Optimization Of Stirring Type Of Sludge Anaerobic Digestion System

There are two factors that can affect the matrix mixing in anaerobic digestion of sludge: the physical characteristics of raw materials used in fermentation digestion,and the mixing device and conditions in the anaerobic fermentation tanks.For the same type of raw materials with the same concentrations,the structure and operating parameters of the mixing device are major factors that affecting the mixing of raw materials,and thus affecting the stability and efficiency of the anaerobic fermentation.In this study,the effects of the total solid concentrations,temperatures and additions of food waste on rheological properties of four kinds of feedstocks(dewatered sludge,waste activated sludge,the mixture of dewatered sludge and food waste,and the mixture of waste activated sludge and food waste)were investigated,the mixing efficiency in the sludge-containing feedstocks under different paddle types and stirring speeds were compared,the gas generation in the anaerobic digestion system under different mixing models were tested through dynamic tests.The four kinds of feedstocks were found to be pseudoplastic fluids,which followed the power law model.When the solid concentration increased from 1% to 10%,viscosities of these feedstocks increased accordingly.The waste activated sludge and the mixture of waste activated sludge and food waste had the maximum rheological indices at TS of 3%,while the dewatered sludge and the mixture of dewatered sludge and food waste had the maximum rheological indices at TS 5%.Addition of food waste improved the rheological characteristics of the feedstocks,increasing the rheological index of the feedstocks to make them more similar to the Newton fluid.As the temperature increased from 15°C to 55°C,the viscosity of these four feedstocks all decreased linearly,but their rheological indices increased gradually.The viscosity of dewatered sludge was most sensitive to temperature among the four feedstocks.Under the top eccentric mixing,the concentration distribution of dry matters became uniform in the anaerobic fermentation tank with increasing stirring speeds.The double flap paddle and double propeller blades,had the critical mixing speeds of 130 r·min-1 and 110 r·min-1 in the digestion of dewatered sludge,and had the critical mixing speeds of 130 r·min-1,90 r·min-1 in the mixture of waste activated sludge and food waste.In mixing the digestion solutions,the energy consumption for stirring the mixture of waste activated sludge and food waste was low,and the stirring efficiency of double pusher propeller type is higher than double baffle vane propeller type.The dynamic tests showed that the stirring speed and stirring time influenced the performance of the anaerobic digestion system.According to the rheological properties of material liquid and apparent reynolds numbers under different stirring speeds and powers,the power curves and the calculation method for the stirring power were obtained.Accordingly,the optimal operation condition was obtained with a mixing rates of 90 r·min-1 and a stirring frequency of 4 times a day(each time stirring 1.5 h).