Influence of sawdust addition on drying of wastewater sludges: Comparison of structural characteristics

ABSTRACT

This work aims to investigate the structural characteristics of wastewater sludges and sludge–wood mixtures, and how the addition of sawdust affects them in a positive way for the drying process. The study was performed for a fixed bed at the pilot scale, with sludges from three different wastewater treatment plants from the Liège province in Belgium, namely, Oupeye, Grosses Battes, and Embourg. X-ray tomography was chosen to evaluate variations in the structural characteristics before and after drying, which included volume, void fraction, and exchange surface of the bed. Results first confirm that the drying rate is raised after the sawdust addition operation for all three sludges, but the increase amount is smaller while the moisture of the sludge is high. With sawdust, the average drying rates increase by factors of 1.475 (Oupeye), 1.342 (Grosses Battes), and 1.162 (Embourg), respectively. It also increases initial volume as well as void fraction and surface exchange, however after drying this addition leads to less shrinkage and void fraction, and higher exchange surface. The initial void fraction increases by 7.25 (Oupeye), 8.30 (Grosses Battes), and 14.01% (Embourg), but the final void fraction decreases 1.56 (Oupeye), 10.83 (Grosses Battes), and 25.61% (Embourg). With high initial moisture content, we observed lower porosity and higher shrinkage.

KEYWORDS:

• Drying kinetics
• image analysis
• mixing
• pilot-scale fixed bed dryer
• X-ray tomography