Optimization of sludge dewatering process by inorganic conditioners under mild thermal treatment

Conditioning sewage sludge with FeCl3 and steel slag under mild temperature has been proved to be an effective mean to enhancing the dewaterability. The Box-Behnken experimental design and response surface methodology (RSM) were chosen to evaluate the combined effects of the three independent variables. Capillary suction time (CST) was used to characterize sludge dewatering. Zeta potential, Fourier transformed infrared spectroscopy, and scanning electronic microscopy (SEM) were employed to explore influencing mechanisms. The results show that after RSM optimization, CST could drop to 7.8 +/- 0.9s at FeCl3=5.89mg/g, steel slag=48.25mg/g, and T=52.06 degrees C. An increase in temperature combined with conditioners led to decrease in zeta potential. The variation in functional groups implied that conditioners interacted with the component of sludge during the hydrothermal treatment, SEM images revealed the rupture of sludge flocs, and formation of block structures as temperature increases. This work offered a new understanding to the role of temperature and conditioners in improving sludge dewaterability.