Experimental Study On Urban Sludge Drying By Applying A Polarized Electric Field With Cathodic Heat Wall

Volume reduction, stabilization, harmlessness and resourcefulness are the fundamental and object of urban sludge treatment and disposal. Dewatering and drying are the basic processes of them. But when the drying process is designed and operated, the problem about sludge adhesion has to be considered, because it can bring some bad effects to drying efficiency and stability, especially for indirect drying. This paper based on the problems of sludge adhesion to the dryer wall, analyzed the effects of sludge properties and working conditions on adhesion, gave and studied experimentally an application of a polarized electric field to reduce adhesion of a biological sludge to the dryer wall by contact drying of a sludge cake deposited on a heated metal plate serving as the cathode. Some theories about fractal and time series were used to analyze the variety of evaporation speed in sludge drying. A symbolic simulation was taken to analyze the microcosmic matter transfer.The sludge properties and working conditions directly affected the sludge-to-wall adhesion, especially including water contents, sticky organic matter and plate temperature. When the sludge contained water above 60% wb, the adhering intensity increased with the decreasing of water contents, and got to the maximum at around 60% wb. After that, it began to decrease following the moisture. The adhering intensities of indigested sludge with more sticky organic matter were more than the one of digested sludge. In the range of low temperatures which were applied on usual hot water or vapour drying, the sludge adhering intensities decreased with increase of plate temperature, for example, the plate temperature of 120℃made the adhering intensities more than the one of 100℃.The experiments were made to research the working conditions of sludge drying by applying a polarized electric field. The results showed, when sludge samples were 10mm thick, the voltage gradient of 4 to 5 V/cm was found to be optimal for adhering intensity, drying time and electric energy. Here, the electric energy consumption amounted to 3.87 kWh/t. By observing and analyzing, the reasons of reducing adhering intensity with a polarized electric field were thought to be following three results: the electro-osmosis changed the water distributing of sludge around the plate, the zeta potential of heat plate became negative, the gas generated owing to electrochemistry on the cathode wall stopped the sludge adhering to wall.The visual judgement and power spectral of drying speed were used to prove the fractal occurred during matter transfering. The methods of variety and hausdorff were used to calculate the dimensions that estimated or supported the kinetics of heat and matter transfer. The R/S analysis showed, the drying speed series belong to positive correlative series and its Hurst index existed between 0.7 and 1.0. The paper also gave a symbolic simulation that results showed applying polarized electric field could boost the water transfer of drying sludge.