Modeling and model-based control of membrane bioreactors

Abstract:
Water is a global resource indispensable for life on earth. Its responsible and sustainable use and reuse is a major challenge of the 21st century. The increasing world population and industrialization lead to a rising demand for potable and process water, and in many areas existing supplies are diminishing at critical rates. Untreated wastewater threatens intact biological systems by introducing large amounts of nutrients, toxic or endocrinous species, heavy metals, and other harmful components. For these reasons efficient water treatment and reuse have become decisive social and economical issues. In many countries legal limits on the effluent concentrations of selected components are tightened, e.g. by the European Water Framework Directive issued in 2000. Strict effluent constraints however together with increasing wastewater loads demand efficient treatment processes. At the same time the increasing privatization of wastewater treatment facilities requires a stronger focus on their economic performance. This context provides the motivation for the research presented here. The technology of interest are membrane bioreactors (MBR) for wastewater treatment, which have increasingly been employed for the last 15 years and which are expected to play an important role in future wastewater treatment. MBR combine classical biological wastewater treatment with subsequent membrane filtration. The membrane unit separates the biomass of the biological treatment from the water. MBR offer high effluent quality, reliable biomass separation, and small space requirements. These properties make them an appealing alternative especially when effluent constraints are tight, when space is limited, and when existing plants need to be upgraded. In general, however, MBR operating cost are higher than those of conventional wastewater treatment plants (WWTP), which employ sedimentation basins for the biomass separation. A large potential to increase the economic feasibility of MBR lies in the improvement of their operational policy. Until today only simple control strategies have been employed. Advanced control approaches frequently used in the chemical process industry have not been applied to MBR due to the large uncertainty in the biological and the filtration processes, in the inflow prediction, and in the limited measurement information. While this is not different from the obstacles in regular WWTP operation, the increased complexity of MBR even more requires efficient online control to exploit their full potential. Hence, this thesis focuses on the process control of MBR. It aims at bringing advanced approaches from many research areas as e.g. modeling, control, and optimization together to provide a capable, flexible, and generic control architecture which takes the characteristics and peculiarities of MBR and MBR operation into account. Due to the process complexity model-based control approaches are proposed. Time and unit scale separation are performed to obtain subproblems of lower complexity for different disturbance dynamics and for both the biology and the membrane system. The subproblems include the scheduling of operational strategies, dynamic real-time optimization, non-linear model predictive control, run-to-run control, inflow prediction, and state and parameter estimation. For each of them suitable models, problem formulations, and efficient solution algorithms need to be formulated. The coordination between the subproblems on different time scales and between the units must be considered. Available solutions are discussed and complemented by new models and algorithms. The framework provides a clear input-output structure in order to enable researchers to easily incorporate extensions and modifications. Due to its modular design, the solutions developed can be applied to WWTP and filtration systems as well as to the MBR process.

Keywords:
model-based control , control