PROJECT DESCRIPTION
BACKGROUND
The long-term disposal of sewage sludge is a major environmental problem, which has so far found few satisfactory or viable solutions. Traditionally, the sludge volume is reduced by centrifuge pressing and then used in agriculture applications or transported to a waste dump. However, environmental directives now limit agriculture use because of risks of contamination to the food chain. As an alternative, the Mayr-Melnhof cardboard factory has drained the sludge through a double-sieve press to a dried matter content of 30% which is then stored in a concrete silo, before being dumped. In volume terms, this amounted to a substantial waste disposal problem for the company. In 1997, 45 tons of sludge a day were generated with a dry content of 32 %. This waste was not only an economic problem, but caused severe environmental problems through seepage, gaseous emissions and the prevalence of unpleasant odors that molested local residents. There was therefore a need to find a way to reduce both sewage sludge weight and volume, in order to make better use of existing waste disposal arrangements. This reduction could only be achieved by an increase in dried matter content.
OBJECTIVES
The purpose of the project was to introduce a fluidized-bed steam drier into its existing wastewater treatment in order to dry the excess sludge. The dryer would be operated with steam from a bio-gas boiler, in a closed steam circuit. The drying would be effected by a combination of contact and convection drying, whilst the accruing water steam would be condensed and returned to the cycle. The new procedure would produce a granulate product which could be stored for an unlimited time and be used as a heating equivalent to brown coal. In this way, the project would achieve substantial volume reduction of sewage sludge through clean technologies, at the same time as producing a new value-added energy product.
RESULTS
The fluidized bed dryer was designed to operate at optimum levels. The moist sludge was first ground down in a special granulating machine before being fed into the fluidized drier bed, thus avoiding the mixing back of the granulate. This fluidized bed was kept afloat by a support blower and a specially designed wind box with a truyere bottom. Within the bed itself, the contact heat exchange was located. This was fed by saturated steam recycled from the sludge heating process. In the heating process,the granulate pellets, as well as the cycle air, were warmed up to 120 C by the heated outer surface. This meant that the actual sludge drying arose from a combination of contact and convection drying. The use of such low temperatures enabled the excess water to be removed from the moist sludge in the most careful way. As a result, the process could achieve high levels of dried matter despite ‘low temperature drying’, providing an economical, low energy procedure. Key innovatory solutions were the use of waste paper in the surface layer of the carton and a tuyere bottom for the drier, increasing the efficiency of the drying process. The accruing water steam was then condensed before being returned to the biological sewage water purification plant, whilst the resulting inert gas was cleaned by a biological filter. This closed system also prevented dust developing during the process. After a period, the granulate pellets were then removed to ensure that no O2 intake could occur, thus eliminating fire or explosion hazards. It was then cooled to storage levels by a cooling screw. These grains or pellets could then be stored outside for an extended period of time or in a silo before being transported away, as even under moist conditions the outer skin would simply seal and prevent any further moisture penetration. The pellets, thus offered a hygenic and easily handled product, which given their high thermal value (12 MJ/kg), could offer themselves to a variety of uses as substitutes for fossil fuels. The fluidized bed drier succeeded in obtaining a dried sludge matter content of between 90 and 95 %, and represented a careful, economic solution to sewage sludge treatment. This would enable the company to substantially reduce the volumes of surplus sludge, enabling the existing waste disposal equipment to be used more efficiently. It was calculated that 8.500 t/year of contaminated waste water and 4.100 t/year of organic substances could be recycled. As the drying process was applied to biologically active surplus sludge, greenhouse gases such as CO2 or CH4 were avoided, as were related odour problems. Moreover, the fuel product produced, “Granulate from the Cardboard Factory Mayr” appeared to offer viable outlets, with interest being shown from a number of sectors, particularly in the cement and brick industry. The project tested out many different kinds of sludges at the pilot facility (municipal, paper sludges, galvanic, non-biological sludge with a high water content), and was able to prove the effective drying results no matter the compound or ingredients of the primary material. It would therefore appear that this systematic circulation method and low energetic processing of moist sludge into highly energetic matter could offer a transferable model for processing different types of municipal and industrial sludges throughout the EU.
