How Rand Water purifies Gauteng’s water supply

Water is made up of 2 hydrogen atoms and 1 oxygen atom (H₂O).  When these H₂Os join together they form water.  When water falls on the earth’s surface from the clouds, many substances dissolve in the water, eg. silt, minerals, bacteria, etc., just like a train picking up passengers. 
Water is the train that moves along the tracks and the passengers are these substances that get on and off the train.  By the time water enters the dams it has many passengers, both good and bad to humans. Purification is required. 

Water collects silt and minerals from the land.  There are germs (bacteria) that may cause people to get sick if they enter the human body.  It is for this reason that the dirty water needs to be cleaned before it is supplied to taps. This cleaning process is called water purification.   Rand Water is the company that cleans (purifies) the dirty water for Gauteng, South Africa.

The Vaal, Klip and Wilge Rivers naturally flow into the Vaal Dam, which has a catchment area of 38 000 km².   Rand Water draws raw water for purification (cleaning) from the Vaal Dam via an intake tower.  This raw water is then transported via canals and pipelines to Rand Water’s two purification stations in Vereeniging.

Raw water from the Vaal Dam contains different “passengers”:

• large water plants & animals (fish, crabs, floating plants)
• sticks, leaves & litter     
• sand, silt and clay particles
• small water plants and animals (small insects, algae, plankton)
• germs (viruses & bacteria)
• bad minerals (iron, aluminum, manganese)

SCREENING

When raw water first arrives at a purification station it passes through metal bars or screens.  These screens trap large water plants, water animals, sticks, leaves & litter, but allows the rest of the water to pass through.

COAGULATION & FLOCCULATION

Now the raw water enters a spiral flocculator where slaked lime is added.  The raw water contains sand, silt and clay particles that have small negative electric charges that repel each other.  The water moves around very quickly so that there is good contact between the slaked lime and the particles.  The slaked lime neutralises these electric charges and causes the particles to attract to each other. This process is called coagulation.  A further chemical, sodium silicate, is added to assist the process. 

As the water moves around the spiral flocculator the sand, silt and clay particles, as well as some small water plants and animals, germs, and all the bad minerals, “stick together” to form floc.  This is called flocculation. Water moves very quickly in the middle of the spiral flocculator whilst on the outside it moves slower making it much easier for the objects to stick together.

SEDIMENTATION

The water, together with the floc, now flows slowly into a large sedimentation tank where the floc settles to the bottom of the tank.  This is called sedimentation.

The floc at the bottom of the tank is now called sludge and is sucked out by desludging bridges and sent to a sludge deposit site.   The top of the water in the tank is now much cleaner.  This clean water flows over the side of the sedimentation tank into the carbonation tank.

CARBONATION

When water leaves the sedimentation tank it has a pH of about 10.5 because of the lime that was added in the spiral flocculator.  This high pH (alkaline) makes the water feel and taste soapy.  In order to make the water less alkaline (a lower pH), carbon dioxide is bubbled through the water.  This is called carbonation.  The pH of the water is now between 8.0 and 8.4.  This makes the water taste and feel much better.  The water is kept at this pH level because it causes Calcium Carbonate to deposit inside the pipes which forms a protective layer for the pipes.

FILTRATION

The water still contains some small water plants and animals, and germs.  The water then flows into filter houses where it flows through sand filters. These sand filters are big flat beds of sand which have particles of varying sizes.  The water flows slowly down through the filter and all the small water plants and some germs are trapped by the sand. This is called filtration.  The water now enters underground pipes.

CHLORINATION

Even after the water has been filtered it still contains some germs.  In order to kill these germs, chlorine gas is bubbled through the water.  This is called chlorination.

WATER SUPPLY

This clean water is then  pumped through underground pipes to booster pumping stations.  As the chlorine is only effective for 6 – 8 hours it is necessary to add chloramine (chlorine & ammonia) to kill any other germs that might get into the water.  From the booster pumping stations the water is pumped into reservoirs and then sold to the various municipalities that supply homes, schools, businesses and factories with clean healthy water.
NOTE:  To make sure that the water reaching the consumer meets the required standards, strict guidelines are set for the quality of the drinking water through all the stages of the purification process and throughout the distribution system.  The quality standards that have to be met are most strict. It is thus unnecessary for you to purchase bottled water or fit home treatment devices to your tap.

Source: Rand Water

Now it can be understood why water tariffs continue to  escalate- it is becoming increasingly expensive for companies to purify water for municipalities who then pass the cost onwards to the consumer.  Rainwater harvesting is one way of reducing water bills and it also provides peace of mind as to the origin and quality of the water (many municipalities and water companies around South Africa are struggling to supply consistent, high quality water).  Rainwater is usually pure but it may pick up contaminants from rooftops and gutters, that is why we always recommend that rainwater from rain water tanks be appropriately filtered if it is used for drinking purposes.  Rainwater is also often closer to a neutral pH of between 7.0 and 7.5 which makes it very good for bathing and for garden irrigation.  Water tanks are the heart of any rainwater harvesting system so choose your water tank carefully.  See JoJo Tanks’ range of water tanks, rainwater tanks, tank stands and other quality JoJo Tanks products by clicking on the links below.

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