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Oct
2007
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Sedimentation

At Midvaal, a secondary chemicals-addition step is sometimes necessary, depending on the prevailing water quality. Relative small doses of flocculants and/or flocculant aid are applied after ozonation to coagulate the oxidised particulate matter into flocs. These flocs then have to be separated from the water, in specially designed sedimentation units. Three different types of sedimentation units are available at Midvaal, namely circular clari-flocculators, a horizontal-flow dam and a pulsator. Sedimentation is the process in which the flocs (formed during coagulation and flocculation) are allowed to settle from the water.

Flocs collect as sludge at the bottom of the sedimentation tanks from where it must be removed regularly. The clean water leaves the sedimentation tanks through collection troughs located at the top of the tanks. Up to here in the treatment process, the bulk of the particulate matter had been separated and removed from the water. Tiny particles and microorganisms may however still be present in the treated water. The water therefore has to proceed to the final process, namely filtration.

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Oct
2007
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Filtration

Sand filtration is a simple process in which the water is allowed to filter through a layer of sand in a specially constructed tank. In the filtration process, the small remaining floc particles are removed by the sand grains and are retained in the bed of sand, while clean water flows out at the bottom of the sand bed. Filters act as sieves and form the final barrier for any particles, whether organic or inorganic. At Midvaal, silica sand is used as filtration medium and the filters are of the rapid gravity type. Two blocks of filters are available at Midvaal.

North / South Filters East / West Filters
Backwash rate ±20-27 m/h 14-16 m/h
Filtration rate ± 4.2 m/ h ± 4.2 – 7m/h
Bed expansion 0 – 22% 10%
Operating time 24-48 hours, depending H²O quality 24-48 hours, depending H²O quality
Media depth 700 – 900 mm (sand and gravel) 500 mm sand
Effective size 600 mm (sand)
Uniformity coefficient
Filtration area 65 m2 ± 46 m2
Floor system False floor False floor
Level Control Actuator on outlet of filters and inlet control valve
Nozzle density 30 nozzles/m2 55 nozzles/m2
Nozzle slot sizes 0.3 mm 0.1 mm

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Oct
2007
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Disinfection

The physical processes of the treatment plant remove a large percentage of bacteria and other microorganisms, especially by sand filtration. However, many bacteria and viruses still remain in water – even at low turbidity levels. It is therefore essential to disinfect water to prevent water-borne diseases from spreading by pathogens (disease-causing micro-organisms) in water.

Disinfection of water entails adding the required amount of a chemical agent (disinfectant) to the water and allowing contact between the disinfectant and particles for a pre-determined period of time under specified conditions of pH and temperature. Other methods of disinfection include boiling water or irradiation with ultra-violet light.

Midvaal applies chlorine for disinfection purposes. Chlorine is a strong oxidising agent and it reacts and oxidises some of the essential metabolic systems of microorganisms, thereby inactivating or destroying them.

Chlorine gas is purchased from reputable suppliers in South Africa, and contained in steel drums of roughly 925kg capacity. It is extracted from these pressurised drums with special chlorination equipment under regulated conditions and dosed into the treated product as the final treatment step. It is imperative that the chlorination step in uninterrupted and that sufficient chlorine is dosed at all times for adequate disinfection of potable water

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Oct
2007
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Water Recovery

All waste streams, generated during the various treatment steps, are accumulated in a balancing dam, from where the waste water is pumped to a sludge treatment process. These waste streams include accumulated froth and sludge from the DAF plant, sludge from the sedimentation dams and filter backwash water. Ferric chloride and polymer are added to the waste water to coagulate solids into flocs, which are allowed to settle out in the dedicated sedimentation unit.

The effluent of the sludge plant is treated with chlorine for disinfection purposes and reclaimed. It is recycled to the inlet of the works to pass through the pre-ozonation process first, before being incorporated with the raw water from the river. Through this process, almost 95% of wastewater is reclaimed at a cost much less expensive than the purchase price of river water.

Type High rate thickener
Polymer Anionic
Maximum flow ±751/s
Thickened Sludge concentration ± 5 – 6% m/v (drying lagoons)
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Oct
2007
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Storage and Distribution

The treated water is stored in sealed storage dams and distributed to almost 500 000 consumers in the Matlosana (Klerksdorp, Orkney, Stilfontein) area on a daily basis. Supply contracts are agreed with all clients, who are expected to submit consumption figures annually in July, for the next ten-year period. Midvaal uses these consumption figures to plan capital and major maintenance projects to ensure available capacity and ability to supply water in accordance with clients’ needs

Water is distributed to the following organisations/institutions in the ratio of about 60% to municipal consumers, and 40% to mining companies and other industries.

  • AFRICAN EXPLOSIVES LTD
  • ANGLOGOLD ASHANTI LTD
  • CANDERO MINING & CONSULTANTS
  • CHEMWES LTD
  • KOPANO
  • DUFF SCOTT HOSPITAL
  • ESCOM
  • CITY COUNCIL OF MATLOSANA (KLERKSDORP, ORKNEY, STILFONTEIN, KHUMA)
  • O M V CRUSHERS
  • PINNACLE PRIMARY SCHOOL
  • SIMMER & JACK (BUFFELSFONTEIN GOLD MINE, HARTIES GOLD MINE, CMTC)
  • A. P. C. TERBLANCHE
  • R. J. BARKLEY
  • J. J. ROSSOUW
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