Due to the growing demand for fresh water, increasingly stringent environmental water quality requirements and the increase of areas prone to water scarcity, MBR technology has become a competitive alternative for municipal wastewater treatment plants (WWTP) or their upgrades. Membrane bioreactors (MBR) are a combination of common bioreactors and membrane filtration units for biomass retention, presenting unique advantages such as high effluent quality and a smaller footprint than that occupied by conventional wastewater treatment plants. Although fouling and its associated operational costs have been known to be a key issue in MBRs, the optimal operation to enhance MBR efficiency regarding biological and physical processes is still lacking. For this reason, the research developed in the framework of this thesis has been focused on the identification of the optimal operational strategies to improve the efficiency of the processes taking place in the MBR in an integrated way, optimizing simultaneously biological and physical processes to minimize fouling and, if possible, operational costs.
This research has been presented in the doctoral thesis entitled “Integrated Operation of membrane Bioreactors: Simulation and experimental studies”, by Montse Dalmau. This has been carried out within the Laboratory of Chemical and environmental Engineering (LEQUIA) group of the University of Girona (UdG), and supervised by Joaquim Comas (UdG), Ignasi Rodriguez-Roda (UdG, ICRA) and Eduardo Ayesa (CEIT). This thesis presents a step towards the integrated operation of MBRs through experimental and model-based studies. Interactions between the biological (nutrient removal and sludge characteristics) and physical (hydrodynamics and filtration) processes in MBRs were studied, with the final aim being to improve their integrated operation and control.
To achieve this aim, a deterministic (or mechanistic) model was developed which describes the processes taking place within the MBR. This was applied to a pilot plant and full-scale MBR to identify the most sensitive parameters with respect to the integrated operation of the biological nutrient removal, filtration and hydrodynamic processes. Two different models were developed to describe complex fouling phenomenon (deterministic and black box, a data-driven model adjusting the output data to the input data based on the process history) to identify the optimal conditions for good filtration performance.
Additionally, two experimental studies complemented the predictions of the models. Firstly, interrelations between biological nutrient removal processes, filtration processes and sludge characteristics determined the strategies for the integrated control of the two most important operating parameters in MBR: biological and membrane aeration. The identification of the optimal aeration conditions led to an airflow rate reduction of 42%, representing an energy saving of 75% compared to the initial operating conditions. Secondly, a novel air-scouring control system was successfully validated for 320 days in a full-scale MBR. The average reduction of the air-scouring flow rate was 13%, with the maximum reduction being limited to 20%, without compromising sludge characteristics and effluent quality. The control actions led to an average decrease in the energy consumption for membrane aeration of 14% and reaching a maximum of 22%.
The results obtained as part of this thesis will improve the integrated operation and the automatic control of the biological and filtration processes simultaneously. Moreover, the reduced energy costs and the better understanding of MBR operation may contribute to making MBR systems a more competitive technology to deal with water scarcity problems.