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Stage 2: Lime dosing, flocculation and filtration

May 28, 2010

Moving right along with my visit to the water treatment station (it’s been a hell week finishing a big assignment for another paper):

After the water is sucked into the pumps from the Oreti River it gets a dousing of lime, which is to increase pH to neutralise the natural acidity of water and stop corrosion of the pipes (the city pipes are made of a variety of substances depending when they were built, including polythene, asbestos mixed with cement -scarily, concrete-lined steel and cast iron in the older parts). The lime dissolves in a few minutes.

We walk up some spindly metal stairs and along platforms to get an overview of the filtration process, below my feet water is flowing along channels where it gets separated into various tanks.

Aluminium sulphate is added as the primary coagulant – that’s to clump all the solids, such as clay and some pathogens, together. The lime helps too. The process, called flocculation starts to get violent here: The water is swishing fast and as it moves along the tank system it gradually slows with the clumps settling out.

More channels for the water to go through (I’m getting slightly confused with all the tanks and channels on either side of the walkway trying to work out what process is happening for each part, while taking notes too). Then pipes distribute the water into eight tanks with an upward flow so decaying leaves and matter floats off the top. It’s called an upward flow clarifier; what a great name. In the middle of the tank is a round suspended cone, sort of like a giant coat hanger made into a round circle. What then happens is the sludge -small particles whose settlement velocity is close or less than the rate of upward flowrate – gets trapped and collects on the cone. Once the cone’s weight gets to a certain level over the space of several hours, the water is automatically flushed away getting rid of the sludge.

That’s enough for now. Let me know: too much detail? Thanks to Martin the plant manager for explaining the process to me.

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