Activity Books Understanding Water Activity Book | страница 64

3
Wastewater flows through the sewer pipes to reach
the treatment plant.
There are
steps to cleaning wastewater:
1 primary treatment 2 secondary treatment and
3 advanced treatment.
1 The first step uses bar screens to filter out large objects like
sticks and rocks. Water flows easily through the bar screens but
big stuff gets stuck and can be removed. Everything that passes
through the bar screens flows into a tank. In this tank, dirt and
particles settle to the bottom. This is called sedimentation.
Solids settle to the bottom of the tank. The solids that collect are
pumped from the bottom of the sedimentation tank into another
settling tank.
2 The second step uses bacteria to break down wastes.
(Bacteria are funny little critters. They can be good or bad.
They are doing good things here.) The wastewater flows
through tanks where air is added and the waste is stirred to help
the bacteria grow. The bacteria and solids settle to the bottom in
the secondary sedimentation tank.
3 The last step is the advanced treatment process. Water flows
through sand and gravel. This is called filtering. The filtered
water is disinfected. The most common disinfectants used are
chlorine, ultraviolet light or ozone.
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