Mixing and Solids Contact Processes

The proper measure of flocculation effectiveness is the performance of subsequent solids separation units in terms of both effluent quality and operating requirements, such as filter backwash frequency. Effluent quality depends greatly on the reduction of residual primary size particles during flocculation, while operating requirements relate more to the floe volume applied to separation units. 

Polymers which already have a long chain structure may provide a good floe at low mixing rates. Often the turbulence and detention in the clarifier inlet distribution is adequate. 

Equipment typically consists of concentric circular compartments for mixing, flocculation and settling. Velocity gradients in the mixing and flocculation compartments are developed by turbine pumping within the unit and by velocity dissipation at baffles. For ideal flexibihty it is desirable to independently control the intensity of mixing and sludge scraper drive speed in the different compartments. 

Operation of slurry-recirculation solids contact units is typically controlled by maintaining steady levels of solids in the reaction zone. Design features of solids contact clarifiers should include  

Rapid and complete mixing of chemicals, feedwater and slurry solids must be provided. This should be comparable to conventional flash mixing capability and should provide for variable control, usually by adjustment of recirculator speed. 

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