Study On Optimization Of Oxidation Ditch By CFD And Experimental Testing

Oxidation ditch (OD) has been widely used as a modified activated sludge biological treatment process in China due to its reliability and good effluent quality. However, successful operation of an OD system may still be challenging for many wastewater treatment plants (WWTPs) in China for economical reason mainly caused by large energy consumption. Since the process efficiency depends heavily on the flow field and dissolved oxygen (DO) concentration profiles in an OD, which also have close relationship with energy consumption, it is necessary to study on flow pattern and oxygen mass transfer characteristics in OD systems. In this study, an experimentally-validated model based on computational fluid dynamics (CFD) simulation technology was proposed to predict flow patterns and oxygen mass transfer characteristics in a lab-scale OD and a full-scale OD in Ping Dingshan WWTP. Full-scale demonstration of system performance under two operating conditions (existing and improved) of the full-scale OD in Ping Dingshan WWTP was carried out and effects of these two operating conditions on effluent quality and energy consumption were compared. Based on the results of the research, an improving operating condition of the full-scale OD, which could not only reduce energy consumption but also satisfy effluent standards, was proposed.The following research work was carried out in this thesis:1. The flow field of the lab-scale OD (0.73 m~3) was simulated and validated for verification and optimization of simulation methods of disc aerators and submerged impellers. The result demonstrated that the flow field of the lab-scale OD with disc aerators and submerged impellers operationg could be simulated well by moving wall model and fan model.2. The flow fields of the full-scale OD (50000 m~3) under two operating conditions (existing and improved) were compared and analyzed. Several suggestions for energy saving were proposed as follows: an operating disc aerator in the entrance of the curve bend could be instead by an operating submerged impeller in the curve bend; the adjacent disc aerators could not be in operation simultaneously.3. Oxygen mass transfer in the typical straight channel of the full-scale OD was predicted by using a unit analysis method and the oxygen consumption was modeled by using modified BOD-DO Model. DO concentrations were measured at two test locations (in the aerobic zone and the anoxic zone) under the existing and improved operating conditions, respectively. Based on the simulation results and field measurements, the operating condition of surface aerators had a significant impact on the DO concentration profiles in the ditch. Under the existing operating condition, the DO concentrations were relatively high that caused waste of energy and the DO concentration gradients were not obvious; while under the improved operating condition, the DO concentrations showed apparent variations along the ditch and provided a more suitable environment for simultaneous nitrification and denitrification (SND).4. A comparison on the performance of two operating conditions was carried out in these two full-scale ODs concurrently for a month. The influent quality and effluent quality were monitored daily and energy consumptions for two operating conditions were calculated. The total energy consumption per hour under the improved operating condition is 98.43 kWh (28.5%) less than that under the existing operating condition.Since the influent flow rate, concentration and composition are likely to fluctuate day to day, an optimal operating condition should be derived from a dynamic model updated to accommodate actual process state and perturbations. The development of such a model is currently under investigation.