Drying of Waste Water Treatment Sludge in Fluidized Bed of Inert Particles

Abstract

The benefits of drying waste water treatment sludge can be seen in two main aspects: the dried sludge can be stored and further as an organic fertilizer, or it can be a source of useful (active) microorganisms that can be reused in waste water treatment. Drying of suspensions and pastes in fluidized beds of inert material is a very attractive alternative to conventional drying technologies for suspensions, due to considerably greater specific capacity expressed as capacity per unit volume, less energy consumption and less drying air consumption. High drying efficiency results from large contact surfaces and large temperature differences between inlet and outlet air. Intensive mixing of inert particles during aggregative fluidization provides approximately uniform temperature over the entire volume of the bed and there is no risk of local overheating. A number of experiments of water treatment sludge were carried out at the pilot plant with a nominal evaporation capacity up to 20 kgH2O/h. The central part of the system is a cylindrical column of 215 mm in diameter and 1200 mm height with a fluidized bed of glass spheres. Influence of the operational conditions on performance of the drying system and on the quality of dried product were examined, in order to determine optimal drying temperature in which the microorganisms present in the dried sludge remain active. The analysis of efficiency of drying, from the aspect of water evaporation capacity and air consumption was performed in order to understand the performance of the drying system. Simple mass and heat balances adequately predict the operating regime of drying system in fluidized bed of inert material.