Chemical-biological Flocculation Process Improve The Quality Of Effluent Of The Secondary Sedimentation Tank

In the system of wastewater treatment, flocculation process is one of the most universal and pivotal techniques. In the actual project, dosing coagulant to the effluent of biological tank has been applied frequently. This process not only can improve settleability of sludge of the secondary sedimentation tank, but also can improve the quality of effluent. There are coagulant and activated sludge in this flocculation process, so it is a process of chemical-biological flocculation. There have been some researches of chemical-biological flocculation now, but it was mainly adpoted as enhanced primary treatment process. Chemical-biological flocculation process which coagulant was dosed to the the effluent of biological tank was studied in this thesis to improve the quality of effluent of the secondary sedimentation tank.In this thesis, FeCl₃, PAC and PAFC were respectively taken as flocculant to conduct chemical-biological flocculation process and effluent quality to the level of the standard A was regarded as the target. Considering the economic and environmental factors, finally PAFC was choosed as the optimum coagulant, the optimum dose is 20mg/L. In order to gain the optimum hydraulic condition, orthogonal experiment was adoped. The optimal hydraulic condition is mid-stirring speed of 100rpm for 4min and slow stirring speed of 30rpm for 6min.Comparing chemical-biological flocculation process and biological flocculation process at the same condition, the removal rates of different kind of pollutants by chemical-biological flocculation process are higher than those by pure biological flocculation process. But the removal rates of TP, SS and COD by biological flocculation process were also obvious. It shows that the presence of activated sludge has cetain effect on the removal of pollutants.The flocs properties analysis shows that the mean diameter and fractal dimension of sludge flocs of chemical-biological flocculation process and biological flocculation process all increased. And the mean diameter and fractal dimension of sludge flocs of chemical-biological flocculation process are all larger than biological flocculation process. For chemical-biological flocculation process, the mean diameter is 247.078μm and fractal dimension is 1.961. For biological flocculation process, the mean diameter is 191.09μm and fractal dimension is 1.824. The floc strength and regrowth ability of chemical-biological flocculation process are better than biological flocculation process. For chemical-biological flocculation process, the floc strengh factor is 0.76 and the floc regrowth factor is 0.67. For biological flocculation process, the floc strengh factor is 0.54 and the floc regrowth factor is 0.46. With the increase of the dosege of PAFC, the partical surface Zeta potential of efflunet shifts to positive potential. But for biological flocculation process , Zeta potential do not chang.On-line monitoring the flocculation process by PDA and analysing the PDA output curve shows: The type and dosage of coagulant can affect the FSI curve. The FSI curves of PAFC and PAC are similar, there is a obvious peak at beginning, then the FSI curve decreases, eventually reach a relatively stable value. But for the FSI curve of FeCl₃, there is no obvious peak like polymer coagulant, it fluctuates but maintains a steady state at all the flocculation process. Under the same dosage, the Ratio value of PAFC is always higher than the other two coagulants, and the Ratio value of PAC is higher than FeCl₃. The Ratio value of biological flocculation process is smaller than chemical-biological flocculation process, but the flocs still have a certain growth.