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Mar
2013
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Overview of the Water Process

The Water Process

12
Oct
2007
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Abstraction of raw water from Vaal River

Due to extensive utilization of water from the Vaal River upstream of Midvaal’s abstraction point, water in this stretch of river is typically high in dissolved solids and contains high amounts of dissolved organic carbon and plant nutrients like phosphate and nitrate. This indicates that the water is “hard” or mineralized (due to the levels of salts like calcium, magnesium, sodium, chloride and sulphate) and contains numerous types of algae and other nuisance water plants like water hyacinths and water grass (due to the high levels of phosphates and nitrates).

The quality of the river water varies seasonally, as rainfall impacts on the turbidity, mineral levels and on the nutrient levels.

Raw water is abstracted from the Vaal River by means of large pumps, situated in the intake tower, and pumped through pipelines to the treatment plant for purification.

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Oct
2007
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Chemical Addition

The type of chemicals added to the water in the treatment process has to comply with nationally approved standards for water purification for human consumption. The quality of raw water at the time of abstraction determines the amount of chemicals to be added to treat the water to the required standard. At Midvaal, the following chemicals are used.

Type of Chemical Example Purpose
Flocculants** Ferric chloride, aluminum sulphate Coagulates suspended particles into floes
Flocculant aid Anionic and cationic polymers Assists with removal of very fine particles and are used mainly during rainy seasons when high turbidity water is experienced.
Stabilization/pH correction Hydrated lime To ensure that the treated water is neither scale forming nor corrosive
Oxidation Ozone gas Removes algae, dissolved metals like iron and manganese, removes color, tastes and odors
Disinfection Chlorine gas Kills harmful bacteria and other micro-organisms

** Flocculation involves the stirring of water to which a coagulant has been added at a slow rate, causing the individual particles to “collide” with each other and with the floes formed by the coagulant. The destabilized individual colloidal particles are agglomerated and incorporated into the larger floc particles

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Oct
2007
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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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Oct
2007
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Ozone

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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