Research On Settling Performance Of Activated Sludge In Rectangular Sedimentation Tank

Rectangular sedimentation tank has many advantages, such as simple constitution, low cost, and better adaptability to the change of influent flow rate. It also can be operated and managed conveniently. So it was widely applied in portable water treatment and primary sedimentation in municipal waste water treatment. When rectangular sedimentation tank is used in secondary sedimentation, influent density current will be formed, because of concentration difference or temperature difference between influent and effluent. How to make use of advantages and overcome disadvantages of circumfluence has become the key to improve treatment efficiency of secondary sedimentation. Firstly, factors affecting the form of density current in sedimentation tank were analyzed. Secondly, hydraulic model which based on similarity guideline was calculated and computational fluid dynamics (CFD) software was used to make a high accuracy numerical simulation of the flow field in the secondary settling tank. Finally, tracer experiment and settling experiment were carried out which used activated sludge. The results of the research are:(1) Sedimentation of activated sludge is a typical density current sedimentation in secondary settling tank. Formation of density current is due to the density difference which is caused by temperature or concentration difference between influent and water in settling tank. Between 10 ℃ and 20 ℃, when the water temperature increases 1℃, the water density decreases 0.16kg/m~3. Between 20 ℃ and 30 ℃, when the water temperature increases 1℃, the water density decreases 0.26kg/m~3. The sludge density increases 0.6 kg/m~3 when the influent sludge concentration increases 1g/L.The influent sludge concentration produces effect on density is greater than temperature. (2) By using computational fluid dynamics simulation software to simulate the flow field in the secondary settling tank. The result indicates that there is a typical recirculation region in rectangular sedimentation tank, and the recirculation region occupies 2/3 of the whole tank volume.(3) The result of tracer experiment indicates that activated sludge can form obvious density current which match up to the result of numerical simulation. After observing the tracer experiment, found that the head of density current will not be broken more easily and the diameter of recirculation will be bigger because of the increasing of influent flow.(4) The result of activated sludge sedimentation experiment indicates that the average effluent concentration at front of the settling tank was lower 10.35mg/L then that at end of the settling tank. It is a good method to collect water at the front-end of settling tank, and the effluent quality will be improved.