Influence Of Back Mixing On Sludge Foaming And Thin-layer Drying Characteristics Of Foamed Sludge

With higher moisture content and bigger viscosity of sludge, its drying has been facing serious challenges. It was found in our previous research that the combined use of foaming pretreatment and thermal drying can reinforce the drying rate of dewatered sludge. However, larger amounts of dried sludge(DS) can be still poured in the end of this process. Hence,foaming and backmixing was jointly applied to the sludge drying process in this paper. This not only reduces the dosage of CaO, and also realizes the reuse of dry mud.Different ratios of (DFS + CaO) were investigated on the influence of foamability and foamstability of raw sludge in this paper. Experimental results indicate that a proper amount of backmixed DFS can greatly promote the sludge foaming when the dosage of DFS is smaller than 10 g. And the best adding ratio is lOg DFS + 10g CaO for lkg of fresh sludge. Meanwhile, backmixing can also significantly enhance the sludge foam stability, which provides a powerful guarantee for the subsequent drying.Based on the best adding ratio, the dosing sequence and the DFS shape were also studied about the influence on the sludge foam-mat drying. It was found that dewatered sludge can foam well for any kind of dosing sequence. And the optimal dosing sequence is first CaO followed by DFS after 5 min. This implies thatCaO is still dominant in the foaming process of sludge. Additionally, the foam-mat drying for dewatered sludge is not greatly subjected to the DFS shape.By the Oven drying experiment, the influence of backmixing was further investigated on the dry characteristics of the sludge. Experimental outcomes showed that the sludge drying occurs mainly in the falling rate drying stage and short constant rate drying stage disappears due to backmixing. The higher drying temperature leads to the faster drying rate. Besides, a high drying rate appears at a high foam density (>0.70 g/cm3) during foam-mat drying. The foamed sludge of 0.80 g/cm3 has the fastest drying speed at 30?, whereas the best drying density is 0.90 g/cm3 at 50?. Moreover, mathematical modelling results demonstrate that the Logistic model is themost adequate model to describe the entire convective drying of thin layer sludge under the best drying density both at 30? and at 50?.Under the best conditions for backmixing,the physicochemical properties of foamed sludge at different densities are as follows:the initial moisture content is declined; pH of the sludge supernatant is all beyond 12;the protein content increases by over 6 times higher than that of raw dewatered sludge;polysaccharide content increases by over 5 times higher than that of raw dewatered sludge; the surface tensiondeclines to a certain degree for the sludge supernatant. The co-existence of polysaccharide and protein, not only ensuresthe strong stability of the foamed sludge, and has no effect on the normal foaming of the sludge.In addition, reduction of the surface tension is also in favor of sludge foaming.