Energy efficient sludge solubilization by microwave irradiation under carbon nanotube (CNT)-coated condition

Microwaves (MW) have great potential for sludge solubilization, and carbon materials can act as good microwave absorbers and heat transfer media because of their high dielectric loss tangent and thermal conductivity. In this study, carbon nanotube-coated MW vessels were developed by preparing a silane-CNT mixture and spray coating. In addition, sludge solubilization by microwave irradiation was performed to evaluate the effects of the CNT-coating at different initial total suspended solid (TSS) concentrations, target temperatures, and MW irradiation times in the uncoated and CNT-coated MW vessels. The sludge solubilization efficiency increased with increasing MW irradiation time and temperature and followed a first-order reaction in both vessels. However, the energy requirement to maintain the temperature was reduced in the CNT-coated MW vessel compared to the uncoated vessel. In addition, the Arrhenius equation revealed the catalytic site in the CNT-coated MW vessel to have a temperature of around 130 °C at an average sludge temperature of 100 °C. The maximum chemical oxygen demand (COD) solubilization and soluble COD (sCOD) increase per MW energy used were 1.64 and 1.67 times higher in the CNT-coated MW vessel than in the uncoated vessel, respectively. The increase in soluble total nitrogen and phosphorus in the CNT-coated MW vessel was attributed to cell wall destruction and intracellular protoplast dissolution, because of the acceleration of the MW thermal effect and high conductivity of CNTs, as well as the MW-induced cell wall and membrane disruption by hot spots on the CNT surface. This suggests that CNTs can be applied to increase the energy efficiency in MW-based pretreatment methods.