Enhancement of alum sludge dewatering capacity by using gypsum as skeleton builder

A previous study demonstrated that red gypsum could be used as skeleton builder in combination with a polymer for conditioning an alum sludge. Sludge filterability in terms of net sludge solids yield (YN) was improved under the mechanism of interaction between polymer and gypsum. In this study, the dewatering behaviour of an alum sludge conditioned dually with a polymer and red gypsum was investigated using a Lab-scale air pressure plate. Groups of experiments were designed to assess the influence of gypsum on the significance of the extent of dewatering. Experimental data showed that the involvement of the gypsum could enhance the dewatering extent by further decreasing equilibrium moisture content (MC) in 7 percentages comparing with the situation of the absence of gypsum. By examining the controlling factors, it is noted that the gypsum associated with polymer can form the more compact sludge floc (evaluated in terms of fractal dimension, DF). This may help to build up a more porous, permeable and rigid lattice structure, which retains solid particles and allows the water to be transmitted. The enhancement of dewatering extent is mainly contributed to the beneficial structure. Interaction of polymer and gypsum is a key factor in the improved dewatering behaviour since gypsum by itself can hardly benefit the dewatering process in the case examined in this study.