Numerical Modeling And Optimization Of Secondary Clarifiers

Secondary Clarifiers are of crucial importance in wastewater treatment systems, particularly in activated sludge systems. A lot of unstable factors exist in the operation of secondary clarifiers due to the reasons such as poor settling properties, denitrification, and turbulence, short-circuiting, density current which occur in the interior of secondary clarifiers. This is true in a wide spectrum of operating conditions. During severe hydraulic loading the clarifier becomes the bottle-neck. In this paper, the main purpose is using modeling concepts to realize the optimal operation, control and design of secondary clarifiers, and to enhance the understanding of the characters of secondary clarifiers.In the first chapter, a review of the history of model development was made, including the initial concept models, one-dimensional layer models, and two or three dimensional computational fluid models. The breakthroughs and shortcomings of each stage of the history have been discussed. The one-dimensional layer models were analyzed from numerical point and the comparison between present models was made from practical point. According to the discussion in this chapter, the potential of model development was presented.In the second chapter, by integrating hydraulic propagation model and secondary clarifier model, the paper made a preliminary investigation and comparison on the effects of activated sludge system by three return sludge control strategies under the severe hydraulic condition. Using the MATLAB programming, we got the results of the changes on the inflow and outflow in the plants and the variation of the suspended solid concentration in the secondary clarifier. The results showed that by the strategies of keeping the return sludge flow constant and controlling the return sludge flow proportional to the influent flow of secondary clarifier, the activated sludge system maintained relatively stably, further the latter showed the best on improving the secondary clarifier to ease the hydraulic impact. While by the strategy of controlling the return sludge flow proportional to the influent flow of activated sludge system presented worse performance.In the third chapter, a modified one-dimensional layer model has been proposed. In the new model, the bio-reaction source was introduced by integratingwith the activated sludge model and written as a stoichiometric equation, thus the variation of the components in the tank can be accurately defined. Even though the accuracy of the model has been proved by the data from full-scale wastewater treatment plants, too many calibrated parameters seem to be the bottleneck of model, especially on online conditions. A proper simplification of the model should be made for the practical purpose.In the fourth chapter, the main topic is the on-line control and diagnosis of secondary clarifiers. The selection of controlling parameters was discussed, and the grey relation analysis method was used to compare the different influences of controlling parameters on the settling properties. Furthermore, this chapter also made a preliminary analysis of the time variation on different controlling parameters and the difficulties on collecting and choosing on-line data.In the fifth chapter, the application of the computational fluid dynamic (CFD) methods and software on secondary clarifiers was introduced. The basic methods and ideas of using CFD methods were recommended. The development and principles of CFD software was presented. Also the modeling method based on CFD was used to evaluate a modified secondary clarifier.Finally the achievements and the further researches on secondary clarifiers were summarized.