Novel sludge drying process for cost-effective on-site sludge management

Pulp & Paper Canada T 93 107:4 (2006) ❘ ❘ ❘ 47 ISPOSAL OF SLUDGES produced from biological wastewater treatment facilities is costly for the pulp and paper industry. A novel biodrying process has been developed that raises the dryness of pulp and paper mill sludge to a solids level where it can be economically combusted in existing boilers [1]. The process involves an aerated biopile, in which a sludge/woodwaste mixture is dried using forced air, and where sludgedrying rates are enhanced by the exothermic metabolic activity of aerobic thermophilic microorganisms. This paper presents the results of a techno-economic study in which the performance of the biodrying process was simulated, and the capital and operating costs for the installed process were estimated. Both the batch (based on the lab work in [1]) and continuous biodrying operations are considered and compared herein. Sludge can be a viable fuel source once dried; with higher heating values (HHV) as high as 18 to 21 GJ/dry tonne (i.e., equal to or greater than the HHV of some wood species) [2,3,4]. Mills often mix their sludge with bark prior to combustion [2]. Combustion, while beneficial to the mill, requires extensive dewatering of the sludge to at least 40% dryness in order to produce an economically viable fuel source. In the biodrying process analyzed in this study, mixed dewatered sludge produced by the mill effluent treatment facilities is combined with bark as a bulking agent, and then fed to the biodrying process. After drying, the sludge is removed and sent for combustion in a biomass or hogfuel boiler.