The Story Behind Tap Water
Rand Water is a water services provider that purifies 3600 megalitres of water per day and distributes clean drinking water, free of harmful microorganisms and chemical contaminants, to 58 reservoirs, via 3000 km of pipelines, to 19 water services authorities. It services 11 million people in Gauteng and parts of Mpumalanga, North West and Free State Provinces.
The quality of drinking water supplied by Rand Water to water services authorities in Gauteng and parts of adjoining provinces is monitored during the purification process and distribution system on a regular basis. The water complies with the requirements of the South African National Standard for Drinking Water (SANS 241) and the World Health Organisation (WHO) guidelines. It is safe to drink water straight from the tap. Let’s explore the story behind tap water.
Rand Water draws raw water from the Vaal Dam via a canal and a gravity pipeline, and by pumping from the Vaal River Barrage Reservoir at the Lethabo weir. It is then purified to ensure that the final product meets the stringent standards set for drinking water. The conventional treatment process removes the suspended material and disinfects the water prior to pumping to the local authorities, the mining industry and other large industrial concerns. The purification process, at each stage is accompanied by changes in the physical and chemical composition of the water. These changes are constantly monitored and corrective action is taken to ensure that the water quality will conform to the prescribed limits.
At the purification station water passes through metal screens to trap large living organisms, sticks, leaves and litter, but allows the rest of the water to pass through it.
Coagulation & 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 very long period. Settling of these suspended particles is aided by the adding of chemical coagulants, such as calcium hydroxide (slaked lime). Calcium hydroxide is produced when raw limestone (calcium carbonate) is heated in a kiln to convert it to calcium oxide and carbon dioxide gas. The burnt limestone is then crushed and water is added, in slakers, to destabilise the electrostatic charges of suspended particles in the water. A small quantity of activated sodium silica is also added to the raw water to promote flocculation. When required, ferric chloride or organic coagulants may be used as well.
Coagulation in Rand Water’s system is the process which destabilises the particles when the coagulants (slaked lime and sodium silica) are mixed with the raw water in about 20 to 30 seconds. This is the first process in removing suspended particles.
Flocculation is the clumping together of the suspended particles, which were destabilised by coagulation, to form heavier visible particles called floc. The floc remains in suspension as the water flows at high velocity through either spiral flocculators or baffled channel conditioning bays. In Rand Water’s system, orthokinetic flocculation predominates resulting from the fluid motion at higher velocity gradients and larger particle sizes.
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 material, bacteria and viruses.
Sedimentation is the oldest known method of water purification and has been employed extensively for thousands of years. The water flows slowly into large sedimentation tanks. The floc then settles to the bottom of the tank to form sludge.
Between 500 and 800 tons of dry slude is produced each day during the purification process. This is removed from the sedimentation tanks as thin slurry containing 3% mass by volume of dry sludge. The sludge is pumped to Rand Water’s sludge disposal site at Panfontein. Here is it dosed with an organic flocculent in gravity thickening plants to aid the separation of the solids from the liquid. The clear supernatant fluid is drawn off and returned to the purification system. Thickened sludge is pumped onto drying beds where it is dried by evaporation.
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 stabilised by adding pure carbon dioxide gas to the water.
This lowers the pH to between 8,0 and 8.4.
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.
Ferric chloride may be added prior to filtration to enhance the process.
The water leaving the purification plant is disinfected with chlorine to kill micro-organisms, bacteria and viruses that may be present in the water.
The free available chlorine concentration must be between 0,8 and 2,5 mg/l depending on the raw water quality to ensure that all pathogens are killed.
This clean water is pumped through underground pipes to booster pumping stations. Although free available chlorine is an excellent disinfectant it does not remain active for much longer than 6 to 8 hours.
At the booster pumping stations monochloramine is produced in situ by dosing chlorine and ammonia at a mass ratio of not less than 4:1. This is a less powerful disinfectant but has the capacity to prevent microbial growth in the water for up to 8 hours to 12 days. From the booster pumping stations the water is pumped into reservoirs. Rand Water then sells it to various local authorities that supply homes, schools, businesses and factories with clean water.
Chemicals used for Coagulation and Flocculation
Rand Water uses hydrated lime for coagulation and flocculation, and activated sodium solicate and ferric chloride as an aid to flocculation. The average dosage rates vary:
• Slaked lime: between 55 and 70 mg/l as calcium oxide;
• Silica: between 1 and 3 mg/l as silicon dioxide; and
• Ferric Chloride: 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
• Lime is not added more than 60 seconds before the point of maximum energy
• Activated sodium silicate is added 15 seconds before the lime. Its use 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.
See also:How is Tap Water Cleaned in Flip Book Format
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