Developing a universal equation to estimate the mass of dewatered wastewater sludge during biological digestion at mesophilic and thermophilic temperatures

A series of dewaterability tests were conducted on various types of sludges to establish a wholistic relationship between sludge water fractions. Sludge samples were obtained from batch and continuous sludge digesters, which were operated anaerobically and aerobically under mesophilic and thermophilic conditions. Dewaterability of the sludge samples and the distribution of water fractions were studied using centrifugation and thermal drying. Thickened waste activated sludge (T-WAS) contained 10-11 g bound water (BW)/g of total solids (TS), and it was more hydrophilic than primary and digested sludges. During anaerobic digestion, BW content fluctuated between 3.2 and 4.2 g BW/g TS. However, aerobic digestion at 55°C reduced the BW content of the mixed T-WAS + primary sludges from 3.7 to 2.1 g BW/g TS. A linear function was developed to correlate supernatant and BW mass fractions (R² = 0.995). An equation was derived from the linear function to estimate the mass of dewatered sludge based on the TS concentration of the initial wet sludge. The developed expression is applicable to different kinds of wastewater sludges. Such an expression would be helpful for the designers and operators of sludge thickening and dewatering systems that use centrifugal separation.