Efficiency And Mechanism Of Sewage Sludge Deatering And Pyrolysis Under Treatment Of Microwave Energy

The increase in production of sewage sludge is a matter of growing concern because it has caused serious environmental pollution. The bottleneck of sludge handing system is dewatering operation, and the pyrolysis has attracted most attention in recent years since the process can improve reclamation of sludge. Since dewatering and pyrolysis were the key technologies of sludge treatment, the microwave was used in dewatering and pyrolysis of sludge with reduction, stabilization and untilization as central tasks. The dewatering improvement and the structural destruction of sludge during microwave pretreatment were investigated, the efficiency and mechanism of microwave assisted pyrolysis was probed, the effect and mechanism of heavy metals immobilization during microwave drying and pyrolysis was revealed, and the utilization of solid from microwave assisted pyrolysis was also estimated in this dissertation.The dewatering of sludge pretreated with microwave within 130s was investigated. To understand the action of microwave, the essential roles played by the particle size distributing, the concentration of exocellullar polymer (ECP) and the disruption of microbial cells in sludge need to be studied. It was observed that the dewatering efficiency was increased with the optimum pretreating of microwave radiation. When 900, 720 and 540W microwave was employed for 50, 110 and 130S respectively, the SVI30 was decreased 48%, 38% and 30% as compared with untreated sludge, the water content of aptly treated sludge dewatered with filtration reached 71%, which is lower than the 85% of untreated sludge after filtration. Further studies revealed that sludge treated by microwave energy was an effective method to remove the ECP from sludge surface and induced structure disruption of sludge flocs, which resulted in the improvement of dewatering. The 30.7% reduction of the supra-colloid number was reached when the sludge was treated by 900W microwave for 50s. And the dewatering was increased when the polysaccharide content in ECP was between 15.8 and 16.5 mg/g MLSS. The increase of DNA concentration in ECP could indicate the disruption of microbial cells in sludge flocs. When the cell wall was destroyed by excessive microwave energy, the flood of intracellular polymer induced the improvement of viscosity and the worsening of dewaterability of sludge.The drying efficiency of sludge in microwave field and immobilization of heavy metals within dried sludge was further studied. It was found that the sludge with 98% water concent could be dried by 900W microwave for 8min.. In different leaching tests and under the condication of simulated natural waters, respectively, the concentrations of heavy metals in the leachate of microwave dried sludge were all well below the concentrations of heavy metals in the leachate of conventional heated sludge. The chemical composition, crystal condition, pore structure were explored by SEM and XRD, and it was revealed that heavy metals could be fixed into the 0.075-0.75μm pores of stabilized matrices by five important immobilization procedures, crack the cell wall in the sludge flocs, pile up the fragments of cell wall, package up the heavy metals in the fragments, fixing the heavy metals, and fusing to form pores.It was also found that if just sludge was tereated by microwave, only 200℃of the sample could reach. However, if the sludge is mixed with a small amount of SiC, Fe2O3, activated carbon, and char as microwave absorberes, temperature of up to 800℃can be achieved, so the pyrolysis took place. Under appropriate operating conditions of microwave pyrolysis, the percentage of organic matter transformation was 10% higher than conventional pyrolysis, The high efficient pyrolysis could be attained.Another issue that must be considered is how safe to utilize the solid that contains heavy metals. The microwave assisted pyrolysis could decrease the volatilization of heavy metals (Cd, Pb, Zn, Cu) efficiently. And the microwave assisted pyrolysis can decrease the leaching concentration of heavy metals from solid by 80.29% more than conventional pyrolysis in TCLP test. Changing leaching conditions such as leachant, ratio of liquid to solid, pH, and under the simulated natural waters condication, respectively, the heavy metals from the solid of microwave pyrolysis exhibited a better immobilizing effect than the heavy metals from solid of conventional pyrolysis. The alkaline characteristics of solid, and heavy metals speciated mainly into a stable form were the important chemical fixation for heavy metals within solid. And the glass-like matrix and formation of microspheres in micron dimension were the physical solidification which occluded heavy metals within solid. It is therefore concluded that both physical solidification and chemical fixation were efficient for immobilization of heavy metals within solid of microwave pyrolysis.Upon the immobilization of heavy metals, the reusing methods of solid was studied. The experimental results showed that the eosin and safranine adsorption capacity of solid from microwave pyrolysis with char as micrwave absorber reached to 73.99 and 79.82 mg/g respectively. Freundich model could describe the adsorption behavior well. The solid also has the character of micrwave absorber, but the adsorption capacity of solid from the pyrolysis with reused solid as micrwave absorber would be broken by fusion and pulverization. So the reusing method of solid should been selected.Through the systemic research on efficiency and mechanism of sludge dewatering and pyrolysis under microwave energy, it can be determined that the microwave have advantages on improving the dewaterability of sludge, assisting the efficient pyrolysis of sludge, facilitating the immobilization of heavy metals, and conducting the untilization of solid. So the inveatigation of dewatering and pyrolysis under microwave energy provided an new approach to realize the stabilization, reduction and utilization of sludge.