FEATURE
the five years to end-2016. It has not slowed since, and
most suppliers say demand is accelerating and they can
scarcely keep pace with demand. One in every ten South
African homes has a JoJo tank, which is reputed to have a
50% market share.
Reduced water availability at source – in dams and
reservoirs – owing to regular drought conditions in one
or another region of Southern Africa has resulted in many
households and companies having to deal with the reality
of water insecurity. There is a nationwide drive to reduce
domestic water consumption to below the current average
of 150ℓ (Cape Town at the height of its drought achieved
55ℓ). Domestic RWH is one such source of alternative water
supply, focused on harvesting rainwater from rooftops. Other
harvesting involves in-field, pavements and roads in the
urban, peri-urban and rural areas.
A new trend?
Collecting rainwater for use in the garden is no new trend;
it’s been done for decades. Placed under a gutter downpipe,
a basic tank arrangement can be used to water plants and
wash the car. A step up from this primitive system are RWH
systems. These collect water in a similar way, then pass it
through a filter to collect any debris or leaves. The water
is stored in a tank, then pumped to feed a toilet, washing
machine or dishwasher. Put simply, rainwater harvesting is
the collection and storage of rain for a variety of uses – but
not for drinking. RWH is a catalyst to dramatically increase
household water security, while reducing the strain on
drinking-quality water in the urban environment.
Fuelling demand for RWH systems is sharply increasing water
tariff hikes, as well as concerns about the reliability, quality
and quantity of water. For every 1m 2 projected roof surface
that receives 1mm of rainfall, 1ℓ of water can potentially
be collected. A city house with a 150m 2 roof surface that
receives 10mm of rain can collect 1 500ℓ of rainfall.
This means, during a decent shower, a company can
easily harvest 100 000ℓ in an hour from an industrial or
warehouse-sized roof, for example.
Payback period
Current constraints for RWH success include justifying the
capital outlay to install a well-designed and fully functional
RWH system. The return on investment period for most
systems at current water tariffs ranges from four to five years.
Wits School of Governance visiting adjunct professor and
former Department of Water Affairs director-general Mike
Muller has previously warned that RWH will not make a
significant contribution to South Africa’s water challenges.
The most important rainwater harvesting we do is capturing
river flows in our dams. In years of drought, household RWH
has less potential, “as it doesn’t solve long-term drought or
mitigate significant water interruptions,” and most domestic
RWH systems store less than 10 000ℓ – usually not enough
to supply a household for more than a few weeks.
Sandy Ballam, director at Ballam-Waterslot, says, “The
extended drought in Southern Africa, along with ever more
stringent water restrictions, makes it very difficult to protect
one’s not insignificant investment in a garden.
The solution is to install infrastructure which will save
rainwater from run-off into the stormwater system and
February 2020 Volume 25 I Number 12
47
Vital information
JoJo Tanks offers the following guide to establishing an RWH system:
The methods used to size the tanks of rainwater harvesting systems
There are many factors you need to consider when choosing your tank numbers and sizes.
Key factors that will influence the rainwater tank size and usage will vary according to the
specific circumstances of each property. These may include:
• Roof surface area;
• The number of occupants in the house;
• If the tank water is being used for internal use (toilets/laundry) or only external use
(irrigation/pool);
• Climate conditions; and
• Rainwater tank size and number of tanks.
In order to know how much rainwater you can harvest from your roof, you need to know your
roof surface area and your local annual rainfall. Once you have worked this out, you will be
able to determine what size tank you require.
As a general rule of thumb, each m2 of roof space collects around 1ℓ of water for every 1mm
of rainfall received.
Rainwater harvesting calculation:
Roof area m 2
x
Rainfall
= Rainwater harvesting volume
Step 1: How to work out your roof surface area
Building plans offer the easiest way to calculate your roof surface area and will also give you
a very accurate result. Google Maps or similar mapping software can also be used, or you can
simply go outside and use a tape measure to roughly measure the size of your roof at ground
level.
Step 2: Calculating annual rainfall
National or local weather services are an excellent source of historical rainfall data for your
area. An example is www.weather-and-climate.com, which provides information about the
weather and climate in almost every country around the world. Select the country and city, and
the site will provide the average monthly and annual rainfall per city.
Step 3: Rainwater harvesting volume
Use the information collected to work out your rainwater harvesting capacity.
For example:
Roof area
150m 2
x
Rainfall in JHB
718mm
=
Rainwater harvesting volume – per annum
107 700L
The size of the property and household or business needs should determine the size of the
rainwater tank you choose. The actual volume and timing will be influenced by the rainfall
in the specific area. Balancing the volume of rainwater you need with the volume you can
harvest, will help with your decisions regarding tank numbers and sizes.
You should also design your Rain Harvesting system with appropriate products and the JoJo
tanks have a 10-year guarantee on material and workmanship. However, properly installed
JoJo tanks have been known to exceed a lifespan in excess of 20 years.
preserve it for use in the dry periods between rainfalls. The
future savings on the cost of municipal water is obvious. Add
to this, a leaf catcher that will prevent leaves and seeds from
entering and rotting in the tank, and the very clever dirty
water flushing device, which greatly reduces the dust and
sands settling at the bottom of the tank.
“With the addition of a small booster pump, this water can
be utilised almost anywhere in the garden. The pressure is
sufficient to run a hand-held nozzle or a sprinkler,” says Ballam.
www.plumbingafrica.co.za