Membrane Dehumidification Technology for Removing Water Vapor from Exhaust Gas in Sludge Drying Process: Full-scale Application and Membrane Autopsy

Abstract Manufacturing solid fuels by drying sewage sludge is an effective method for increasing the production of renewable energy sources; however, the existing technology presents considerable economic challenges owing to high energy consumptions of the drying process. Here, an innovative sludge-drying process that combines microwave drying and membrane dehumidification technologies, using a gas-separation membrane, was developed and applied for the first time. Additionally, a membrane autopsy study was conducted on the dehumidification membranes. The 10 t/d system was continuously operated, and after the stabilization period, a highly efficient sludge drying process was observed; the consumed energy for drying was maintained at ≤ 650 kcal/kg water. Mean water vapor removal ratio in the membrane dehumidifier and dry air circulation rate was 82.2% and 85.2%, respectively. After 80 d of continuous operation, membrane autopsy was done to compare the performances of the contaminated and virgin membrane modules. We observed that long-term operation did not induce any significant change in membrane performance, except for slight organic contamination; this is a very meaningful result. Thus, we showed and verified that even under specific adverse conditions during the sludge-drying process, the combined membrane drying process was effective during long-term operation.