Evaluating the contact angle as an indicator of sludge dewatering performance: Limitations and methodological insights

Abstract

This study evaluates the utility of contact angle (CA) as an indicator of sludge dewatering performance, addressing inconsistencies in previous findings and the lack of a clear standard. We systematically compared four pretreatment methods—agar coating, in-situ filtration, lyophilization, and hot air drying—to assess contact angle efficacy. Measurement reliability was determined using standard deviation and coefficient of variation (CV), revealing that hot air drying (74.85° ± 4.24°, CV 5.67 %) and lyophilization (71.48° ± 4.74°, CV 6.6 %) exhibited significantly lower variability than agar coating (42.54° ± 12.24°, CV 28.7 %) and in-situ filtration (53.35° ± 9.66°, CV 18.1 %). Correlation matrix and Variable Importance in Projection (VIP) analyses indicated that contact angle only partially correlates with dewatering performance indicators, including water content, capillary suction time (CST), and specific resistance to filtration (SRF). Importantly, the predictive power of CA measurements varies across different pretreatment methods, underscoring its limitations as a standalone metric. Over-reliance on CA measurements may lead to biased conclusions. This study offers key perspectives on refining CA measurement methods and highlights the importance of comprehensive evaluation of sludge dewatering performance, guiding the development of more efficient treatment technologies.