Membrane Bioreactor (MBR) Research at UNSW

The membrane bioreactor (MBR) has become a legitimate alternative to conventional activated sludge processes and an option of choice for many domestic and industrial applications. At the University of New South Wales (UNSW), the UNESCO Centre for Membrane Science and Technology investigates many aspects of MBR operation, maintenance and optimisation in collaboration with the Centre for Water and Waste Technology (School of Civil and Environmental Engineering). Some areas of focus include:

 

Although many studies have assessed fouling behaviour in MBRs, in-situ or direct observation of the fouling layer has not yet been possible. The observation of the fouling layer resulting from the filtration of model solutions allowed better understanding of MBR fouling intensity and mechanisms. Three visualisation techniques, confocal laser scanning microscopy, environmental scanning electron microscopy and direct observation will be tested to observe the fouling behaviour of alginate in MBR.

Vicki Chen and Richard Stuetz examine a bench-scale membrane bioreactor used in fouling studies.

Detailed studies of EPS filtration under controlled operation can provide a better understanding of fouling propensity and mechanisms process in MBR operation. In preliminary studies, alginate is used as model foulant for polysaccharides, bovine serum albumin (BSA) for proteins analogue while bentonite and yeast (washed and unwashed) were representing suspended solids content. The fouling mechanisms of these mixture provide insights into the interactions of these components in complex feeds found in MBRs.

Following a detailed protocol for the fractionation of the real fouling layers into three distinct sections, biopolymeric analyses of the foulant is possible to elucidate fouling layer formation on membrane surfaces. Preliminary results indicated that the upper fouling fraction consists of a porous, loosely bound cake layer with a similar composition to the biomass flocs. The intermediate fraction, which consists of equal parts of soluble molecular products (SMP) and biomass aggregates, features a higher concentration of carbohydrates and possibly acts as a linkage between the cake layer and irreversible fouling layer. The lower fraction, representing the irreversible fouling fraction and predominantly consisting of SMP, features a relative higher concentration of strongly bound proteins.

 

In an effort to better understand membrane stability and to optimise its lifetime, new physical and chemical cleaning strategies are to be assessed for MBR operation. The benefits and limitations of relaxation, continuous and backwash modes are compared with more complex filtration cycles. A series of enzymatic cleaning agents are also to be assessed to limit fouling in MBRs.

 

The optimisation of MBR units requires knowledge of biological treatment, membranes and hydrodynamics/mixing. Good mixing can ensure the effective use of the entire reactor volume and can affect nutrient removal efficiency. The degree of mixing and membrane configuration (e.g. flat sheets and hollow fibres) affects the output response describing the system’s flow regimes and expressed by the residence time distribution (RTD) profiles. The authors_ research group has investigated the mixing efficiency of pilot scale MBRs [1] and full-scale MBRs [2] with different membrane configurations via RTD analysis. Recently, we have developed a CFD model that has been validated with field experiments to show how membrane configurations can affect mixing conditions in the reactor.

CFD simulations were conducted using the commercial software package Fluent® on a 2.2 MLD hollow fibre membrane MBR in Sydney and a 2.5 MLD double deck flat sheet membrane MBR in South Australia. A 3-dimensional flow field consisting of the interacting phases of water and air were computed using the Eulerian-Eulerian multiphase model. The simulation results showed good agreement with the measured field RTD data. The hollow fibre MBR has a Peclet number of 0.24 and number of completely mixed tanks in series of 1.08, while the flat sheet MBR has a Peclet number of 0.37 and 1.13 of completely mixed tanks in series, which showed that the two MBRs were both close to completely mixed conditions. However, the mixing energy contributed by the mixer, bioreactor and membrane aeration, and recirculation pumps was 55.8 kW in total of the flat sheet MBR while 42.9 kW of the hollow fibre MBR, which indicated that the use of flat sheet membranes was 20% higher in mixing energy to create the same degree of mixing.

In conclusion, the development of MBR CFD model can provide the access to evaluate the effects of membrane configurations on energy consumption with the view of achieving the optimum mixing conditions at the lowest possible energy inputs for the design of large installations.

Liquid velocity distribution of Victor Harbour flat sheet MBR (left) and North Head hollow fibre MBR (right).

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