08-01-2013, 12:49 PM
Mixing and Flocculation
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Mixing …?
Mixing is an important unit operation in many phases of wastewater treatment including:
(1) mixing of one substance completely with another,
(2) blending of miscible liquids,
(3) flocculation of wastewater particles,
(4) continuous mixing of liquid suspensions,
(5) heat transfer.
Continuous Rapid Mixing in Wastewater Treatment
Continuous rapid mixing is used, most often, where one substance is to be mixed with another. The principal applications of continuous rapid mixing are in
(1) the blending of chemicals with wastewater (e.g., the addition of alum or iron salts prior to flocculation and settling or for dispersing chlorine and hypochlorite into wastewater for disinfection),
(2) the blending of miscible liquids,
(3) the addition of chemicals to sludge and biosolids to improve their dewatering characteristics
Flocculation in Wastewater TreatmentObjective..?
The purpose of wastewater flocculation is to form aggregates or flocs from finely divided particles and from chemically destabilized particles.
Flocculation is a transport step that brings about the collisions between the destabilized particles needed to form larger particles that can be removed readily by settling or filtration
Maintaining Material in Suspension
In biological treatment systems the mixing device is also used to provide the oxygen needed for the process. Thus, the aeration equipment must be able to provide the oxygen needed for the process and must be able to deliver the energy needed to maintain mixed conditions within the reactor.
To homogenize the contents of the digester to accelerate the biological conversion process, and to distribute uniformly the heat generated from biological conversion reactions.
Energy dissipation in Mixing and Flocculation
Mixing intensity and residence time determine whether the stated goals will be met.
To determine mixing intensity define as the average shear intensity (mean velocity gradient) in the rapid-mix tank.
The Camp-Stein equation is often used to compute mixing intensity, however it is an equation which is based on laminar flow -- a case seldom found in rapid-mix or flocculation basins so it’s an “average” approximation.