Studies On Hydrodynamic Characteristics And Functions Of Secondary Clarifier In Activated Sludge System

As a necessary unit in activated sludge system, secondary clarifier plays an important role in sewage treatment process. Experimental method and numerical simulation were both used in this paper. Distribution of flow velocity, SS, N03--N, NO2--N and P043--P in secondary clarifier were monitored. The flow pattern, sludge settling characteristics and the function of biological reaction were studied. The results indicated that the range of flow velocity in secondary clarifier was1.63e-3-38.74e-3m/s. The flow velocity was very small (1.63e-3m/s to28.09e-3m/s). The feed and takeoff style of the clarifier had great influences on flow velocity distribution in different positions. According to SS distribution, the secondary clarifier was divided into quarters:depth above1.59m called settling section, depth between1.59m-2.39m belongs to flocculation zone, depth between2.39m～3.99m belongs to stratified sedimentation zone, and depth below1.59m called compression settling zone, in which the sludge concentration was the highest. The existence of biological denitrification was confirmed and secondary clarifier could remove nitrogen.On the basis of experimental research results, the mathematic model of secondary clarifier was studied. One-dimensional of denitrification model of secondary clarifier was created, considering biological denitrification reaction. Compared with the actual results, the effectiveness of this model was verified. It was confirmed that denitrifying bacteria maximum specific growth rate values in this study should be0.2d-1.The two-dimensional model of the clarifier was also studied. Fluent, a popular software for computational fluid dynamics was used to simulate the flow pattern inside of the peripheral feed and takeoff secondary clarifier, with sludge discharged in different ways and volumes. According to the actual sludge discharge way of the clarifier, three different geometric models were constructed with three different kinds of sludge exports. Studies showed that when sludge discharging through round holes that uniformdistributed at the bottom of the clarifier, the average simulation error betweensimulation values and the actual velocity magnitudes was minimum, and the simulationwas the most accurate. With the sludge emissions increased, fluid could flow muchmore widely inside of the clarifier, and the flow pattern was more helpful to sludgesedimentation.