Thermal and non-thermal mechanism of microwave irradiation on moisture content reduction of municipal sludge

Both huge volume and high moisture content of municipal sludge have brought great troubles and attracted extensive concerns in the world. The bound water of sludge and pore clogging in the process of pressure filtration dewatering are two key factors influencing the deep-dewatering effect of municipal sludge. The results of this study suggest that microwave irradiation (MI) can be used to synchronously evaporate water, reduce the bound water of sludge and solve the blockage of sludge pore channels during the process filtration dewatering, which can greatly reduce the moisture content of municipal sludge. Low-field nuclear magnetic resonance, carbon dioxide gas absorption and desorption, and scanning electron microscope were synthetically used to detect the pore structure of sludge cakes. Thermogravimetric differential scanning calorimetry was used to detect binding energy to calculate moisture distribution. Thermal mechanism was revealed by dielectric constant, thermal conductivity, fractal dimensions, water saturation and porosity in different layers of sludge cake. The results showed that the bound water in sludge samples after MI was reduced by 19.4%, the pore fractal dimension was reduced from 2.955 to 2.867, the surface fractal dimension increased from 2.099 to 2.362 with the conditions of microwave power (700 w) and treat time (5 min). When the water saturation was close to 1, the dielectric constant of middle and outer layers was the main factor of heating, while the water saturation was close to 0, the main factor became the difference of thermal conductivity in middle and outer layers. Moreover, after MI, moisture content of sludge was greatly reduced from 68.4% to 40.1% for its thermal effect, further reduced to 34.1% by the continuous mechanical dewatering for its non-thermal effect which change the pore structure and moisture distribution. These conclusions are helpful to understand microwave thermal and non-thermal mechanism and shed light on new dewatering-drying integrated techniques of municipal sludge.