Ultrahigh pressure filtration dewatering of municipal sludge based on microwave pretreatment.

Municipal sludge is difficult to treat and dispose of because of its high moisture content (MC) and volume. In this study, a novel dewatering method that utilizes ultrahigh pressure filtration (UHPF) and thin-cake-press (TCP) coupled with microwave pretreatment was proposed to reduce the MC of sludge cake. The influence of microwave contact time (MCT), microwave intensity (MI), initial MC of sludge, dewatering time, applied pressure and sludge weight (Ws) on the MC of a sludge cake was investigated by the single-factor experiment. Moreover, a water discharge path model was developed to understand the dewatering mechanism and explain the relationship between thickness and MC of the cake. The key factors affecting the MC of the cake were explored by the orthogonal experiment. The experimental results showed that microwave irradiation could effectively improve the dewatering performance and reduce the MC of the sludge cake. The MC of the cake reached its lowest value of 28% at MCT of 120 s and MI of 400 W, which is much lower than the value obtained by the traditional dewatering method. Among the parameters mentioned above, Ws has the most significant influence on MC because a large amount of sludge leads to a thicker cake, which seriously hampers the flow of filtrate and inevitably increases the MC of the cake.

Pore-scale model and dewatering performance of municipal sludge by ultrahigh pressurized electro-dewatering with constant voltage gradient.

The disposal of huge municipal sludge with high moisture content has led to numerous energy consumption and brought extensive concerns in the world. In this paper, three dewatering modes, ultrahigh-pressure mechanical dewatering mode (UMDW), pressurized electro-dewatering (PEDW) with constant voltage mode (U-PEDW) and constant voltage gradient mode (G-PEDW) were performed on a self-designed pressurized electro-dewatering apparatus for municipal sludge. The pore structures and moisture distributions were detected by low-field nuclear magnetic resonance technology. Meanwhile, the moisture distributions and quantitative bound strength were analyzed by the thermogravimetric differential scanning calorimetry test. Moreover, a pore-scale electro-osmosis model was accordingly developed based on the fractal characteristics of pore size distribution. Finally, the effect of dewatering modes and operating parameters on moisture content and energy consumption were examined in detail. The results indicate that the pore-scale electro-osmosis model show good consistency with experimental data. The electric field can drive the middle-layers-water to overcome the capillary pressure, and make G-PEDW removing more water than UMDW. The moisture content of dewatered municipal sludge by G-PEDW and U-PEDW reaches to 28.41% and 27.33%, respectively. Furthermore, the energy consumption of G-PEDW is 189.62Wh/kg.H2O, it is much lower than that of U-PEDW. Therefore, the G-PEDW mode with low moisture content and less energy consumption indicates best dewatering performance compared with UMDW and U-PEDW modes. The present work is helpful to understand the dewatering mechanisms of G-PEDW and provides useful guidelines for G-PEDW dewatering technology.

Meso-mechanism of mechanical dewatering of municipal sludge based on low-field nuclear magnetic resonance.

Both huge volume and high moisture content of municipal sludge have brought great troubles and attracted extensive concerns in the world. In this paper, the mechanical press filtration (MPF) dewatering was performed under ultrahigh pressure in order to improve the dewatering performance of municipal sludge. Low-Field Nuclear Magnetic Resonance (NMR) technique was used to study the effect of MPF parameters on dewatering performance. Based on the pore characteristics of municipal sludge, a capillary bundle model was developed to explore the mesoscopic mechanisms of MPF dewatering. The results indicate that moisture content of sludge cake decreases gradually with the increase of compressed pressure and dewatering time as well as the decrease of sludge weight , and the moisture content of municipal sludge can be reduced to as low as 30% with MPF dewatering. According to the peak and envelope area of relaxation time curve in NMR, it can be found that the size of pores decreases and the percentage of small pores increases during the dewatering process. Therefore, the capillary water cannot be removed by MPF method because the capillary pressure in very small pores increases evidently. Furthermore, the compaction degree of the outmost layer is generally higher than that of the middle layer, and the difference of moisture content between the middle and outmost layers of cake is 25%-28.4%. The present work may be helpful to understand the MPF dewatering mechanism and shed light on the new dewatering techniques of municipal sludge.