5 Things to consider when installing a rainwater harvesting tank

The prospect of installing rain water tanks or rain barrels for home rainwater harvesting is exciting but do not rush the process.  There are professional installers but simple rainwater harvesting systems can be installed with a minimum of DIY skills.

Here are some tips:

1. Determine how much rainwater you can harvest from your roof

Here is a simple online calculator that you can use to calculate your rainwater harvesting potential

Or use the method below:

Collection area (roof size):

To calculate your roof runoff, 1 millimetre of rain on 1m² will deliver 1 litre of water into your water tank. As a rough guide:

• An average single carport is approximately 25m²
• The roof of an average 3 bedroom house is approximately 150m²
• The roof of an average 4 bedroom house is approximately 200m²
• The roof of an average 5 bedroom house is approximately 300m²

2. Rainwater tank size

The size of your residence and your household needs should determine the size of the rainwater tank you choose. There are a range of products available in different shapes, sizes and materials. Generally, rain water tanks can be classed as:

• Small water tanks – less than 2000 litres
• Medium water tanks – between 2000 litres and 10000 litres
• Large water tanks – greater than 10000 litres
• The number of occupants in the house: It makes sense to assume that the larger your household, the faster you will use your rainwater supply.
• Your internal and external water use practices: By connecting your water tank for internal use in the toilet and laundry you will increase your rainwater tank’s yield.
• Local climatic conditions: The climate where you live will have an important impact on the yield of your rainwater tank. You should consider the characteristics of your seasonal rainfall zone when deciding to purchase a rainwater tank.

3. Type and brand of water tank

In South Africa, water tanks can be found made from fibreglass, stainless steel, galvanised steel, concrete and polyethylene (‘plastic’ or ‘poly’ water tanks).  We recommend polyethylene water tanks because of their ease of transport and installation, relatively low cost vs good longevity.  Always choose a well-known brand such as JoJo Tanks – JoJo water tanks are guaranteed for 5 years, are rotational-moulded  (no seams to split and crack), are available in an array of sizes and colours and are lined with an FDA-approved food-grade black inner lining to keep stored water safer and fresher for longer and inhibit algae growth.

4. Installation of the water tank

Most simple rainwater harvesting systems require the rain barrel or rainwater tank to be placed at the lowest point, under the eaves to collect rainwater from the gutters and downspout using gravity.  A simple leaf separator can be bought or made – this is attached to the top of the downspout to separate water from leaves and other debris that may be in the gutter.  Another sieve or fine net may be place over the entrance of the hole that enters the rain tank to further remove finer debris.  Some rainwater tanks, such as the JoJo 750 litre Slimline rainwater tank, are supplied with a stainless steel sieve as well as a tap/faucet at the bottom of the tank.

JoJo Tanks 750 litre slimline rainwater tank with simple downspout and sieve

More sophisticated rainwater harvesting systems using small pumps allow the rain water tank to be situated away from the building, out of site or underground water tanks may be an option too (the JoJo 6000 litre underground tanks are popular in South Africa).  Ensure that the water tanks are placed on a firm, level surface or water tank stand.  The larger the water tank, the more critical the base strength.  A concrete base is recommended or a certified water tank stand such as those supplied by JoJo Tanks is even better.

5. What will you use the rainwater for?

Most people start off installing a rain barrel or rainwater tank to supplement garden irrigation – very little filtering is needed for this purpose; a fine sieve will remove most debris.  Rainwater is much better for plants than chemical-laden municipal water.  If the rainwater from the rainwater tank is to be used for household use, the rainwater should be free of coarse particles and should be appropriately filtered to ensure it does not clog up washing machines, shower heads, etc.  If the rainwater is to be used for drinking, ensure that you filter and/or purify the rainwater first (rain is pure before it reaches your roof but it may have collected contaminants from your gutters).

Rainwater harvesting is very rewarding – it not only decreases dependence on Mains water supplies but it saves money on water bills, ensures emergency water supplies during water supply outages and is good for the environment by reducing stormwater runoff in urban areas.