Sludge rheology: semi – empirical correlations to predict the apparent viscosity and yield stress of sludge mixtures

Abstract

Anaerobic digestion is a biological process in which municipal and industrial solid wastes are converted to biogas and anaerobic digested sludge. In this study, the rheological behaviour of different types of sludge as well as sludge mixtures are studied. Primary sludge, secondary sludge, primary – secondary sludge mixtures and primary – secondary – digested sludge mixtures behaved as non – Newtonian, shear thinning materials exhibiting a yield stress. A dimensionless form of the Herschel – Bulkely model was employed to develop master curves to predict the rheological properties of primary sludge, secondary sludge or sludge mixtures. Below the yield stress, primary and secondary sludge (with 2.8 – 9.2% TS) exhibited viscoelastic behaviour similar to soft glassy materials. Primary sludge experienced abrupt yielding such that viscosity bifurcation was detected similar to highly thixotropic colloidal suspensions. Secondary sludge transitioned smoothly into the liquid regime similar to gels. Primary and secondary sludge flowed steadily into the liquid regime. Increasing the solids concentration of primary and secondary sludge increased the hydrodynamic and non – hydrodynamic interactions so that the apparent viscosity and yield stress increased following exponential and power law models as a function of total solids concentration, respectively. The apparent viscosity and yield stress of sludge mixtures prepared by mixing primary sludge (with 3, 4, 5, 6.5 and 7.1%TS) to secondary sludge (with 3, 4, 5, 6.5 and 7.1%TS) increased with increasing volume fraction of secondary sludge. This was attributed to the deflocculation of the weakly bound structure of primary sludge so that the weak colloidal particles of primary sludge became entrapped and entangled into the gel like network structure of secondary sludge. These parameters followed a power law model as a function of secondary sludge volume fraction. When thickened primary sludge (5.4% TS) was mixed with dilute secondary sludge (2.8%TS) and vice – versa, the apparent viscosity and yield stress of the mixed sludge increased with increasing volume fraction of the thickened sludge – regardless of sludge type following a power law model as a function of the volume fraction of thickened sludge. The apparent viscosity and yield stress of primary – secondary – digested sludge prepared by mixing a 50:50 (v/v) primary – secondary sludge mixture (with 3, 4, 5.1, 6.3, 7.1%TS) to digested sludge (with 3, 4, 5.1, 6.3, 7.1%TS) increased with increasing volume fraction of digested sludge so that the solid interactions within the sludge mixture increased – highlighted using the shear compliance and shear modulus. When a thickened primary – secondary sludge mixture (50:50 v/v and 5% TS) was mixed with dilute digested sludge (1.8%), the apparent viscosity and yield stress decreased as the volume fraction of digested sludge increased. This was attributed to the dilution effect. As such, the apparent viscosity and yield stress followed a power law model as a function of volume fraction of digested sludge. The parameters of these models were predicted using the pH of the sludge mixtures. Finally, procedures were presented to demonstrate how the developed correlations may be used to optimize the power and energy requirements of unit operations such as pumps and mixing systems