Uncertainty Analysis during the Multi-criteria Evaluation of Wastewater Treatment Plant Design Alternatives
Systematic methods and tools for managing the complexity
Process Synthesis & Design - II (T4-1b)
Keywords: Process Design, Wastewater, Multicriteria Analysis, Uncertainty, Monte Carlo Simulation
As in many other engineering applications, the evaluation of design alternatives in wastewater treatment plant (WWTP) design must take into account several objectives i.e. it is a multi-criteria problem. Moreover, in WWTP design there is generally a high uncertainty associated to the values that can be assumed for economical, environmental, technical and legal parameters.
Nowadays, the analysis of activated sludge WWTP design options is supported by the use of numerical simulators. However, commercial process simulators assume deterministic rather than stochastic parameters and thus do not consider the influence of parameter variance on simulation results. Uncertainty is a central concept when dealing with biological systems such as a WWTP, a plant that is typically subjected to large natural load variations. Thus, both identification and understanding of the influence of parameters presenting uncertainty is essential for the correct assessment of a WWTP design and for the analysis of existing designs.
The objective of this paper is to present a systematic procedure to evaluate WWTP design alternatives under uncertainty.
The set of new results are illustrated with a case study, where the bioreactor of a WWTP is retrofitted to achieve simultaneous carbon and nitrogen removal. This plant is comprised of five aerobic continuously stirred tank reactors (CSTRs), a settling tank, a recycle and a purge. A number of simulations are carried out coupling a deterministic model with a Monte Carlo engine to explore the WWTP response with respect to the evaluation criteria. The knowledge generated during this process is extracted automatically and is quantified in terms of risk. This estimated risk provides a wider picture of the design space and the resulting decisions based on such information are more solid, since more information has been taken into account.
The significance of the proposed procedure resides in the advantages of integrating deterministic and probabilistic methods for multi-criteria decision making. Detailed numerical models are used to investigate the effect of parameter uncertainty on the overall process performance. As a result, it is possible (a) to provide the designer with a tool to handle the uncertainty inherent in the early stages of WWTP design, b) to demonstrate how the different uncertainties are propagated through the model and how they affect the effluent streams, and (c) to make more informed and rational decisions on whether or not to implement a specific wastewater treatment technology by quantifying its influence on the design under uncertainty.
Presented Tuesday 18, 15:20 to 15:40, in session Process Synthesis & Design - II (T4-1b).
