Study On Influence Mechanism Of The Settling Performance Of The Circular Secondary Clarifier In Sewage Plants

As the main structures of the clarification process, the Circular Secondary Clarifier’s treatment effect is influenced by many factors, resulting in bad sedimental and mud water separation effect. This thesis studies the flocculating settling characteristics of water mixture and the influences of flow regime and density current on effluent quality with the the Circular Secondary Clarifier of Hefei Wang Tang Sewage Plant as an example. It aims at the problems of Secondary Clarifier, and put forwards effective improvement measures.The thesis first studies the flocculating settling characteristics of the water mixture in a Secondary Clarifier. Results show that the effluent SS values increased with the water mixture of the SVI value increasing. When the SVI value reaches180mL/g, the effluent SS value surpasses the point of20mg/L. In winter time, the SVI value of the water mixture is between130mL/g and230mL/g; the SV value of free sedimenting for2hours is less than50%; activated sludge’s flocculating settling characteristics are poor.In order to improve the poor flocculating settling characteristics of the water mixture in the Secondary Clarifier, the analysis employs a mixing test with low G value. The test probes into the influences of different mixing G value on settling velocity and particle size of activated sludge as well as the water quality. The result shows that:(1) Mixing test with low G value can effectively improve the settling velocity of activated sludge, and shorten the settling time. The G value which gives better settlement effect is0.035s-1～0.076s-1, and the GT value is13.68～68.4. Among them, the muddy surface settling the fastest when the G value is0.054s-1and the GT value32.4. High G value and low G value will result in decline of the settling velocity.(2) Stirring at appropriate low G value can effectively promote the flocculation of particles. The most appropriate mixing G values is0.054s-1～0.076s-1, and the floc particle diameter can be up to170um. Oversized G value will lead to floc fragmentation, resulting in an increase in the supernatant of small and medium size floc. When the G value reaches0.215s-1, the supernatant’s SS index is beyond A standard, and the SS index goes over B standard with the G value increased to0.215s-1Through analyzing the water flow regime, suspended sediment concentration and particle diameter in the the Circular Secondary Clarifier, the result shows that the density current caused by high-density water mixture leads to uneven distribution of the flow velocity and velocity gradient in the Secondary Clarifier, thus, limiting the use of the effective volume of the Secondary Clarifier and affecting the flocculating setting of the activated sludge. In order to improve the stability of the water flow and prevent the occurrence of density currents, this thesis presents additional horizontal baffles in the bottom of the central cylinder to improve the pool flow regime. It also studies the the design of draft tube dimensions. The result shows that the draft tube’s diameter of the the Circular Secondary Clarifier in Hefei Wang Tang Sewage Plant is2.978m, and best flow regime can be get when the distance of the bottom of the draft tube from the horizontal baffle is0.619m.