Study On Optimization Of Flow Field In Improved Oxidation Ditch And Preparation Of Building Materials From Sewage Sludge Based On Energy Saving And Emission Reduction

In the background of the policy"Energy saving and emission reduction"of the national eleventh five-year plan, many issues in municipal wastewater treatment plants have been focused on, such as high energy consumption, difficult dispose of sewage sludge and so on. The thesis took oxidation ditch process of the second-stage of Pingdingshan wastewater treatment plant as object of study and made a tentative exploration for energy saving and emission reduction of wastewater treatment plant from two aspects as below:(1) Optimization of flow field of oxidation ditch was studied by using computational fluid dynamics (CFD) modeling technology and appropriate models for simulation of disc aerators and submerged impellers of oxidation ditch were chosen. By comparison of field data and simulation results, it indicated that the optimized operation was reasonable.Because there are 13 groups of disc aerators (each group contains 45 discs) and 9 groups of submerged impellers in the oxidation ditch, it brings about complexity in simulation run. The thesis applied moving wall model and fan model to simulate disc aerators and submerged impellers for the first time, and the simulation of flow field in oxidation ditch was carried out finally. By comparison of field data and simulation results, the simulated results in some places were smaller than the field data, but the whole trend of flow field matched well.The following two conditions, existing operational condition and optimized operational condition, were simulated respectively to obtain the flow field distribution of mixed liquor in the oxidation ditch of Pingdingshan sewage treatment plant. The simulation result indicates good consistency with the measured data. Based on the simulated and measured results, following suggestions are proposed for energy saving: in existing operating condition, 9 disc aerators and 3 submerged impellers are in operation and in optimized operating condition, 6 disc aerators and 3 submerged impellers are in operation. According to the analysis of simulated and measured results, it can save 98.43 kWh per hour in optimized operating condition than existing operating condition, and it saves about 28.5% energy.(2) Preparation of building materials from sewage sludge based on skeleton builder was proposed for solving the issue of high water content and difficult dewatering of sludge, the problem of sludge compression due to high organic content was solved effectively, and the feasibility of autoclave brick made by dewatered sludge was discussed.The dewatering technology for sludge based on skeleton builder effectively solved the problem of sludge compression due to high organic content. The result showed that the optimized formula of using fly ash and calcined lime as conditioner to sludge dewatering is add 5 g fly ash and 5 g calcined lime into 100 mL wet sludge, and the specific resistance of sludge decreased to 10 7s2/g after conditioning and the water content of sludge decreased to about 45% after dewatering.The thesis explored recycling process based on skeleton builder, carried out the experiment of making baked brick and autoclave brick by dewatered sludge and analysed strength and durability of bricks by using XRD, SEM and other methods. Results showed that the compression strength and freezing resistance of the baked brick and fly ash-clay baked brick, which were made by adding 40% dewatered sludge into the raw material, could reach the national standard of baked stock brick (GB5101-2003).The thesis used 50% filter cake, 39% slag, 5% fly ash and 6% cement to make autoclaved brick with 20% water content, 20MPa pressure, 180℃conservation regime and 0.8 MPa steam curing for 8 hours. The strength, leaching toxicity and durability of obtained sample were tested and the strength and freezing resistance could reach the MU15 requirement of the standard of autoclaved lime-sand brick (GB11945-1999). The carbonization factor of sample was 0.83 and the sample was conducted drying and watering cycle and thermal cycle under 27℃,40℃,60℃respectively and the rupture strength can reach more than 2.5 MPa, the compression strength can reach more than 10 MPa and the strength and freezing resistance can also reach the MU15 requirement of the standard of autoclaved lime-sand brick (GB11945-1999). The analysis of XRD and SEM indicated that the main phases of sludge autoclaved brick are CSH gel, granular hydrogarnet, mullite, quartz and calcite, especially CSH gel and granular hydrogarnet are main contributors to the strength. Drying and watering cycle and thermal cycle in different temperatures have a certain impact on the phases of sample.In conclusion, the thesis studied the optimized operation of flow field in oxidation ditch by using CFD modeling technology, explored the feasibility of baked brick and autoclaved brick made by dewatered filter cake through the technology of sludge dewatering based on skeleton builder and recycling of filter cake. The thesis explored the optimization of energy consumption, sludge dewatering deposal-recycling for building materials for second-stage oxidation ditch in Pingdingshan wastewater treatment plant, and the results have reference value and meaning of guidance for promotion of energy saving and emission reduction in other wastewater treatment plants in our country.