Shear and solid-liquid separation behaviour of lagoon anaerobic digested sludge and scum

Anaerobic lagoons are used in wastewater treatment facilities for the sedimentation of solids and anerobic digestion of settled sludge. Computational fluid dynamics (CFD) modelling is utilized for the troubleshooting and optimization of anaerobic lagoon processes. However, due to the large variation of solids concentration and operating conditions in different sections of the lagoons, efficient CFD modelling requires accurate correlations for the prediction of the shear and solid-liquid separation properties across a large solids gradient, which is currently unavailable in existing literature. This is the first study on the simultaneous characterization of the shear and solid-liquid separation properties for an anaerobic lagoon sludge, which has applicability in other wastewater treatment plants with anaerobic digesters. The outcomes of this study are the correlations for the shear and solid-liquid separation properties as continuous functions of solids concentration for an anaerobic lagoon sludge, which were used as input for CFD modelling of an anerobic lagoon. The identification of the appropriate model to characterize the sedimentation velocity for dilute sludge suspensions has been an outstanding problem and is a requirement for accurate CFD modelling of the anaerobic lagoon process. In this study, the appropriate model for the prediction of settling velocity for sludge suspensions was identified through the analysis of dilute and concentrated gravity batch settling tests. In addition, the necessary adjustments to the model fitting parameters with the variation of solids concentrations were indicated. The outcome of this study was a settling velocity model for the sludge suspensions with solids concentration of 3-16 g/L, which was used as an input for CFD modelling of an anaerobic lagoon. Although the impact of anaerobic digestion on the shear and solid-liquid separation behaviour of anaerobic digested sludge has been widely studied, there is an existing research gap in the study of the evolution of these parameters in continuous laboratory scale anaerobic digesters under psychrophilic temperature range, which has relevance to operating conditions of anerobic lagoons. In this work, the shear and solid-liquid separation properties of anaerobic digested sludge from two continuous digesters operated at psychrophilic temperature were measured across a wide range of solids concentrations and linked to the volatile solids destruction (VSD).  It was observed that the viscosity of anaerobic sludge increased with the increase in VSD. The simultaneous impact of solids concentration and VSD on the viscosity of the digested sludge was illustrated from the analysis of a hypothetical digester. In contrast to the impact of VSD on shear rheology, insignificant impact of VSD on the solid-liquid separation behaviour was observed. However, there was a general increase of final average cake solids concentration with the increase in VSD, indicating better solids-liquid separation behaviour at higher digestion. The negative impact of floating scum layer on the surface of covered anaerobic lagoon is a critical operational problem resulting in significant maintenance and capital costs. To mitigate the adverse impact of scum formation, it is necessary to understand the composition and formation and consolidation mechanism of scum. In this study, the physiochemical and mechanical properties of scum from two different lagoon systems were measured and compared to those for settled sludge to identify the formation and consolidation mechanism of anaerobic lagoons. This study concluded that scum contains a significant portion of undigested sludge which floats due to biogas generated from anerobic digestion. The mechanical properties indicated that the scum network becomes stronger as it consolidates.  The industrial implication of this study is that scum formation is an inevitable outcome of anaerobic digestion in the lagoons and the scum must be actively managed through mechanical or hydrodynamic disruption before it can consolidate.