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Back-mixing is an operation that can be used to reinforce the texture of soft and pasty sludges to improve the drying effect. In this study, three mass fractions in a back-mixing operation (40%, 60%, and 70% on a dry basis) were chosen for convective sludge drying in a fixed bed. The structural characteristics including the total exchange surface, object volume, and shrinkage of the sludge bed were measured by X-ray macrotomography during the drying process. The average drying rates are 0.076 g/s (original sludge), 0.079 g/s (40% back-mixed sludge), 0.092 g/s (60% back-mixed sludge), and 0.099 g/s (70% back-mixed sludge), respectively. For the original sludge and three back-mixed sludges used, both the object volume and total exchange surface decrease in a quasi-linear manner with decreasing water content in the sludge. The final total exchange surface increases by 97.1% for the 70% back-mixed sludge compared with that of the original sludge. After drying, the shrinkage ratio of the object volume of the original sludge is 82.1%, however it is decreased to 50.8% when the 70% back-mixed sludge is used. The average convective mass transfer coefficient increases by 60% for the 70% back-mixed sludge compared with that of the original sludge. It was assumed for belt drying that, the lengths of the belt used for the 40% back-mixed sludge, 60% back-mixed sludge, and 70% back-mixed sludge will be nearly 0.96, 0.83, and 0.77 times, respectively, as long as that used for the original sludge.

Influence of back-mixing on the convective drying of sewage sludge: The structural characteristics