Study On Response Characteristics Of Sludge Aggregates To Shear Conditions And Coping Strategy Of Flocculation Dosing

The additional shears created in pipes, pumps and other dewatering devices will lead to sludge aggregates breakage. In this study, shear tests were conducted to obtain the variations of capillary suction time (CST), floc size, fractal dimension, zeta potential and suspending solid(SS) in supernatant of raw and conditioned sludge. The parameters of shear sensibility, strength factor and the relationships between floc size or strength and average velocity gradient (G) were determined. The process of aggregates breakage and re-growth and main influencing factors of re-growth capability were investigated. After considering the reaction of overdose conditioned sludge aggregates to shear, a comprehensive conditioning dosing scheme can be proposed.The results showed that shear could deteriorate the dewaterability of both raw and conditioned sludge, the fractal dimensions of raw sludge roughly kept constant and its floc sizes decreased slightly. For conditioned sludge aggregates, the increase of shear speed led to the rising of the floc fractal dimensions and the decrease of the floc size. The stair-stepping change occurs after multi-pulse shear in floc sizes of conditioned sludge aggregates, high speed play a major role in this process. Shear sensitivity (Kss) of raw and conditioned activated sludge (AS) are4.73×10-2、8.33×10-2, as for raw and conditioned anaerobic digested sludge (ADS), the Kss are3.38×10-2and12.94×10-2. Furthermore, the raw sludge has higher strength factor values, which all showed greater shear stability on the raw sludge. Additional shear also cause the exposure of more fresh surface with negative charge. Surface erosion was the main mechanism for the raw sludge, while the fragmentation mechanism became more and more obvious for the conditioned sludge beyond the critical shear strength. The more additional flocculant dosage, the better recover ability of broken aggregates. With2.5g/kg flocculant dosage, re-growth factor of ADS aggregates was132%for lmin continuous shear mode,110%for5min continuous shear while87%for multi-pluse shear mode. Generally, new formed aggregates have looser internal structures. Additional flocculant dosing points make a great difference in the re-growth process, if added after the start of breakage, the new formed aggregates were broken again by the following shear, but added in the end of breakage period can make the broken aggregates complete re-growth. Overdosed conditioning contributed to decrease the effect on dewateribility of sludge aggregates. In conclusion, both of overdosed condition and additional dosage method can solve the problem of shear breakage. To compare the two methods, former one is more convenient to operate, but it cannot eliminate the effect on floc size, while latter one is more effective but requires of operation and facility relatively complex.