Effect of mixing intensity on polymer conditioning of sludges

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Virginia Tech


Conditioning studies were conducted on several water and wastewater sludges using polymers to determine the effect of high-intensity mixing on floc formation and breakup. Mechanisms acting in particle coagulation were also investigated. A rapid mixing apparatus was used in conditioning the sludge to simulate the high velocity gradients encountered in most dewatering equipment. Anionic and cationic polymers of varying molecular weights and an inorganic coagulant were chosen as the conditioning agents. Dewatering rates were measured by the capillary suction time (CST) apparatus. The effect of rapid mixing on the polymer efficiency was evaluated by premixing the polymer, using the same mixing apparatus, prior to applying it to condition the sludge.

Data analyses revealed the great impact the primary floc breakup has on the sludge dewatering rate, although this was shown to be dependent on the nature of the sludge particles. Some minor changes were imparted to the polymer chains by intense mixing but there was no evidence that particle polymer bonds were fractured. The mechanisms of coagulation were observed to be related to sludge type. Overall, successful conditioning of sludges by polymers requires a proper determination of the sludge characteristics, a correct choice of polymer type and dose, and a knowledge of the mixing intensities.