ARMS & SCRAPERS

Traditional design can be characterized by:
  • small number of arms
  • slow revolutions
  • too shallow scrapers (and too shallow clarifiers)
  • inappropriate shape of scrapers

Advanced design combines full length (radius) and partial length arms to provide sufficient energy to overcome tixotropic properties of activated sludge and sufficient transport of sludge to minimize the difference between the bottom and the recycled TSS and thus sludge blanket depth. Two-speed or variable speed arms are used as well.

Some principles of new design are shown in the figure.

Mean interval of rotation at peripheral velocity 0.04 m/s
  1. Straight plow scrapers were popular but not effective for any type of solids.
  2. Curved plow scrapers are convenient at many situations. Fixing them to the arm is relatively complicated and thus spiral scrapers took over.
  3. Variable angle spiral scrapers are popular in Europe. They are easy to assemble and less expensive than previous types.
  4. Logarithmic spiral scrapers are increasingly popular. Angle of attack is typically 30-40º.
Industrial clarifier for periodically heavy, periodically light particles, with peripheral drive, two full and two short arms, equipped with logarithmic spiral. Single weir effluent launder mounted on the inner side of the wall; Stamford baffle. Weir loading rate was not considered to be a valid parameter for design. NHscrapers.gif

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