12
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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