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Dissolved Air Flotation (DAF)

Flotation is an effective process to remove relatively light types of particles e.g. organic material and algae. This process involves the formation of small air bubbles that are passed through flocculated water, which had been treated with chemicals. The bubbles attach to the light-weighted flocs causing them to rise to the surface where they are collected as froth and removed from the top of the flotation unit. Heavier particles will settle as sludge and is normally decanted once a day. The froth and sludge are treated in a separate process where the water is reclaimed and the concentrated sludge discharged to drying lagoons. DAF is an effective process which produces effluent with reduced suspended solids and reduced amounts of algae.

Design specifications of the DAF Plant

Design Modular, 5 x 50 Ml / day
250 Ml / day 250 Ml / day
Retention time ± 1 hour
Recycle stream 7 – 10 % v/v
Bubble size 0,5 mm
Pressure vessels 500 kpa
Sludge concentration 1.5 – 2 %
Flocculation Serpentine channels; adjustable outlets
Air : water 1:1
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Ozone (O3) is an enriched form of oxygen (O2). It is a powerful oxidant and can oxidise organic matter (e.g. algae, humic acids, etc.) as well as dissolved metals (e.g. manganese and iron). Because it is so powerful, ozone is a very good disinfectant and can kill harmful microorganisms like bacteria and viruses.

Ozone is generated on-site at Midvaal by using specialised ozone generation equipment and natural air. In nature, air contains about 18% oxygen. In the ozone generation process, this oxygen is concentrated through a method called Pressure Swing Adsorption (PSA), and then passed over an electrical discharge to form an ozone concentration of 9%.

The ozone in the form of gas is added to the water in specially designed contact reactors, where the ozone dissolutes (dissolves in the water) and reacts with the targeted constituents. At Midvaal, ozone is added at two stages of the process:

Stage 1

Pre-ozonation, where ozone is added at a concentration of maximum 1.5 mg/l to deal with algae in the raw water prior to any other treatment process.

Stage 2

Intermediate ozonation is performed to deal with the more complex constituents, at concentrations ranging between 1.5 and 2.5 mg/l. Intermediate ozonation is applied after most of the algae and suspended particles have been removed through the DAF process.

O3 concentration 9% v/v O3 in air stream
Pre-ozone reactor Radial flow type, contact time 2 minutesTransfer efficiency: approximately 92%
Intermediate ozone reactor Deep U-Tube, contact time 4 minutesTransfer efficiency 98%
U-tube 15 m deep
Control Frequency control; voltage control (±60 Hz)
Generators 3 x 8 kg/h each
Dew point Minimum – 80°C
Oxygen supply PSA method
Energy consumption ± 40 kWh / kg generated
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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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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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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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