorly stored water may also be detrimental to the land it is used on.
Your choice of fittings is also important. For drinking water, polyethylene and PVC pipes are probably the safest since they don’t leach toxins into your water. Brass and copper fittings can pollute your supply and are more prone to corrosion. Metal pipes often contain traces of lead.
In most cases, provided you have installed a clean and sealed system and your tank has a food grade liner, your water will be fine to drink.