Charles Sellick, Gordon’s Bay
The article “Several shades of grey water” by Tony Totten (Bolander, June 28) refers.
The numbers used in the article to describe amounts or quantities of water involved, are confusing and inaccurate. As well intended as this article is in trying to quantify the amounts of fresh water consumed and waste water disposed of, it confuses and misleads the readers instead of enlightening them about the magnitude of the current water crisis faced by the City of Cape Town.
Units of measurement commonly used to measure quantities of fluid or liquid are:
1 litre = one sachet of milk
1 Kilolitre or 1 cubic metre = 1 000 litres (i.e. 1 000 sachets of milk)
1 Megalitre = 1 000 Kilolitre or 1 000 cubic metres =
1 000 000 litres (i.e. 1 000 000 sachets of milk)
1 million Kilolitres or
1 million cubic metres = 1 000 Megalitres = 1 000 000 000 litres (i.e. 1 000 000 000 ( 1 billion) sachets of milk)
The following are examples of inaccuracies in the article.
Firstly: The article says the current water usage is “640 million Kilolitres a day”.
This means that in only 18 hours, the City will empty a full Theewaterskloof Dam, which has a storage capacity of 480 million Kilolitres.
It also means that each of the 3 500 000 citizens supplied with water by the City is using 183 Kilolitres or 183 000 litres of water each day.
Secondly: The article says that the City was using “well over 1000m Kl (per day) in the days before the drought”.
This means that in under 12 hours, the City will have emptied a full Theewaterskloof Dam.
Thirdly: The article says that “450m Kl of treated waste water (is poured) into the sea every day.” This means that this rate of discharge would fill an empty Theewaterskloof Dam in under 26 hours.
Furthermore, the statement that “Unfortunately for our dams, torrential downpours do not collect as much water in the catchment areas due to the quick run off.” is incorrect.
Torrential rains in the catchment areas of the dams will fill the dams quickly.
However, the groundwater is best recharged by more gentle prolonged rainfall.
The groundwater sustains the springs and river flows during the generally dry summer months of the year.
The article also refers to the benefits of rainwater harvesting. It was and could still be common practice in many parts of South Africa. This is what I grew up with until I was 11years old.
However, the lifecycle cost per Kilolitre of harvested rainwater is very high when compared to the convenience and cost of purified municipal water supplied in the home or garden under pressure.
Part of the cause is the high initial cost of storage tanks – in excess of R1 000 per Kilolitre of storage capacity. If 2 Kilolitres or 2 000 litres of domestic storage was provided for each of the
3 500 000 citizens supplied with water by the City, this would provide only 7 million Kilolitres of additional storage.
This is only 0.8% of the existing 898 million Kilolitres of storage capacity provided by the six major dams that supply water to the City. The initial cost of these storage tanks will be in excess of R7 000 million.
Thank you, and Bolander apologises for the inaccuracies.
