FACTS, NOT FADS, ABOUT WATER-SAVING

I like Daniel Louw’s letter “Water Conservation: help or hazard?” (October 2017), as I share his concern that some products and practices may just be a fad.

I live in a large residential complex in Cape Town, where management recently urged owners to install a rainwater tank, pointing out that just 10 mm of rain on a 100 square metre roof yields up to 1 kilolitre of water. That is true, but let’s take a closer look.

A hundred square metres is the average size of a cottage in this complex, excluding the garage. Probably all the garages have a separate gutter and downpipe, which would therefore not feed into the tank. The cottages have different designs, with different sizes and differing numbers of downpipes. The residential portion has at least two gutters and the associated downpipes, one for the front of the roof and one for the back. So, without altering the gutters, each tank would be fed by just one gutter and downpipe, and the average catchment area of that tank would probably be only around 33 square metres.

Based on past rainfall figures in Cape Town, and taking into account the reduced likely future rainfall, and allowing for a high degree of evaporation from the roof during the dry summer months, I guesstimated that the total water harvest, without altering the guttering, would be maybe just 12 kilolitres per cottage per annum.

If one kilolitre costs, say, R10 if supplied by the Cape Town municipality, then that represents a saving of only around R10 per month. The cost of purchasing and installing a tank would probably be R3 000 or more. It would never pay for itself. There would have to be a huge increase in the charge for water for this to become a cost-effective solution.

However, I’m not advocating against rainwater harvesting. It would reduce the water supply burden on the City. So it would be a good idea for people with sufficient money to spare to invest in one or more tanks. And this would increase the number of manufacturing jobs, thereby relieving the chronic unemployment problem. People should just be aware that they probably won’t recoup their costs.

There is another aspect I’m concerned about: some of the downpipes currently empty directly into drains. What will the effect be if that water is redirected onto the lawns and gardens?

I foresee two effects. It will reduce the amount of water flowing into the drains, and the redirected water could result in more soil and leaves being ultimately washed into drains. I know that not flushing the toilet for an extended period of time causes the sewage in the pipes to solidify and results in a blockage. So if less water goes into the drains, and more debris goes into them, could this result in a blockage?

I have a similar concern if water from the bath, shower, hand basin and sink is similarly removed for use elsewhere, though I doubt the consequences would be as serious.

I have referred the above specifically to my own complex. However, from what I’ve seen of houses in the suburb, my observations also apply to them: they are just a bit bigger.

C STEVENSON CAPE TOWN