Positive Operations on Wastewater Sludge Drying: Comparison of Back-Mixing and Sawdust Addition

Abstract

This work compares two positive operations, back-mixing and sawdust addition, on the drying of wastewater sludges. The influences of back-mixing (with mass percentages of dried sludge of 40, 60, and 70% on a dry basis) and sawdust addition (40% on a dry basis) operations on the drying rates of sludge in a pilot-scale fixed bed were studied. X-ray tomography was used to monitor the evolution of structural characteristics of the samples during drying. When using the same mass percentage of dried materials (40%), the sawdust–sludge mixture has a higher drying rate than the back-mixed sludge as a result of higher initial and final bed volumes together with higher total exchange surface areas. To reach a similar rate increase with a back-mixing operation, 60% dried sludge must be back-mixed.

Effect of Bulking Agents on Dewatered Sludge Biodrying Followed by Thermal Drying

“…Thermal drying has been well developed and widely accepted as a fast and controllable method, but this process consumes much energy when it is directly used to remove moisture from DS on account of its structural features and high bound water content [5]. It was verified that SD addition improved the thermal drying characteristics of DS because of higher total exchange surface areas [17,18], and SD was also commonly used as a bulking agent in organic waste biodrying [9]. Besides, Cai et al [10] proposed that biodrying resulted in a multi-porous micro-structure and the transformation from bound water to free water of biodrying samples, which implied that the thermal drying characteristic of the biodried product might be improved.…”

Influence of back mixing and sawdust addition on convective drying of wastewater sludge: Temporal evolution of structural characteristics

“…The pilot-scale dryer used mainly includes a fan, an electrical heater, a sample holder, and a balance, as shown in previous studies [5][6][7]. The ambient air is drawn into the pipe by the fan and heated by the electrical heater.…”

“…Li et al [5,6] added sawdust to sludge and obtained higher volume, exchange surface, and drying rate. Li et al [7] also compared these two operations, back-mixing and sawdust addition, on sludge drying and found that sawdust addition has a better effect than back mixing using the same mass fraction of dried materials (40%). However, the structural characteristics were only given before and after sludge drying, and their change regularity hasn't been investigated during the drying process.…”