Impact of hydrodynamics on the precipitation efficiency--application to HARDTAC reactor.

Precipitation of gypsum is studied in a HARDTAC (High-Aspect Ratio, Draft-Tube, Agitated Crystallizer) reactor, which is considered as the core crystallization unit of lots of wastewater treatment systems. Coupling Computational Fluid Dynamics (CFD) and population balance modelling to simulate precipitation can be a useful tool to come to a decision about upstream and downstream units. In the present study, we aim to validate such approach by investigating gypsum precipitation in a HARDTAC pilot unit and comparing experiments results with simulation.

One-dimensional modelling of the secondary clarifier-factors affecting simulation in the clarification zone and the assessment of the thickening flow dependence.

This paper presents a one-dimensional (1-D) model of the secondary settling tank (SST), and the drive for model development is discussed using steady-state simulation results generated with a 2-D computational fluid dynamics (CFD) model. Concentration profiles measured in a flat-bottom circular (fbSST) that is equipped with suction-lift sludge removal, served for CFD model validation. We investigate the factors that may deteriorate the 1-D simulation performance of clarifier models, which incorporate dispersion both in terms of the effluent sludge concentration and in terms of the sludge blanket height.

Modelling of wastewater treatment plants--how far shall we go with sophisticated modelling tools?

Several levels of complexity are available for modelling of wastewater treatment plants. Modelling local effects rely on computational fluid dynamics (CFD) approaches whereas activated sludge models (ASM) represent the global methodology. By applying both modelling approaches to pilot plant and full scale systems, this paper evaluates the value of each method and especially their potential combination.

Computational-fluid-dynamics (CFD) modelling of an industrial crystallizer: application to the forced-circulation reactor.

The CFD (computational fluid dynamics) technique is used to describe the mixing conditions in a pilot-scale FCC (forced-circulation crystallizer) and to study the impact of flow rate and aspect ratio on local flow conditions and RTD (residence-time distribution) in the crystallizer. The analysis adequately predicts the oscillating flow and two-phase (gas-liquid) interaction at the free surface. A comparison has been made between the CFD predictions and models of RTD.