Effect of total suspended solids and various treatment on rheological characteristics of municipal sludge

Abstract

Rheological characteristics play an important role in the design and optimization of sludge treatment processes that are dependent on the total suspended solids (TSS). The present study systematically investigated the impact of various treatment processes on sludge rheology. The effect of TSS on rheological properties was comprehensively explored. Rheological tests were performed at (20 ± 0.1) °C, physicochemical parameters of the sludge were measured for compositional analysis. The composition of sludge is altered when subjected to both heat treatment and AD. All sludges exhibit shear-thinning behavior, the results of a t test (with significance level of 0.05) indicated that the Herschel–Bulkley model provides a valid description of sludge flow behaviors. The anaerobically digested sludge (ADS) is the least viscous while thermal hydrolyzed sludge (THS) shows slightly higher viscosity than that of fresh mixed sludge (FMS) at high shear rates. The yield stress and thixotropy decrease gradually as the treatment proceeds. Both infinite viscosity and yield stress of THS increases dramatically when the TSS exceeds 7.30%, hence 7.30% may serve as a critical level for THS to preliminarily determine whether it is economically viable to implement a thermal pretreatment. The TSS threshold for ADS is determined to be 7.12% given the sharp increase in infinite viscosity and yield stress, which are undesirable for dewatering. The thixotropy increases with increasing TSS, and TSS only has a slight influence on the thixotropic properties of THS relative to FMS, which demonstrates the advantage of thermal pretreatment for the purpose of achieving high solid sludge AD.