Floe separation by settling-sedimentation

This process is hardly ever used any more but should still be cited. Because floe densification is necessary, lime or CaC03 mg I ) were used and scraped 

Internal or external recycling of sludge to enhance flocculation is not necessary as long as the floe produced has at least 150 to 200 mg l of SS. The flocculator can be incorporated in the center of the settler. The sludge produced is rich in CaC03 with 40 to 60 g-I DS and this may correspond to 0.25 to 0.45% of the volume of treated water. 

These fines air bubbles are produced by recycling a fraction of the water clarified by flotation (20 to 50%) at a pressure of 4 to 6 bar. Compressed air is dissolved in the water to saturation (in what is called a pressurizing drum), then the water is expanded in contact with the flocculated raw water and at flotation cell backpressure (0.2 to 0,4 bar). Expansion and mixing with flocculated water should be performed with care. The delicate floe must in no case be sheared. This is why the design of potable water flotation cells (where the use of organic flocculants is sometimes prohibited, whereas the floe is labile), is very different from the design of WW flotation cells. 

The flotation cell is cylindrical or rectangular. It is equipped with a device to scrape off surface scum, or more commonly to scrape bottom sludge (Fig. 29). 

The flocculated water goes in the lower central part and comes out under a peripheral siphoid wall. A fraction (20 to 50%) of the floated water is sent back to a pressurizing system that includes  

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