Experimental study on the effect of sludge thickness on the characteristics of ultrasound-assisted hot air convective drying municipal sewage sludge

Abstract

Due to the stratification of sludge particles in the ultrasonic field, the acoustic interaction forces the particles to agglomerate on a plane perpendicular to the direction of ultrasonic propagation. Therefore, the thickness of sludge can significantly influence the characteristics of ultrasound-assisted hot air convective drying municipal sewage sludge. In this paper, the stratified aggregation phenomenon of sludge with different thicknesses was observed in the ultrasonic field using the experimental method. It was found that the stratification of the internal structure of sludge became more obvious with the increase of its thickness. The effects of ultrasound on the drying time and the drying rate of sludge with various thicknesses were studied. Meanwhile, the effective moisture diffusivity (Deff) was analyzed. The experimental results demonstrated that within a certain range, the larger the sludge thickness, the longer the time length of the first falling rate stage, and the promotion of the drying rate is worse. The situation is opposite at the constant rate stage. Among the sludge with the thicknesses of 5 mm, 10 mm, and 15 mm, the total drying time and energy consumption of the sludge with a thickness of 10 mm decreases the most substantially at different power ultrasound-assisted. The smaller the thickness of sludge is, the less obvious the effect of ultrasound on the effective moisture diffusivity (Deff), and vice versa.