(614a) Review on Rheological Studies of Sludge from Different Sections of Municipal Wastewater Treatment Plants for Improved Process Performance

Wastewater treatment process generates large quantities of sludge from different operational steps. The rheological behaviour of sewage sludge at different steps of wastewater treatment plant (WWTP) is useful for the design, operation, optimization of polymer dosing, hydrodynamics and mixing in anaerobic digesters, and for efficient sludge management. Rheological characterization of sludge is also known to be an essential tool to optimize flow, mixing and other process parameters in WWTPs. In this review article, the impacts of different parameters like solid concentration, temperature, pH, floc particle size, mixing ratio, ageing and dose of sludge conditioning agents on the rheological behaviour of sludge from different parts of WWTPs were analysed. Raw primary sludge, excess activated sludge, thickened excess activated sludge, mixture of raw primary and thickened excess activated sludge (mixed sludge), digested sludge, and biosolid rheological and flow behaviour under the influence of different operating parameters and the impacts on process performance were reviewed in this study. Solid concentration, temperature, primary to thickened excess activated sludge mixing ratio, and sludge age were found to affect the viscosity, yield stress, flow index and flow consistency of mixed sludge., viscosity also showed a decreasing trend with decreasing total solid concentration and percentage of thickened excess activated sludge in the mixture. In contrast, yield stress and viscosity generally showed reduction with increasing temperature. The effect of dose of conditioning agents on the rheological behaviour of thickening of excess activated sludge and digested sludge was also addressed in this review. The relationship between activation energy and viscosity for varying temperature has also been discussed. The rheology of digested sludge affects the flow hydrodynamics, dewaterability and the polymer consumption in WWTPs. The rheological characteristics of digested sludge are highly dependent on changes in total solid concentration, temperature and polymer dose. The rheological characteristics of digested sludge during polymer conditioning and flocculation process was significantly affected by temperature and solid concentration; hence, polymer dose can be reduced by operating the dewatering process at optimum temperature condition and varying the polymer dose as a function of the total solid concentration and viscosity of the digested sludge. Volatile solid content of digested sludge fed to centrifuge has inverse relationship with total solid content of biosolid or biosolid capture rate. Hence, improving the volatile solid and COD removal/reduction by enhancing the performance of anaerobic digesters or by using other digested sludge conditioning methods will significantly improve the flow behaviour of digested sludge and dewatering performance. Investigating the relationship between rheological and physico-chemical characteristics of sludge can also serve as a tool to optimize essential process parameters. The applicability and practical significance of various rheological models such as Bingham, Power Law (Ostwald), Herschel-Bulkley, Casson, Sisko, Careau, and Cross models to represent experimental rheological characteristic of different sludge types was also investigated in this review.