Biosludge is comprised primarily of bacterial cells, which are very small particles, and drainage of water through this matrix occurs at a very slow rate. In order to improve the drainage rate, sludge is normally conditioned with metal salts and/or organic polymer electrolytes. The polymers are high molecular weight materials that consist of a long strand with multiple charged sites along the length of the molecule. The bacteria in the sludge have the opposite charge and are attracted to, and become attached to the charged sites on the polymer. The net result is that the small particles are agglomerated into larger ones and the drainage characteristics of the sludge/water matrix are much improved.
Historically, contact between the polymer and sludge generally relied on creating turbulence through some kind of restriction in the pipe, or static mixer. This resulted in a significant head loss for the system, and the effectiveness of mixing became dependent on the flow rate.
The dynamic in-line mixing pioneered by the Optifloc™ technology is dramatically more effective. The Optifloc™ mixers are variable speed mixers, which can generate high-shear mixing with a minimum head loss. The improved performance generally demonstrated by the Optifloc™ mixers is attributed to the fact that the high-energy mixing results in utilizing a higher proportion of the active sites on the polymer.
Why it Works
The key to effective deployment of the Optifloc™ mixers is recognizing that modern dewatering practices benefit from high-energy mixing. The more robust organic dewatering polymers that have evolved to withstand the forces in dewatering centrifuges lend themselves to more energetic mixing for short periods
The Optifloc™ technology is the first system designed specifically to provide a high-shear mixing zone for contacting polymer with sludge. The high-intensity mixing zone generated by the rotating impeller provides almost instant dispersion of the polymer solution within the sludge matrix. This results in a fundamentally more efficient use of the active sites on each polymer molecule, significantly reducing polymer demand while producing smaller, more compact floc. These small, well-conditioned flocs
- are more resistant to break-up by subsequent shear in the dewatering system
- have better water release characteristics than floc formed conventionally
- result in higher solids in the final cake and fewer solids in the filtrate/centrate.
Extensive testing shows that mixing energies (rpm x time) are not born equal. Short bursts of high energy outperform longer mixing times at lower energy. It should be noted, however, that there is an optimum mixing energy, beyond which damage to the polymer occurs. The impeller speed of the Optifloc™ mixers is variable and can be optimized for each individual sludge/polymer combination. The net effect of optimised mixing can be a substantial reduction in polymer requirements and improved dewatering performance.
