A Study For Foaming Ability Of Dewatered Sludge And Drying Characteristics Of Foamed Sludge

As the dewatered sludge is highly viscous and sticky, sludge drying is one of themost challenging technical tasks in the field of wastewater engineering. In this study,the foaming drying was employed for sludge drying. Meanwhile, the foamingmechanism of dewatered sludge and drying characteristics of foamed sludge werefocused, which could provide a new efficient method for sludge drying.The effect of CaO addition on the foaming performance of dewatered sludge wasstudied in this research. Results showed that the CaO addition at2.0wt%relative tothe total weight of dewatered sludge in wet basis presented the maximum foamingefficiency, which was also supported by the maximum values of protein,polysaccharide, SCOD, and the minimum value of surface tension. Thus, the reasonfor foaming sludge with CaO addition may be that the structure of sludge was beendestroyed with the increasing of pH, then the surface tension of sludge was reducedbecause of lots of bio-surfactants dissolved in sludge. At last, the foamed sludge couldformed with appropriate mechanical whipping.The foams in dewatered sludge were highly stable. The total number of bubblesof sludge with different densities had big differences. However, the bubbles sizedistributions were insignificantly different amongst the samples with higher and lowerdensities. The diameters of bubbles concentrated on the range of0.1mm-0.3mm. Thehomogenous bubble size distributions in a foam structure could lead to stability offoam because of balance forces at interface. Moreover, as a three-phase foam system,the solid particles in sludge foam system improved the foam stability because theparticles could bridge gas bubbles in close contact and increased the viscosity ofsludge.The drying experiments were carried out by drying oven in the convective mode.The results indicated that foamed sludge at0.70g/cm3had the best dryingperformance in each level of temperature. The drying rate and effective moisturediffusivity of foamed sludge were improved with the increasing of drying temperature.The affection of sample thickness to drying rate was not obvious as the samplethickness increased from2mm to8mm.The shorter drying time of foamed sludgemay resulted from the larger surface area exposed to drying air, more water frombound type into free type, and the lower internal resistance to moisture and heat transfer of the foamed sludge.The values of the effective diffusivity were found to increase with the decreasingof sludge moisture content. Meanwhile, it was also increasing at higher dryingtemperature and sample thickness. The Wang and Singh model represented the dryingcharacteristics better than others, with the highest values of γ2, the lowest χ2and SR.