We had an opportunity to visit the waste water treatment facility in Viikki, Helsinki.
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Waste water treatment process starts with sand and big particles removal 
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Some examples of the stuff people have flushed down the sewers
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Good example of the "not-belonging" stuff is butt-plugs...
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Mechanical grids are picking up unwanted things from the waste water
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Primary sedimentation and thickening
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All of the process is done underground
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Oxygen is added to the process so the bacteria can eat the waste
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Final cleaning is done with bacteria before the cleaned water is pumped to sea