Preparation of binder-less activated char briquettes from pyrolysis of sewage sludge for liquid-phase adsorption of methylene blue.

Binder-less activated char briquettes from sewage sludge were prepared and used for the liquid-phase adsorption of methylene blue. The properties of sludge char briquettes prepared under the different initial sludge moisture content, compression pressure, and heating rate were systematically investigated through the tests of thermogravimetric analysis (TGA), scanning electron microscopy (SEM), surface and mechanical properties, burn-off rates, methylene blue adsorption kinetics and isotherms. All of the prepared briquettes presented hierarchical structures and microporous/mesoporous characteristics, and the increase of initial sludge moisture content from 10 to 30 wt% resulted in a great increase of surface area (SBET), total pore volume (VT), apparent density, and a slight decrease of mechanical performance. The decrease of compression pressure markedly enhanced the equilibrium adsorption capacity (qe, exp), owing to the decreased diffusion resistance and blockage of diffusion pathways inside briquettes. In consideration of the mechanical performance and adsorption capacity, the optimum preparation condition was obtained at the initial moisture content of 30 wt%, compression pressure of 25 MPa, and heating rate of 10 °C/min, in which the axial compressive strength (ACS) and qe, exp of the prepared briquettes were as high as 22.2 ± 3.1 kg/m2 and 316.9 mg/g. The results also showed that the equilibrium adsorption data fit well into the pseudo-first order model system, and the adsorption isotherms followed the Langmuir isotherm model, suggesting that the adsorption process was attributed to physical adsorption, and was inclined to happen on the adsorption sites with the same energy level. Finally, the thermal regeneration tests demonstrated that the binder-less briquette had a good regeneration performance and was worthy of reusing for industrial applications.

Effects of frequency and duty cycle of pulsating direct current on the electro-dewatering performance of sewage sludge.

Electro-dewatering of sewage sludge with pulsating direct current (PDC-dewatering) was conducted to investigate the effects of pulsating frequency (0.01-60 Hz) and duty cycle (DTC) (20-100%) on sludge dewatering. The results indicated that both the frequency and DTC showed marked influences on electro-dewatering. Compared with the condition under the stable direct current (SDC-dewatering) of 30 V, the filtrate discharged from PDC-dewatering (at DTC of 40-60% and frequency of 30 Hz) was about 8% higher than that from SDC-dewatering. At DTC of 40%, the sludge electro-dewatering performance was promoted when the frequency increased from 0.01 Hz to 30 Hz. Compared with SDC-dewatering, PDC-dewatering can effectively mitigate ohmic heating. Layered tests were also conducted to investigate the differences of SDC- and PDC-dewatering in the distributions of water, pH, organic matters, zeta potential and conductivity in the upper, middle and lower layer of sludge cake. The results indicated that the variation tendencies of these parameters were similar between SDC- and PDC-dewatering, but the water, organic matters and charged ions in sludge cake were more homogeneously distributed during PDC-dewatering than SDC-dewatering. In addition, the anodic pH of PDC-dewatering was higher than that of SDC-dewatering, suggesting the potential of mitigating anodic corrosion during PDC-dewatering. Finally, energy consumptions of PDC- and SDC-dewatering were calculated and compared. The effects of frequency and DTC on energy consumption were investigated. PDC-dewatering was found to be more energy efficient than that of SDC-dewatering, making PDC-dewatering a promising electro-dewatering technology in future.