Technological data for processes for water contents reduction in sewage sludge

Sludge from sewage treatment contains a number of biological constituents with a heating value not to be neglected. The sewage sludge may be used as a fuel, but not until a large part of the water, which also forms part of the sludge, has been removed. A number of technologies exist to reduce the water content of sewage sludge, and more technologies will be developed within the next year. In order to determine which technologies should be aimed at, it is necessary to evaluate the different methods from a technical as well as an economic point of view. In this review a method for evaluation of these technologies is presented together with a spread sheet in which the method has been used for a number of the present conventional technologies. For the comparison, two key figures are calculated expressing each their aspect. Key figure A expresses economy as cost in a method compared to the energy increment added to the sludge. In the review the calculations have been made exclusive of taxes and grants, but there is nothing to prevent the inclusion of the tax problematic in the spread sheet. Key figure B expresses the environmental influence of a method - the CO2 emission increase during dehydration - compared to the energy increase. From the figures of the review is appears that dried sludge is not CO2 neutral, but that there is a potential CO2 gain by application of sludge as fuel when the alternative is coal for heat and power production. The project is carried out on behalf of the Danish Energy Agency as an internal review. The review has resulted in the appointment of a task group to continue working on the subject of energy efficient sludge utilisation