| With the rapid development of sewage treatment facilities,the amount of sludge has been rising and the toxic and hazardous substances contained in sludge can lead to serious environmental pollution if the sludge can be handled inproperly.Based on the wide application of membrane technology in the field of wastewater treatment,the high efficiency of solid-liquid separation of membranes has been gradually applied to sludge treatment.However,due to the high concentration of sludge and the serious membrane fouling in the process of using membrane for simultaneous sludge thickening and digestion(MSTD),hindered the practical application of membrane technology.In this paper,a novel sludge treatment process-the membrane coupled rotatable electrodes for simultaneous thickening and digestion of sludge(MRESTD),which combined the electric field with MSTD technology,was investigated the feasibility of MRESTD,so as to provide a new idea for the sludge treatment.First,in this study,the effect of electric field on the properties of sludge was investigated,the surface morphology and particle size of sludge before and after electrolysis was also analyzed.The experimental results show that the CST of sludge is shortened,sludge dewatering performance better and SCOD value rise under the conditions of voltage of 30 V,anode material of titanium plate,initial concentration of sludge of 6 g/L and pH of 10,indicating that the appropriate electric field conditions can improve the sludge dehydration,lysis and the stability.At the same time,through the microscope analysis of the surface morphology and the change of particle size of the sludge,it was found that there was a large sludge flocs group without the electric field.The sludge agglomeration was obvious and the flocs increased,the particle size of the sludge rises slightly after 60 min.Also,the flocs of sludge gradually crumbled,and the original mass-like material was broken indicating that the continuous action of electric field would destroy the colloid of sludge.The anti-fouling of conductive microfiltration membrane under different voltage and sludge concentrations was also studied.According to the experimental results in the third chapter,the critical flux of the conductive film measured at 30 V is 10-12 L/(m~2·h)and30-34 L/(m~2·h)without electric field,indicating that 30 V voltage aggravated the degree of membrane fouling.In the condition of 10 V,it was found that the transmembrane pressure increased more rapidly in the electrified group than without electricity.The main reasons as follows:1)The pH value of the sludge mixture and the negatively charged particles gradually decrease under high voltage condition,while the cathode membrane attracts the positively charged particles and the particles continue to deposit on the membrane surface to aggravate the membrane pollution.2)Voltage conditions have exceeded the critical field of the conductive microfiltration membrane,the sludge particles in the mixture continue to gather in the membrane surface,membrane fouling intensified.In the study of different sludge concentration and voltage gradient of the conductive film critical flux changes,it was found that as the sludge concentration increases,the voltage value that influence the conductive film critical flux increases gradually,the sludge concentration from 10 g/L rises to 30 g/L,the voltage increases from 3 V to 6 V,but the critical flux does not rise during the entire experiment.Finally,the paper continued to explore the voltage and rotation speed conditions of the MRESTD.The experimental results showed that when the voltages were 1 V,2 V and 3 V,the membrane pressure was slightly lower than 0 V condition,the conductive film showed some anti-fouling properties.However,the EDS spectra at different voltages showed that the higher voltage,the more metal ions adsorbed on the membrane surface,the greater impact on the membrane.As the rotation speed,it was found that compared to 0 rmp/min,under conditions of 30 rmp/min and 60 rmp/min,the membrane pressure is slightly lower and the current at 60rmp/min is relatively higher,so the optimal voltage and speed conditions for the MRESTD process are 1 V and 60 rmp/min.To further verify the feasibility of the process,the long term operation of MRESTD process for 15 days is monitored with the reference of MSTD.The experiment found that MRESTD process showed good anti-pollution,the TMP of the MSTD rose to 20 kPa,while the MRESTD was only 6.5 kPa at the end of the experiment,;while the MLSS of the MRESTD and MSTD increased from the initial 2-3g/L to 36 g/L,MLVSS also increased to about 28.13 g/L.The cumulative digestion rates of MLSS and MLVSS in MRESTD process reached 53.09%and 45.37%,respectively,the MSTD process reached49.86%and 38.60%,indicated that the MRESTD and MSTD process can be used to simultaneously thicken and digest activated sludge.In contrast,the sludge digestion rate of MRESTD process was slightly higher.In the digestion mechanism of sludge,the DO of the MRESTD and MSTD decreased from 4.7 mg/L to 0.8 mg/L within 15 days of operation,sludge is digested by aerobic digestion in the first stage and microaerobic in the end.Also,the SCOD content of MRESTD was slightly higher than the MSTD,indicating that the cell disruption increased and release of organic matter with electric field,providing a source of energy for microorganisms and improve sludge digestibility further;MRESTD anti-pollution mechanism mainly includes the role of the external electric field so that there is electrostatic repulsion between the pollutants in the sludge mixture and the cathode membrane,and the effect of the applied electric field on the properties of the sludge mixture is also affected.The viscosity of the sludge and the slightly decrease of EPS and SMP may be beneficial to the delay of membrane fouling. |
