How does water get from the Vaal dam to your tap at home?


All this wondrous rain bucketing down across Gauteng today has us thinking: what does it take to get water from the Vaal Dam to our offices here in Johannesburg?

Yes, there are a series of pipes and depots scattered around the city but the journey the water itself takes from the dam to our taps is an incredibly interesting one.

It all starts at the Vaal Dam where Rand Water extracts the water using a canal and a gravity pipeline. Water is also pumped from the Vaal River Barrage Reservoirs at Lethabo, Zuikerbosch and Vereeniging and a small amount of water comes from underground water in Zuurbekom.

Once this water reaches Rand Water it needs to be purified and as it turns out this is an incredibly intensive task.

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The first stage is called coagulation and involves removing suspended particles in the water using slaked lime.

Erm, what is slaked lime?
Slaked lime is created from raw limestone that comes from the Northern Cape. It is burned in a shift kiln at 1200 degrees centigrade where it is converted into calcium oxide and carbon dioxide. The burnt limestone is then added to water in rotating slackers and produced into slaked lime also known as calcium hydroxide. As for the CO2 that’s created, that happens to be used as well.

During this process a small amount of sodium silicate is added to the water which allows the aforementioned particles to clump together. This mixing process is done in periods 20 – 30 seconds long because speed helps along the process of forming clumps, also known as flocks.

This is a very important process because not only does it bring all the particles together so they can be removed later but it also limits the growth of algae and removes heavy metals, organic materials and bacteria and viruses. Once the water has gone through coagulation and flocculation it moves one to sedimentation.

Letting nature do its thing

Once the particles have been flocked they need to be removed and Rand Water uses something incredibly simple to do this; gravity.

Water containing these flocks is pumped into special tanks where the flock settles and separates itself from the water. This process can take four hours and removes 95 – 97% of suspended particles in the water.

Measuring water purity
Water is graded against turbidity which is how much transparency is lost due to suspended particles. The World Health Organisation has said that drinking water should not have a turbidity rating of higher than 5 Nephelometric Turbidity Units (NTU) but ideally it should be below 1NTU. The water leaving Rand Water’s sediment tanks has a turbidity of 5 NTU before it goes on to purification.

The sludge that is formed during the seperation process is pumped to Rand Water’s disposal site in Panfontein. Here the sludge is pumped with organic flocculent to thicken it further so that any excess fluid can be separated and then returned to the purification process. As much as 1300 tons of sludge is produced each day.

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Stabilising

Those of you who understand chemistry will know that adding lime to water can raise its alkaline. In fact Rand Water says the aforementioned process can raise the pH of the water to 10.5 and that can lead to corrosion. To bring the pH level of the water down the water is pumped into carbonation bays where the CO2 that was created while making slaked lime is bubbled through the water to bring the pH down to 8.4 – 8.0. This allows Rand Water to produce a more chemically stable liquid that won’t corrode the pipes it will eventually flow through.

Once the water has been carbonated it is then filtered.

Rand Water uses a filter that is comprised of a 600mm layer of fine sand that is supported by 500mm of gravel. These filters are covered to protect them from light which prevents the growth of algae. Once water passes through this filter its turbidity rating falls from 5 NTU down to 0.5 NTU.

To insure that these filters work properly they are cleaned regularly using air to loosen up the fine sand after which water is pumped through them at a speed of 32 metres per hour.

Purifying your water

The final step of the process involves purification. This is done with chlorine but its not as simple as sprinkling a bit of HTH on the surface of your pool at home.

The amount of chlorine used for instance must be between 1.5mg/l and 4.0mg/l depending on the quality of the water. There is however a catch, chlorine is only effective for six to eight hours, the amount of time it takes water to be pumped from the purification plant to booster pumps around Gauteng.

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For this reason the water at these booster pumps has additional monochloramine (a mixture of chlorine and ammonia) added to it to prevent the growth of bacteria. While this solution is not as active as chlorine it can protect the water for up to eight days and according to Lenntech it actually improves the flavour of the water after its been treated with chlorine.

Once this process has taken place all that’s left is you opening your tap at home and taking a sip of some incredibly high quality H2O.

[Source – Rand Water]

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