Effects of configuration and operating conditions on turbidity removal of jet clarifier

Abstract

In water treatment, flocculation creates large and weighty flocs enough to be removed by the downstream processes of sedimentation and filtration. Among the various existing technologies, the jet clarifier is considered as an effective and compact system as it couples flocculation and clarification in a single unit. For the design of jet mixing, much experimental work has been done and many correlations have been proposed. However, these correlations are case specifics, and, to date, there is no comprehensive view for the flocculation aspect.

In order to evaluate the performance of the jet clarifier for turbidity removal and understand hydrodynamics to propose the optimal operating conditions and design criteria, two different configurations of the continuous jet clarifiers are figured out. The first one is a prototype of a 3D jet clarifier studied at two scales and implemented at Samsen Water Treatment Plant, Thailand; these two reactors were designed to investigate the performance and mean Residence Time Distribution (RTD) for various injected flow rates. The results indicated no effect of reactor sizes, and a reduction of the initial turbidity (50 NTU) was achieved with an efficiency of approximately 80% under optimal conditions.

Moreover, the second jet clarifier configuration was designed as a Quasi-2Dimensional (Q2D) jet clarifier at the TBI-INSA-Toulouse, France allowing the application of optical metrological methods used to understand better local phenomena controlling the efficiency of the jet clarifier. Hence, measurements of instantaneous velocity field were performed by means of Particle Image Velocimetry (PIV). The processing of experimental PIV data highlighted a strong circulation induced by the jet in the flocculation zone. At this location, the range of velocity gradient (G) is 3 to 13 s-1 whereas the residence time decreases from 4 to 1 hour. Based on the hydrodynamic analysis, the Camp number (Gt) in the flocculation zone is shown to be constant at around 7,000 for different jet flow rates (from 11L/hr to 49L/hr). The efficiency of such the jet clarifier can thus be foreseen. Plus, measurements of the number of flocs and their size distributions were performed by means of shadowgraphy and image analysis. Thanks to a coupling between the different experimental results obtained in the Q2D jet clarifier, it was possible to relate the evolution of the number of flocs along the jet to the recirculation loop present in the flocculation zone. The relative independence of the floc size distributions on the flow rate is discussed in light of the Camp number, which can explain the efficiency of the jet clarifier in terms of flocculation.

Finally, due to reactor design, the simulations using CFD code showing encouraging results were presented at the end of the manuscript. Here as well, characteristic time scales and velocity gradient were used to perform the first comparisons.