One-Dimensional Flux Model For The Secondary Settling Tanks And Its Application

The secondary settling tank (SST) is one of the most important compositions in the activated sludge process. It makes great sense to model and simulate the SST behavior or the activated sludge process, especially for their design, process control and optimization. In this paper, the one-dimensional (1-D) flux model for SSTs based on mass conservation law and its application were studied in following aspects: (1) the settling velocity of activated sludge; (2) improvement of the 1-D flux model; (3) establishing and applying the coupling model of activated sludge process by the 1-D model and ASM2. The main outcomes include the following aspects:(1) Based on batch settling tests, a mathematical model describing the compression settling process of activated sludge was developed. Furthermore, a theoretical equation for determining the critical point when the compression settling stage commences, and a new velocity function for the compression settling were deduced from the model. Dividing the complete settling process of activated sludge into two settling stages (zone settling stage and compression settling stage), and describing them by respective velocity functions were more reasonable for characterizing the complete settling process of activated sludge in the SST.(2) A new 1-D flux model with the varying cross-sectional area was developed based on mass conservation law. The settling velocity functions adopted in the model is able to describe three types of settling of activated sludge in SST, namely settling at low concentration, zone settling and compression settling. The solutions for avoiding the shock wave (numerical instability) were advanced. And to obtain the ideal results, the number of divided layers in the SST was suggested at 30. Results show that this model is capable to simulate the concentration of suspended solids in effluent flow, the concentration of return sludge, the concentration profile of sludge and the sludge blanked height (SBH) in the SST. Furthermore, it keeps the solids mass conservation during each layer, and has a strong stability even when the disturbing pulses appear caused by changes of the influent flow rate, the sludge concentration in influent flow and the recycle ratio.(3) The simulation to a wastewater treatment plant (WWTP) in Chongqing was conducted by using the coupling model. Results show that the coupling model could simulate the process well. The relative errors of return sludge concentration between...