How the nature of the compounds present in solid and liquid compartments of activated sludge impact its rheological characteristics

Understanding the Influence of Diverse Non-Volatile Media on Rheological Properties of Thermophilic Biological Sludge and Evaluation of Its Thixotropic Behaviour

In this study, the rheological properties of thermophilic biological sludge (TBS) have been investigated evaluating the influence of non-volatile solids (NVS). Calcium carbonate, sand, and sodium bentonite were separately added to the sludge to evaluate the effect of concentration and type of NVS. Results show that TBS consistency coefficient significantly enhanced increasing sodium bentonite concentration. On the contrary, calcium carbonate and sand showed relatively small influence on the rheological properties of TBS. Thixotropic behaviour of TBS has also been investigated and is more pronounced at higher shear rate (1000 s−1). Double exponential fitting model was the best choice to represent thixotropic behaviour in case of low (100 s−1) and high shear rate (1000 s−1), while a single-exponential model represents the best option in case of medium shear rate (400 s−1).

Viscous and filamentous bulking in activated sludge: Rheological and hydrodynamic modelling based on experimental data

Although achieving good activated sludge settleability is a key requirement for meeting effluent quality criteria, wastewater treatment plants often face undesired floc structure changes. Filamentous bulking has widely been studied, however, viscous sludge formation much less investigated so far. Our main goal was to find relationship between sludge floc structure and related rheological properties, moreover, to estimate pressure loss in pipe networks through hydrodynamic modelling of the non-Newtonian flows in case of well settling (ideal-like), viscous and filamentous sludge. Severe viscous and filamentous kinds of bulking were generated separately in continuous-flow lab-scale systems initially seeded with the same reference (ideal-like) biomass and the entire evolution of viscous and filamentous bulking was monitored. The results suggested correlation between the rheological properties and the floc structure transformations, and showed the most appropriate fit for the Herschel-Bulkley model (vs. Power-law and Bingham). Validated computational fluid dynamics studies estimated the pipe pressure loss in a wide Reynolds number range for the initial well settling (reference) and the final viscous and filamentous sludge as well. A practical standard modelling protocol was developed for improving energy efficiency of sludge pumping in different floc structure scenarios.

Rheology improvement in an osmotic membrane bioreactor for waste sludge anaerobic digestion and the implication on agitation energy consumption

Sludge rheology is an essential factor for anaerobic digestion (AD) processes to control the agitation energy consumption. In this study, the sludge rheology was characterized for an osmotic membrane bioreactor and a conventional sludge anaerobic digestion reactor as the solid content being increased from 3.5-3.7% to 7.5-7.7%. The flow curves were fitted using different rheological models and the mechanism was discussed. The sludge from the osmotic membrane bioreactor exhibited obviously better rheological properties than that of the conventional reactor at a solid content of 7.5-7.7%. Larger particles induced by less negative zeta potential and higher extracellular polymeric substances, together with the higher conductivity resulted by reverse salt flux in the osmotic membrane bioreactor, improved the sludge rheology due to reduced interactions between particles. As a result, the agitation energy consumption of the osmotic membrane bioreactor can save up to 34-39% compared with the conventional one at total solid content of 7.5-7.7%.