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 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.
Sedimentation is the oldest known method of water purification. It has been employed extensively for thousands of years. Although it is a natural phenomenon, it is aided by the addition of chemical coagulants to produce flocs which are allowed to settle in specially designed tanks, also engineered to remove sludge.
Rand Water uses horizontal flow tanks with retention times of 4 hours and produces water with a turbidity of 5 NTU at the outlet weirs which is considered acceptable for filtration. Depending on the turbidity of the incoming raw water, between 95% and 97% of the suspended particles are removed during sedimentation.
Between 500 and 1300 tons of dry sludge are produced each day during the purification process. This is removed from the sedimentation tanks at Zuikerbosch and Vereeniging in thin slurry containing 3% mass by volume of dry sludge.
The sludge is pumped to Rand Water’s sludge disposal site at Panfontein. Here it is dosed with an organic flocculent in gravity thickening plants to aid the separation of the solids from the liquid. The thickened sludge is pumped onto drying beds where it is dried by evaporation and the clear supernatant fluid is drawn off and returned to the purification system.
The sludge consists mainly of calcium carbonate, magnesium hydroxide and complex silicates containing aluminium and iron.
Phase separation of the sludge takes place in a thickening plant. The separation is enhanced through adding between 0.3 and 0.8 kg of polyacrylaride per ton of sludge and the thickened sludge is then spread in 200 mm layers over an area of 200 hectares and sun dried.
The use of lime as a coagulant raises the pH of the water to about 10,5 which is very unstable and conducive to scale forming. After sedimentation, the water flows into carbonation bays where it is stabilized by bubbling carbon dioxide gas, obtained from the lime-burning kilns, through the water. This reduces the pH to between 8.0 and 8.4 which is necessary to produce chemically stable water that will not cause excessive scaling or corrosion.
Following carbonation, the water passes into the filter houses where it flows through rapid gravity sand filter beds of finely graded silica sand and pebbles. The remaining suspended particles are removed at this stage.
The latest filters constructed have a fine sand layer, 600 mm thick, supported on a 500 mm gravel layer. They are washed by using air to first loosen the sand and then water, at an up wash rate of 32 m/h, to wash away all the collected dirt. Filters are covered to exclude light to less than 25 lux to prevent algal growth on the filters. After filtration, the water has a residual turbidity of 0,5 NTU or less. The filter backwash water is treated to remove organics, heavy metals, bacteria and other contaminants which concentrate in this water before being recycled back to the inlet of the filters.
The water leaving the purification works is disinfected with chlorine to kill micro-organisms, bacteria and any viruses that may be present in the water. The required concentration of chlorine is determined so that the number of colony forming micro-organisms per milliliter, as determined by the standard plate count technique after 48 hours incubation at 37°C, is less than 10 after 20 minutes contact with chlorine.
The chlorine dosage must be between 1,5 and 4,0 mg/l depending on the raw water quality to ensure there is minimal re-growth of any micro-organisms during the 6 to 8 hours that the water travels to the booster pumping stations. The free residual chlorine at these dosages will vary between 1,0 and 2,5 mg/l after 20 minutes contact time. There are no chlorine contact chambers and the mixing takes place in the pipelines.
Secondary disinfection by chloramination
Chlorine, although an excellent disinfectant, does not remain active for much longer than 6 to 8 hours. Disinfection needs to be repeated but this time with a less powerful agent that will remain active for long periods so that the water may be protected right up to the end consumer. This is achieved by dosing chlorine and ammonia at the booster pumping station in the correct mass ratio of not less than 4:1 and forming the monochloramine in situ. The monochloramine, although less active than chlorine, then protects the water against bacterial regret for periods of up to 8 days.