Sedimentation is the term applied to the separation of suspended particles that are heavier than water, by gravitational settling. In the case of the proposed biological wastewater treatment, it is used for the removal of the activated – sludge flocs, and it is carried our in a settling basin, called secondary clarifier. The main factors affecting the sedimentation process are the treated flow, the area and the particle setting velocity. The area (and thus the diameter) are obtained using the flow and the recommended settling velocity, which for the biological treatment processes is about 0.33 m/s. A scraping bridge is installed in the secondary clarifier tank, which has a girder bridge that acts also as a flying bridge or catwalk; the top scraper removes the floating material and foam, while the bottom scraper sends the settled sludge to the center where it is pumped back to the process. Most of the removed sludge is sent back to the process, in order to keep a constant concentration of activated sludge in the biological oxidation or aeration tank, so that the process efficiency is also constant. The activated sludge return is a key part of the process, and this is done through the activated sludge return pumps. The excess sludge that is produced is sent to the thickening tank.
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