Some typical questions about Rainwater Harvesting

Check out these typical ‘frequently asked questions’ about rainwater harvesting and rain water tanks:

1. What do you say to people who tell you, “I like the idea of harvesting rainwater, but it’s too difficult for me.”?

Rainwater harvesting systems need not be complex nor difficult to install.  In its simplest form, a rainwater harvesting setup can consist of a small rain barrel with a downspout  from a gutter.  Some form of ‘first flush’ mechanism should be fitted or simply a gauze covering that will prevent leaves and coarse debris from entering the rain barrel.  There are also specialist rainwater harvesting companies that can install a simple rain barrel for you or a more complex system that integrates with your existing water supply.  Most people start with a simple rain barrel or small water tank and then once they realise how beneficial and satisfying it is to harvest free water, many progress to more complex, automated systems.  Sceptics should start small, the financial investment is not much, then decide later whether an upgrade and more water storage capacity will suit you.

2. Is rainwater harvesting just a fad, or has it been going on for awhile? 

Rainwater harvesting has been practised for hundreds of years, with earliest records dating back 2000 years.  Many rural dwellers all over the world harvest rainwater as a matter of course due to necessity.  Urban dwellers have been spoilt by the easy availability of municipal water but uncertain times and rising water tariffs have created a renewed interest in rainwater harvesting.

3. What are some of the uses for harvested rainwater? 

Rainwater can used for irrigating gardens and lawns, filling swimming pools, flushing toilets, laundry and bathing/showering.  Rainwater can also be used for drinking purposes but it should be appropriately filtered to remove any potential contaminants that may have been collected off the rooftop or gutters.

 4. Talk briefly about the types of rainwater harvesting system/apparatuses that are commonly used for homes. 

As mentioned earlier, the simplest rainwater system consists of a rain barrel or water tank, a downspout from the gutter and some kind of device that separates the leaves from the rainwater.  If leaves and other debris is allowed to enter the tank, maintenance and tank de-sludging will become a frequent necessity.  Gauze should be used to prevent mosquitoes and other insects from entering the tank and laying eggs.  More complex systems can involve more storage capacity and underground water tanks that have integrated filtration systems and pressure pumps.  The rainwater can be diverted for irrigation, washing and laundry while the existing water supply is used for drinking.  Alternatively, the rainwater can be used for all purposes after it has been filtered by a whole-house water filtration system.

5. How much rainwater can a household harvest and use in a typical year?

This is highly dependent on the local rainfall patterns, roof area and space available for water tanks. In certain high rainfall areas, a household can harvest enough rainwater for its total annual requirement.  In many cases, the storage capacity is simply not enough and is the limiting factor.  It is surprising how quickly a single, good rainfall event can fill large water tanks in a very short space of time.

6. To what extent must harvested rainwater be filtered before it is stored or used? 

If the rainwater is to be used for drinking, then it must be filtered or purified.  There are many types of water filters that can be employed; your local water filtration specialist will be able to advise.  If chemicals such as chlorine are used for sterilising the rainwater in the water tank, these should be removed with an appropriate filter too (chlorine, even in low doses, has been found to have detrimental effects on human health over the long term).  Rainwater need not be finely filtered for other uses; just remove the coarser particles and leaves with gauze or a device made for this purpose.

7. What’s the best way to determine how big of a storage container is needed for a rainwater harvesting setup?

This is determined by your roof area and average annual rainfall.

A simple calculation is to multiply your roof area by your annual rainfall to get your maximum possible amount of rainwater you can expect to harvest.  Estimate the maximum number of days you could expect with no rain and multiply this by your average daily amount of water that would be drawn from the water tank. This will give you the amount of water you need to have stored for the dry period.  For example, if you expect a dry period of 30 days, and your household uses 500 litres of water per day, you will need a water tank of at least 15000 litres.  In reality, some rainwater is lost to evaporation and certain roof surfaces absorb more rainwater than others.  There are more complex equations that take these factors into account but it is better to err on the side of more water storage capacity than less.

8. In addition to conserving water, how does rainwater harvesting help the environment? 

Rainwater harvesting, in urban areas especially, reduces the amount of water entering stormwater drains.  As the rainwater passes over streets, yards, industrial areas, etc. it collects contaminants such as oil, fuel, pesticides, fertilisers and other chemicals as well as topsoil.  This stormwater inevitably ends up in rivers and lakes where it pollutes aquatic ecosystems and, in many cases, the water supplied by municipal services.  The paved/tarred nature of the urban landscape results in very little rainwater infiltration into the soil and is channelled to water courses in unnaturally high volumes in a very short space of time- in effect flooding the local water systems causing stream bank erosion and other problems.  So harvesting rainwater really is beneficial to the environment.

Rainwater harvesting can also take the form of diverting rainwater to ‘rain gardens’ keeping much of the rainwater on the property.

Depending on where you are in the world, there may be various by-laws that apply to rainwater harvesting and even some restrictions so check with your local authority before investing in a system.

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