Drying biosludge with a high-velocity cyclone dryer

Abstract: Sludge disposal is a worldwide problem due to increasing volumes and respective environmental regulations. Currently the most common ways to dispose sludge are agricultural use, disposal in landfills, incineration and sea dumping. By drying sludge volumes can be decreased and the respective heating value increased. Sludge mainly originates from wastewater treatment. The Swedish pulp and paper mills produce approximately 400 000 - 500 000 metric tons of dry sludge each year. This includes roughly 60 000 metric tons of dry biosludge which mainly consists of bacteria and is difficult to dry due to high amounts of intercellular water. New drying methods for biosludge are hence needed. The purpose of this project is to investigate the possibility to dry biosludge in a high-velocity cyclone dryer since the technology has been shown to be able to handle sticky materials. The sludge used in this project originates from the biological wastewater treatment at the pulp and paper mill Metsä Board Husum, located in Husum, Sweden. The achieved dry matter contents and the specific energy consumption have been examined and compared with established drying methods.  Design of experiments has been used during the trials for optimizing the information attained from a limited number of trials including effects of inlet air temperature, sludge feeding rate and recycling of drying air. The respective factors were varied in three steps and relevant temperature, pressure, humidity, flow and material mass values were measured for analysing the process.  Models for attained dry matter content, specific energy consumption (total and of the fan), removed water and fan power were developed and had high statistical significance. The models showed that sludge dry matter content up to 80% could be achieved. The total specific energy consumption was over 1.6 kWh/kg H2O, which is higher than for established drying methods, but the specific energy consumption of the fan was as low as 0.7 kWh/kg H2O. The specific energy consumption of the fan was used to approximate dryer operation with excess heat during which it could compete with all established drying methods. A case where the excess heat available at Metsä Board Husum would be used to dry generated biosludge was investigated and supported a need for a priority order between maximizing sludge dry matter content and minimizing specific energy consumption of sludge drying. If sludge dry matter content is prioritised alternative drying methods may prove more suitable. To attain a better understanding of the high-velocity cyclone dryer and drying of biosludge the evaporation energy of sludge at different dry matter contents, the effects of various sludge inflow temperatures and the effect of fan rotation speed should be included future investigations.