DIRECT FEED TECHNOLOGY FOR THERMAL DRYING OF BIOSOLIDS IN A FLUIDIZED BED

The trend towards use of thermal drying as a means for producing a material which: meets Class A requirements (under 40 CFR Part 503), is relatively easy to handle, and can be used in a variety of downstream applications (i.e. as fuel, as fertilizer, etc.) is growing, and becoming of increasing importance worldwide. As the trend is neither new nor just emerging, the history is one of evolving technology, and is at least consistent in that has been colourful and frequently fraught with difficulties. The difficulties can essentially be categorized in terms of operating & safety issues, such as: inherent level of safety for the particular process, operability, reliability/system availability, and the degree to which the system must be attended.One approach introduced in Europe in 1990, based primarily on two principles -- direct-feed of dewatered cake (without a back-mix, addback or recycling process), and use of a fluidized bed with indirect heating & closed gas loop – has been successfully implemented now in over 20 municipal sludge drying installations. A number of these are configured as regional facilities receiving a variety of sludges, digested/undigested, mechanical dewatering via filter press or centrifuge, with a wide range of incoming moistures – and therefore, must readily & continually adapt to constantly changing characteristics in the dewatered cake. The demands for a technological solution to deal with these variations led to the introduction of these innovations. This paper presents the background information, comparative results, utility consumptions, and installation particulars for many of these plants, some of which have been in operation for 10+ years.