Hydrodynamic CFD Simulation of a Two-Phase Flow in a Single Tube of an Ultrafication Membrane for a Side-Stream Membrane Bioreactor

Membrane bioreactors (MBRs) combine biological treatment with membrane separation technology. There are two types of membrane bioreactors, one has the membrane inside the bioreactor, commonly called immersed membrane bioreactor (iMBR); and the other has the membrane in series with the bioreactor, named side-stream membrane bioreactor (sMBR). This work focuses on the last type: the sMBR configuration, that uses a tubular ultrafiltration membrane (X-FLOW) manufactured by Norit (The Netherlands). A common problem encountered with MBR’s is the fouling of the membrane resulting in frequent cleaning and even membrane replacement that increases the operational costs considerably. The membrane fouling happens due to three principal factors (Mulder 1998): 1) Membrane material, 2) Feed water quality: the bulk organic type and concentration, the solution conditions (pH, ionic strength), microorganisms, inorganic and organic substances (EPS); 3) Operating conditions: transmembrane pressure (TMP), cross-flow velocity and temperature. In terms of operation, three aspects are considered to be important with regard to the fouling of the membrane: particle size distribution (PSD), biological parameters and process hydrodynamics (figure 1). Due to the complexity of the MBR process, the modelling exercise is broken down in the development of several sub-models. For the hydrodynamic process it is important to account for the interaction between the biology and the membrane. Therefore, a model is being developed by means of computational fluid dynamics (CFD) using Fluent© (Ansys Inc.). The CFD model should consider several factors, among which: the geometry of the membrane, the boundary conditions (pressure, cross flow velocity and gas flow rate), the feed concentration and physical properties of the sludge like density, surface tension and viscosity. To reduce the fouling on the membrane air is often introduced in the sludge flow to create a gasliquid two-phase cross-flow (Cui et al. 2003). This is done for the following reasons: 1) increase the permeate flux; 2) improve membrane rejection characteristics (reduction of fouling) due to the early transition from laminar to turbulent flow (Ghosh and Cui 1999), 3) increase the surface shear stress to remove foulants that are already attached and 4) increase the mass transfer between the cake layer and the bulk region. In literature it has been observed that the slug flow pattern is the most effective as well as the least energy consuming process in ultrafiltration membranes (Bellara et al. 1996; Cuiet al. 1997; Smith and Cui 2004). Other studies suggest to operate the system in laminar rather than in turbulent regime to decrease the energy consumption (Ndinisa et al. 2005).