Numerical Simulation And Optimization Of Rectangular Secondary Sedimentation Tanks With Peripheral Inlet And Outlet Based On CFD

In this paper, a small model was created in order to use PIV(particle image velocimetry) method to analyse the flow field distribution of rectangular sedimentation tanks. The experimental data proved the feasibility of Mixture two-phase flow model and RNG k-? turbulence model in rectangular sedimentation tanks. The validated model was then applied to the rectangular sedimentation tank in Hedong Sewage Treatment Plant in Xiangtan, Hunan. Computational fluid dynamics(CFD) was used for the three-dimensional numerical of simulation two-phase flow of the tank. The distribution of flow regime and suspended particles was analyzed and compared with that of common circular settling tank with the same flow rate and surface load. The surface load and sludge concentration of inflow was adjusted in order to figure out its suitable range. Numerical simulation were done respectively on different parameters(length of inlet and outlet b affles, reflux ratio and diameter of suspended particles, etc.), solutions to optimize and improve the operation of the plant were proposed after analyzing the distribution of flow regime and sludge concentration in the tank.Results from the numerical simulation tests showed that there were usually 3 circumfluence areas in rectangular sedimentation tank: under the inlet baffles, the middle of the tank and behind the inlet baffles. The existence of these circumfluence areas reduced the effective sedimentation volume of the settling tank, and therefore was unfavorable for the settling of suspended particles. On the other hand, obvious stratification of the suspended particles was observed in the settling tank, which increased the removal rate of suspended pa rticles and better purified-water could be obtained.It is compared with the traditional circular settling tank with the same designed flow rate and surface load. Because of the consistency of the water-crossing section area of the rectangular sedimentati on tank, the uniformity of velocity distribution increased and the flow regime gets better. The effectiveness is thus better than circular settling tank. Moreover, when dealing with the same amount of sewage, rectangular sedimentation tank required 30% les s of volume, and its operation was easier comparing with that of circular settling tank, thus had higher economic efficiency.Without changing other parameters, numerical simulation tests were done with increased surface load and sludge concentration of in flow respectively. The results indicated that rectangular sedimentation tank had very high impact resistance: when surface load was increased for 30%(1.365), or sludge concentration of inflow raised to 4500mg/L, the standard of the GB18918-2002 could still be met.Without changing other parameters, various lengths of inlet baffles were used for numerical simulation to calculate the volume fraction of suspended particles at the outlet in order to study its impact on ef Fluent sludge concentration and removal rate of rectangular sedimentation tank. The results showed that the removal rate of suspended particles increased when the inlet baffles were lengthened. After comprehensive comparison, the best-fit length for inlet baffles of rectangular sedimentation tank is 1.5m.Without changing other parameters, various recirculation ratio were used for numerical simulation to study its impact on ef Fluent sludge concentration. The results demonstrated that within certain range, increasing recirculation ratio enhance d flocculation, therefore increased the removal rate of suspended particles. However, the removal rate decreased instead of increased when recirculation ratio went higher than 100%. After comprehensive comparison, the recirculation ratio of rectangular sedimentation tank should be set between 80% and 100%.Without changing other parameters, suspended particles with various sizes were used to study its impact on sedimentation. The results of simulation tests showed that the minimum diameter of suspended part icles required for desired separation is 100?m. In actual operation of rectangular sedimentation tank, in order to get desired separation, the size of suspended particles can be enlarged by adjusting the recirculation ratio or adding flocculant if particle size of inflow is too small.