Drying and incineration of wastewater biosolids: Experiences and perspectives based on the development in Denmark

With the advent of class A biosolids (U.S. EPA 40 CFR Part 503) and challenges to find suitable biosolids disposal technologies, alternative methods for biosolids treatment need to be developed. The purpose of this paper is to analyse the recent international developments where the traditional disposal for agricultural use has changed to other disposal routes, especially for smaller wastewater treatment facilities where energy efficiency, operational ease, and safety are of utmost importance. One of the main routes is the thermal treatment, drying and/or incineration. Drying concepts adapted to suit these demands have been developed and utilize waste heat recovery for indirect drying at low temperatures to impart energy efficiency and safety since drying is accomplished at temperatures that do not allow combustion. A modular drying unit can be coupled to a modularized incineration system to provide a compact system for handling biosolids. Recent operating experience has shown that the technology and concepts are sustainable in all main respects, i.e. energy utilisation, environment, operation etc. Coupled with a modular incineration unit the BioCon belt dryer has proven to be one of these concepts that also produces a class A product. The use of heat recovery and air recirculation allows the system to operate with a favourable net energy balance while providing good results. The combination of the low temperature dryer with recirculation of drying air and an efficient combustion ensures that the latest EU Directive requirements for incineration plants for solid waste can be met with a good margin of safety. Results from facilities in Denmark will be presented that highlight the new directions and advancements in biosolids handling.