Household rainwater capture – practical and economical

What can you do to personally cope with this, and future, water shortages?

The fact is that the SW Cape typically enjoys good rainfall each year varying between 350mm and 1000mm depending upon one’s location. In other words rain that falls in areas right where people live is actually sufficient to sustain the population with little need to draw from the dams.

The problem is that because of the Cape topography, most of this rain falls at very low elevations making it practically impossible to capture this water on a big scale and store it in large dams.

One solution would be to install rain water capture and storage systems in every house that has sufficient space and a large enough roof area. This is exactly what is done in most country areas to supply fresh drinking water to rural households. So why not do it in the cities?

We propose that the provincial government could assist by:

– negotiating economical prices of certain standard tank sizes and piping/connection fittings.

– encourage banks to permit people with bonds to finance the installation through extending bonds.

– help to train and form enterprises of people skilled to install and plumb in such systems.

– provide financial support to low income households to enable them to access rainwater tanks.

We feel extremely strongly that if possible every house should be equipped with a rainwater capture system with at least one tank connected to the house gutters. The rule of thumb for rainwater harvesting is that approximately 0.85 litre is captured per 1sqM of roof per 1mm of rain; i.e. a 10mm shower falling on a 200sqm roof area will capture about 1700 litres of water.

Here is an example of how it could work out for a two-person household who first reduced their overall consumption to 100 litres/day/person; have a 200 sqM roof area; and install a 5000 litre tank. It assumes that the rain showers occur evenly across the month.

They would consume 72,000 litres per year but capture 56,000 litres, only drawing 16,000 litres pa from the system (dams). Theoretically, they would only draw municipal water during 4 months and should never exceed the basic 6000 litre allowance so the cost would be zero. [Note that in survival circumstances with water use cut to 40 litres/day/person, a full 5000 litre tank could supply fresh water for the 2 people for two months.]

Another case would be a 4-person household also using 100 litres/person/day with a bit more roof area (say, 250 sqm) and a 5000 litre tank. They would consume 144,000 litres per year but capture 87,000 litres, only drawing 57,000 litres pa from the dams of which only 14,400 litres pa would be in excess of the basic allowance and be paid. [Note that in survival circumstances with water use cut to 40 litres/day each, a full 5000 litre tank would supply fresh water for the 4 people for a month.]

Or, consider a 3-person household also using 100 litres/person/day with a roof area of, say, 180 sqm and just a 3000 litre tank. They would consume 108,000 litres per year but capture 64,000 litres, only drawing 58,000 litres pa from the dams of which only 5100 litres pa would be in excess of the basic allowance and need to be paid.

And the overall outcome?

A 5,000 litre tank and piping could probably be installed for R7000 or less (I have assumed gravity feed but if a pump is needed then add R1200-R1900 or so depending upon its complexity). Assuming households could consistently reduce consumption into the range of 100 litres per person per day, or less, and save water in this manner then the net draw of water from the dams could be reduced to, say, average 30,000 litres of water per annum per household vs the 270,000 litres drawn on average by those households per annum under “normal” circumstances, a net reduction of up to 240,000 litres of water drawn per household.

Read about D-I-Y rainwater harvesting and tank installation here.

Lastly, assume that 700,000 households achieved this goal. Savings could total 168 McuM of water per year, equivalent to the output of the desalination plant now under consideration.  Compare:

– cost of 700,000 tank installations @ R7,000 each = R5 billion capex; annual operating cost = nil.

– versus cost of a desalination plant = R15 billion; annual operating cost R1.2 billion.

– and in so doing, water captured and stored in the existing dams would be sufficient for 3+ years.

This could sustainably solve the problem for ever at a fraction of the cost of a desalination plant. Most of the cost would be covered by individual households and effectively be paid for out of future water cost savings (pay-back should be 7-8 years). And installations could start straight away.

Rainwater harvesting by households in SW Cape should be a key part of any solution.