Evaluating the potential of off-line methodologies to determine sludge filterability from different municipal wastewater treatment systems

Abstract

This study aimed to evaluate the capability of different methodologies for determining sludge filterability and estimate filtration resistance during real filtration processes. Three filterability methods were applied during this study: capillary suction time (CST), time to filter (TTF), and specific resistance to filtration (SRF). These methods were evaluated using three different sludge sources: aerobic activated sludge, supernatant from a primary settler further concentrated by ultrafiltration membranes (PSE), and digestate from the anaerobic co-digestion of microalgae and primary sludge. These sludge sources were taken from systems treating municipal wastewater entering to a full-scale wastewater treatment facility. The capability of CST, TTF and SRF to estimate total suspended solids (TSS) and soluble microbial products (SMP) concentrations was also assessed, while validating the results obtained with the real filtration process of the operated membrane-based systems. The results suggested that both TSS and SMP concentrations significantly affect filterability. However, each sludge filterability was mainly dominated by one of these parameters (TSS or SMP), being the biological sludge more influenced by the SMP content, while the PSE filterability was mainly controlled by the TSS concentration. SRF method resulted in poor correlations between filterability and TSS and SMP concentrations, especially regarding SMP. CST method resulted in good correlations for all treated sludge regardless the TSS and SMP concentration. However, when treating sludge without an important biological activity (e.g. PSE), TTF method was identified as the best option due to its better correlation with the experimentally determined sludge filtration resistance.