Wastewater treatment plant primary clarifier hydrodynamic modelling
The operating performances of wastewater treatment plant primary clarifiers essentially depend on hydraulic conditions. The aim of this paper is to characterize and to simulate the hydraulic flow regime of an existing sedimentaton tank. This study was carried out on a wastewater treatment plant primary clarifier. It is a circular clarifier of 3900 m3 in volume, 42 m in diameter, 2.1 m in straight useful height, and, 2.1m in height for the conical part. The inlet flow rate is between 1400 and 2700 m3/h, whereas the sequental (3 primary clarifiers) sludge extraction flow rate is about 20 m3/h. To characterize the liquid phase residence time distribution (RTD) in the clarifier, we did a tracing operation (pulse injection) with lithium chloride as tracer (20 kg of lithium chloride dissolved in 120 litres of tap-water). After the tracer injection, the lithium concentration at the outlet of the clarifier was measured as a function of time. During this experiment we also followed the evolution of inlet flow rates and the suspended solid (SS) retention efliciency ofthe clarifier. For 350 min of experimental time, the recuperation of lithium was 92.7%. The experimental RTD curve allowed us to develop a model for the primary clarifier hydrodynarnics by association of elementary units (completely stirred reactors, CSTR) in series or in parallel. The transfer function in the LAPLACE domain, G (s), and the mathematical expressions of the first order and second central order moments were determined for this model. The nurnerical simulation of RTD between the inlet and the outlet of the system was performed with the help of a general computer code which numerically calculates the RTD, using Fast Fourier Trandormation techniques, from a given transfer fucnction. The hydrodynamic model proposed in this study matches very well with the experimental results. It comprises two trranches associated in parallel. The first one, which is drived by an effluent flow rate fraction α (α=0,14), is treated as 7 CSTR's in series of 77 m3 in total volume. The second branch, which is drived by the remaining eflluent flow rate fraction (1-α), is composed of two modules associated in series. The first module, which characterizes a recycling zone, is treated as a CSTR of 645 m3 in volume exchanging with a second CSTR of 1935 m3 in volume, the exchanged flow rate fraction, ß, being equal to 28. The second module of this branch. which characterize a uniform flow zone, is represented by 7 CSTR's in series of 1183 m3 in total volume. These results are confirmed by the comparison of the first order and the second central order moments. Indeed, between t = 0 and t = 350 min., we find for the experimental curve μ1=l27 min and μ2=6957, and, for the proposed hydrodynamic model curve μ1=129 min and μ2=7154 which are equivalent values. Finally, the proposed hydrodynamic model makes possible the analysis of the primary clarifier performance with respect to suspended solid removal.