DAF flocculated particles optimum size

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Hello

In SWRO pretreatment consisting of DAF as a stage, proper size of flocs preceding this stage has been always challenging. On the one hand, coagulants  and flocculants appropriate dosing rate is usually defined using jar test in order that flocs growth is executed to the fullest.

On the other hand, there should be proper relation between the size of particles created in flocculation and the size of air bubbles formed in DAF unit in order to float suspended solids on the surface. In other words, if particles are too large, there might be some difficulties regarding flotation of them using tiny air bubbles. I was wondering if any member of this network has any idea about the issue has had related experiences? 

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1 Answer

  1. Bottom line if you flocculate particles too much and they become to large, then you open yourself up for shearing the big particle and leaving straggler floc carryover.  Particle size should be almost homogeneous if you are flocculating correctly.  You gain experience by watching this in the mixing areas and observe the laminar areas to see the clarity of water from the immediate settling.  The water pool above the particles showing clarity and all the floc particles look about half the size of an old pencil eraser.  An art for sure.  If the water is clean enough you can explore an instrument measuring zeta potential and set a sweet point that the coagulant adjusts while the flocculant normally stays constant.  Once you have 2 variables, it becomes more complicated.  If you are tired of going out and checking on the mix zone for proper flocculation, we install led lights and a camera to remotely dial in the chemistry.  Works well when you can’t be live to “feel” it.  Sean