SHIELA TERRY / SCIENCE PHOTO LIBRARY SHIELA TERRY / SCIENCE PHOTO LIBRARY
Final sedimentation tank at a sewage treatment works, with the pumping station in the background. Incoming raw sewage is first pumped to a comminuter which shreds the larger solid material. It then flows into the primary sedimentation tanks, where 85% of the solids are removed, & then into the biological treatment units, where micro- organisms break down and remove fine organic material & dissolved pollutants. The sewage then passes to the final sedimentation tanks (humus tanks) where excess biomass is settled out as sludge and is swept into a central hopper by the rotating arm. The effluent is then discharged into a nearby stream.
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