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Stage 3: the rapid gravity sand filter: water treatment

May 30, 2010

view of tanks and walkway at the Branxholme water treatment station

Next on from the upward flow clarifer, Martin, the plant manager at the Branxholme water treatment station, shows me the filtration process. A rapid gravity sand filter is used, common in many water treatment plants. Alternatively a slow sand filter can be used, which makes excellent water but it’s downfall is in the name; it’s slow and labour intensive.

After the clarifer, the flow of water is separated into four areas with rapid gravity sand filters. That’s upped to a maximum of 8 in summer  because demand for water is higher.

“See that ledge,” Martin says, pointing towards a faint line beneath the water. “That’s the top of the filter. It builds up a layer of dirt that assists in the filtration because it’s finer than sand (in the filter)”.

Each filter is designed so 30 litres of water a second goes through the filter, although flow is dictated by water consumption in town. Every 48 hours there is a controlled backwash lasting about an hour, which aerates and loosens and rids the filters of all the sediment buildup.

We’re finished the outside walkway tour and head back inside. Just inside the door is a marvelous control panel (didn’t get a photo, sadly) which looks like it is from a 1960s science fiction space control station. Opps, it turns out it was designed in the 80s. Still, I loved the big buttons and diagrams. It’s function? Controlling much of the automation of the plant.

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