A Study for the Thermal Treatment of Dehydrated Sewage Sludge with Gas-Agitated Double Screw Type Dryer

Abstract

A specially designed dryer has been developed for the thermal treatment of dehydrated, highly viscous sewage sludge with moisture content more than 80 wt.% by an gas-agitated, double-screw type dryer system. The treatment capacity of sludge dryer was about 100 kg/h. It consists of burner, feeding hopper, hot gas supplying ducts with double screw conveyor, damper and outlets of drying gas and dried material. The sewage sludge was transported by the revolution of the cylinder conveyor together with the tumbling and mixing action of the screw and lifters. The heating of the sludge was made efficient by the combination action of conduction and convection modes together with the gas-agitation process. The conduction occurred across the surface of the combustor cylinder, the convection was made by the flue gas interaction over the sludge and the agitation action through holes of conveyor cylinder into the bottom of the sludge. The number and location of gas-agitation holes and thereby the fraction of drying gas into the agitation holes were evaluated by numerical calculation of turbulent reacting flow. To evaluate the performance of the dryer developed in this study, a series of parametric experiments were performed in terms of important variables. In general drying process occurred successfully even for the highly agglomerating municipal sewage sludge through the first lumped plastic phase to a fine granulate state when the heat and mass balance was matched by adjusting rpm of the conveyor, burner capacity, and the fraction of gas-agitation. Further the avoidance of initial sludge agglomeration was considered to be important by the combination action of screw and lifter. The weight of the sludge was reduced approximately by 60% and therefore the volume of sludge was decreased to approximately 75% of initial volume with the water content of 10–20%. Energy efficiency of dryer was evaluated in the range of 70–75% at the sludge feed rate, 100 kg/h. The results obtained show the development of a robust, drying action applicable to highly viscous and high moisture content sewage sludge by the novel design of conveyor system and gas-agitation process developed in this study.

Keywords:

• Dehydrated sewage sludge
• Gas-agitation
• Double-screw and lifter Dryer