Modelling of membrane bioreactor systems

Membrane bioreactors (MBR) first developed in the late 1960s and commercialised in the last 20 years, uniting membrane technology and biochemical degradation processes (activated sludge) for wastewater treatment. It has been a subject of keen interest, and rapid development in the past 4-5 years and has extensively penetrated the market in the recent years due to increasingly stringent effluent regulations and continuously decreasing costs of membrane systems. By year 2000, over 500 membrane bioreactor plants have been commissioned to treat both industrial and municipal wastewater, as well as for in-house treatment and reuse of grey water.

Process design and simulation using activated sludge model (ASM) has been wildly adapted as a powerful tool in the design and optimisation of biological wastewater treatment plants. However, Due to complete retention of suspended solids and partially retention of colloids and very high sludge concentration and sludge age, some modifications have to be made to build models and run simulation for MBR systems. Furthermore, membrane fouling is a major constrain of efficient biomass separation, and membrane fouling has its own unique characteristics in MBRs since its feed water might be manipulable via the control of the biology in the activated sludge process. Hence an integrated model incorporating both biodegradation in bioreactor and fouling in membrane modules are very interesting for the MBR system design and optimisation, but such a model is currently still not available. Finally, increasing global water scarcity problems demand safe and cheap technology for wastewater reuse. The excellent effluent quality of MBR and the fact of decreasing cost appear to be interesting.

The main goal of this Ph.D. research is to develop an integrated model to predict and simulate pollutant removal and membrane fouling in MBR systems. The developed models can be used to assistant MBR system design and optimise its operation. Model development will be performed in laboratory scale systems using synthetic wastewater and evaluated in pilot scale MBR installations with intention to eventually go to full scale application. The detailed objectives of this research include:

1. Trace and model the “life cycle” of colloids and slow or none-biodegradable solutes (e.g. soluble microbial products) in MBR systems including their production and degradation in bioreactor, rejection or adsorption by the filter cake, adsorption and blocking membrane pores and finally passing the membrane. Finally a model will be developed to predict and simulate the effluent COD quality and the process of membrane blocking.

2. Model the formation and flocculation of mixed liquor suspended solids (MLSS) in the bioreactor and its effect on the build up of membrane filter cake by integrating activated sludge model hydrodynamic particle deposition model and cake filtration model.

3. Quantify and model the extent of organic matter removal due to biodegradation in the bioreactor and membrane rejection and evaluate the role of membrane rejection in MBRs compared settling with conventional activated sludge.

4. Propose possibilities of MBR effluent reuse for various purposes (e.g. drinking, industrial, bath, irrigation, recharge ground water) combined with other technology (e.g. Reverse Osmosis) with respect to closing the water cycle.