Reasearch On Simulation Of The Flow Pattern And Designing Optimization Of The High-sludge Sedimentation Tank In Water Treatment Plants

At present, as a representative of "sludge recycling" settlers, High-sludge sedimentation tank has a notably positive perspective of various applications. It is suitable for areas where land resources are scarce, where the raw water quality altered considerably, where the standard of outlet water is rigid, or where condensation and dehydration of produced sludge is demanded. Therefore, the study on optimization of High-sludge sedimentation tank design has important practical significance in the development of water treatment process in China.In this study, based on a projected major waterworks in Chongqing, the optimization of the High-sludge sedimentation tank is studied and organized as follows:①The domestic and international researches and related issues on settlers are summarized and reviewed;②Work principle, operation processes, advantages and application prospects of High-sludge sedimentation tank are analyzed;③Based a projected Waterworks in Chongqing, an appropriate numerical model have been built to simulat the flow pattern in the settling zone of High-sludge sedimentation tank by utilizing FLUENT. The simulation result suggests that the flow pattern in the settling zone of empirical designed High-sludge sedimentation tank is less than ideal, which evolves density current, short-flow, vortex and backward flow;④According to similarity criterion, a physical model has been built, which validated the flow pattern in the settling zone of High-sludge sedimentation tank simulated by FLUENT on comparing the flow consistency between the numerical model and the physical model.⑤The design of the settling zone of High-sludge sedimentation tank is optimized. Optimal investigations have been taken with respect to the following 3 solutions: increasing inlet flow, extending the length of the baffle and placing an appropriate bar screen at the inlet of the pre-settling- thickening zone. By applying average velocity and average kinetic energy value of turbulence at the section of outlet water as the assessment indicators, combined with analysis on flow charts and velocity distribution at the section of outlet water, these 3 solutions are verified to effectively improve the flow pattern of the sedimentation zone of High-sludge sedimentation tank. Finally, the correlations between flow pattern and optimization factors of these solutions are concluded as following:1) Density current can be effectively eliminated by increasing inlet water velocity. It is illustrated in the experiment that density current vanishes when the inlet water velocity at the settling zone reaches 0.025m/s. It is also suggested that the inlet water velocity should be retrained within a certain range: stagnant velocity results in density current; while highly kinetic water tends to convey alum, consequently increasing turbidity in outlet water.2) Improvement on short-flow can be realized by adding a baffle before lamellae zone. This study's result demonstrated that there is a negative correlation between the length of the baffle and the average water velocity at the section of outlet water. However, when the length of the baffle exceeds 1.0m, the tank bottom would be flushed by reflected inlet water, which has adverse effect on sludge concentration. In addition, outlet water turbidity would be severed since upper layer alum tends to be brought into clean water zone.3) The backward flow in settling zone of High-sludge sedimentation tank can be alleviated by adding a bar screen at the inlet of pre-settling- thickening zone. This study concludes that while maintaining the integrity of alum, the denser and finer the bar screen is, the more desirable uniformity of inlet water flowing into pre-settling-thickening zone would be achieved,accordingly the more effectively backward flow would be eradicated。⑥By analysis and comparison of outlet pattern of lamellae area, the design of lamellae area in settling zone is optimized. And a conclusion is drawn that the aperture water manger is most ideal pattern.The conclusions and experimental results of this study can be implemented into future design of High-sludge sedimentation tank in waterworks, thus the performances of High-sludge sedimentation tank could be enhanced in economy, efficiency and adaptability prospects.