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

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/day 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 days 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 and inorganic (Al) contamination. 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.