How Much Rainwater Can You Collect and What Are the Challenges?
Rainwater Harvesting – How Much Can You Collect?
In the Helderberg and Stellenbosch areas, the average annual rainfall is about 750mm. That means for every 10m² of roof space, you could collect around 7,500 litres (7.5m³) of rainwater per year. If your roof is 120m², you could harvest up to 90,000 litres (90m³) annually.
That’s a lot of water!
It’s not always practical to collect rain from every part of the roof, but the potential is impressive.
Seasonal Considerations
Most of our rain falls in winter—when we don’t use much water. Unfortunately, the dry Cape summers follow, and that’s when gardens need the most irrigation. To carry stored rainwater through the summer months, you’d need massive storage capacity.
So, using harvested rainwater for toilets, washing machines, or general cleaning during the rainy season and into the summer months is often more practical than irrigation.
Is Rainwater Safe to Use?
1. Rainwater Is Acidic
Rainwater in the Western Cape is naturally acidic and lacks calcium and magnesium. This makes it corrosive—it can damage copper pipes, geyser elements, and even kettles.
Corroded copper can leach into the water, and copper in drinking water is toxic. Replacing pipes and equipment can be expensive, so it’s best to avoid using untreated rainwater indoors.
If you want to pipe rainwater into your home, treat it first. The pH must be adjusted to bring the Langelier Saturation Index into a safe range (In South Africa, the safe range is between -0.5 and +0.5).
2. Contamination from Roof Runoff
Birds and animals often defecate on roofs. When it rains, their droppings wash into your gutters, and then into your tank.
That runoff contains fecal bacteria (like coliforms), and possibly other harmful microbes. While screens and first flush diverters can reduce contamination, they won’t make the water safe to drink.
We’ll cover these pre-filtration options for rainwater collection in more detail in a future blog.
3. Organic Waste and Tank Conditions
Leaves and debris on your roof add more organic material to the rainwater. This discolours the water and feeds bacteria.
Inside the tank, where bacteria, heat, and humidity are present, the bacteia multiply quickly. A warm tank with a high-humidity air gap above the water creates perfect conditions for bacteria to thrive.
This makes untreated rainwater unsafe, especially if it contains high levels of organic carbon.
4. Problems with Disinfection
While Chlorine is an effective disinfectant if the water is not going to be used for drinking purposes, it does have negative side-effects due to the organic carbon in collected rainwater due to harmful by-products like trihalomethanes, which are known carcinogens. Trihalomethanes are released when chlorine makes into contact with organic carbon.
Ultraviolet sterilisation is often ineffective if the water is not crystal clear, which is often the case with collected run-off rainwater due to tannins, organic carbon and impurities.
But do not let the above information scare you away from rainwater harvesting, we’ll explore safe and effective rainwater treatment methods in a follow-up post.
