Rand Water abstracts its raw water from the Vaal Dam via a canal and a gravity pipeline, and by pumping from the Vaal River Barrage Reservoir at Lethabo, Zuikerbosch and Vereeniging. A small quantity of water is also abstracted from underground sources at Zuurbekom

It must now undergo the purification process required to ensure that the water meets the stringent standards set for drinkable water.

Conventional treatment processes remove the suspended material and disinfect the water prior to pumping to local authorities, the mining industry and other large industrial concerns.  Each stage in the purification process is accompanied by changes in the physical and chemical composition of the water.  These changes are constantly monitored and corrective action is taken to prevent the water quality from deviating from the prescribed limits.

The purification process involves seven stages which are:

·  Coagulation

·  Flocculation

·   Sedimentation

·  Stabilization

·  Filtration

·  Disinfection

·  Chloramination

Coagulation and Flocculation

All raw water contains suspended particles which need to be removed.  The water from the Vaal Dam contains highly dispersed particles which, because they are colloidal, tend to remain suspended for a long period.  This colloidal property, with minute diameters varying from 0 to 100 nanometers, is more significant in this first stage of Rand Water’s purification than the quantity of suspended matter as, under normal circumstances, it can remain suspended for two or more years.

Raw limestone (calcium carbonate) from the Northern Cape is fired in a shift kiln at Zwartkopjes at 1200°C to convert it to calcium oxide and carbon dioxide gas.  The burnt limestone is crushed and slaked with water in rotating slackers to produce slaked lime or calcium hydroxide.

The slaked lime is then added to the water as the main coagulant to destabilize the electrostatic charges of suspended particles in the water.  A small quantity of activated sodium silicate is also added to the raw water to enable the suspended particles to clump together to form larger clusters or flocks which can then be settled out by gravitation.

·   Coagulation in Rand Water’s systems is the process which destabilizes the particles during flash mixing periods in about 20 – 30 seconds.  This is the first process in removing the suspended matter and involves adding sodium silicate and slaked lime to the water.  The rapid mixing is very important as it achieves instantaneous and complete homogenization of the coagulant with the specific volume of raw water being dosed.

·   Flocculation is the clumping together of the suspended matter destabilized by coagulation to form heavier visible particles called flock.  The flock remains in suspension as the water flows at high velocity through either spiral flocculates or baffled channel conditioning bays.  In Rand Water’s systems, orthogenetic flocculation predominates resulting from the fluid motion at higher velocity gradients and larger particle size.

Chemicals used for

coagulation and   flocculation. 

Rand Water uses hydrated lime for coagulation and flocculation, and activated sodium silicate and ferric chloride as an aid to flocculation.  The average doses rates for:

·  Slaked lime vary between 55 and 70 mg/l as calcium oxide

·  Silica vary between 1 and 3 mg/l as silicon dioxide and

·  Ferric chlorides vary between 1 and 5 mg/l as ferric chloride.

 Low energy conditions are required for optimum coagulation.

  ·  A .G value of 600 per second with a Camp number (Gt value) of 18 000 is ideal in Rand Water’s system.

·  Lime is not added more than 60 seconds before the point of maximum energy dissipation.

·  Activated sodium silicate is added 15 seconds before the lime.  It allows a reduction of up to 50% in lime dosage for proper coagulation and flocculation.

·   Ferric chloride is used as a secondary flocculent to aid filtration.

 The high pH of between 10,5 and 11,0 which is obtained during the lime coagulation limits algal growth and is very effective in removing heavy metals, organic materials, bacteria and viruses.