Thesis Abstract

Thesis Overview

<p>1.0 INTRODUCTION&nbsp;</p><p>1.1. MOTIVATION&nbsp;</p><p>The increasing pace of industrialization, urbanization and population growth that our planet has faced over the last one hundred years has considerably increased environmental pollution and habitat destruction, and negatively affected water, air and soil qualities. In this the context within which wastewater treatment has become one the most important environmental issues of the day, insofar as it reduces or prevents pollution of natural water resources - i.e. inland surface waters, groundwater, transitional water and coastal water -promotes sustainable water re-use, protects the aquatic environment and improves the status of aquatic ecosystems. The implementation of EU Directive 91/271/EEC concerning urban wastewater treatment promoted the construction of new facilities and the introduction of nutrient removal technologies in areas designated as sensitive. The need to build at a rapid pace imposed economically sound approaches for the design of the new infrastructures and the retrofit of the existing ones. These studies relied exclusively on the use of heuristic knowledge and numerical correlations generated from simplified activated sludge models (e.g. ATV, HSA Principles, Ten State Standards and Custom Models). Nevertheless, some of the resulting wastewater treatment plants (WWTPs) were characterized by a lack of robustness and flexibility, bad controller performance, frequent microbiology-related solids separation problems in the secondary settler, high operational costs and/or partial nitrogen and phosphorus removal. This made their performance far from optimal. Most of these problems arose because of inadequate design, making the scientific community aware of the crucial importance of the design stage (Vanrolleghem et al., 1996; Vidal et al., 2002; Dominguez et al., 2006; Rivas et al., 2008).<br></p><p>  The new EU Water Directive (2000/60/EEC) establishes a new framework for Community action in the field of water policy. The Directive requires the development of management plans, where the major pressures and impacts on the receiving water are shown and measures to reach quality objectives are decided. In addition, one of the main characteristics of this new directive is the shift away from control of the point sources of pollution to integrated pollution prevention and control at river basin level, with the receiving water quality based on upstream pollution limits. This approach results in more freedom during the evaluation procedure – due to the expansion of the management limits – which can lead on the one hand to a better allocation of economic resources in pollution abatement, but on the other hand introduces a higher degree of complexity during the evaluation procedure because additional factors must be taken into account (Benedetti 2006). <br></p><p>  For this reason, traditional design approaches have to become more complex assessment methodologies that address design/redesign problems with respect to multiple objectives and multiple performance measures. WWTPs need to ensure a sufficient degree of pollution removal in terms of organic matter and nutrients to comply with the legislative limits on water discharge while, at the same time, keeping construction and operating costs to a minimum. Also, the need to increase their effectiveness and reduce their environmental impact has led to consider some additional criteria to evaluate a plant’s technical reliability and the potential damage to the water body caused by the treated effluent. Further, more attention has to be paid at the conceptual design level in order to ensure better WWTP performance. Decisions made during the conceptual design stage - e.g. sequence of aerobic, anoxic and aerobic sections, addition of certain chemical compounds and extra recycles - are of paramount importance in determining the whole plant performance – e.g. adaptation to short term and long term perturbations, potential risk of solids separation problems, aeration costs and effluent characteristics. Finally, a more reliable decision making procedure is necessary in which the selection of alternatives is based on communicable, systematic, objective and transparent procedures that allow a subsequent analysis of why one alternative was selected with respect to others. Considering the importance of the conceptual design/redesign of WWTPs, the existing literature in the field is still sparse and only a few systematic methodologies that tackle the complex task at the heart of the design problem are available to support the decision maker. The importance of conceptual design for WWTPs using multiple objectives and the lack of systematic methodologies to handle this complexity are the main motivations for this research work. <br></p><p> <b>1.2. CHALLENGES IN THE CONCEPTUAL DESIGN OF WASTEWATER TREATMENT PLANTS&nbsp;</b></p><p>Certain key challenges have to be confronted to promote the further progress of conceptual design in wastewater treatment facilities:</p><p>&nbsp;• Reducing the number of process alternatives. Conceptual design is complex and ill-defined because of the large number of potential solutions - e.g. modifications of existing equipment, addition of new equipment, piping - that might be considered in order to accomplish the same goal (Douglas 1988). However, after thorough evaluation, a very high percentage of these alternatives prove to be unsuitable.&nbsp;</p><p>• Dealing with multiple criteria during the evaluation of alternatives. The different conceptual alternatives have to maximize the degree of satisfaction of different objectives (Hoffman et al., 2003).&nbsp;</p><p>The purpose of wastewater treatment is to remove pollutants that can harm the aquatic environment if they are discharged into it. Thus, the selected alternative needs to comply with current regulatory standards as well as minimize the environmental impact on the receiving water body (Copp 2002). Furthermore, both construction and operating costs have to be minimized. In particular energy savings must be looked at - e.g. aeration, pumping, heating and mixing. Chemicals such as metal salts for phosphorus precipitation, the external carbon source to enhance denitrification efficiency and the costs related to the collection and disposal of sludge (Vanrolleghem and Gillot, 2002) must also be considered. Finally, when technical reliability is maximized several additional more factors must be considered. First, the plant adaptation to different types of perturbations, i.e. good disturbance rejection. Very few WWTPs receive a constant influent either in quantity or quality, but are subject to daily, weekly and annual variations (Gernaey et al., 2006).&nbsp; Secondly, when the plant has instrumentation, control and automation, it is important to evaluate the performance of the controller and the degree of adaptability to different perturbations under different design&nbsp; or operating conditions (Olsson and Newell, 1999). Thus, the selected alternative must maintain the operating variables within an operating space delimited by a set of constraints, which may be process (biomass, oxygen requirements), equipment (maximum pumping rates) or safety (effluent requirements) related. Last of all, it is important to include all naturally occurring microbiology-related solid separation problems caused by microorganisms population imbalances between filamentous and floc-forming bacteria, leading to problems of bulking and foaming or causing undesirable operating conditions which could, for example, lead to rising sludge (Wanner 1994, Jenkins et al., 2003). Consequently, wastewater engineers have to take into account design factors (organic load, the anaerobic ratio, anoxic and aerobic time) and operating factors (sludge retention time) that could have a crucial influence on changes to multispecific populations integrating activated sludge systems.&nbsp;</p><p>• Handling critical decisions arising during the conceptual design of WWTPs. Certain decisions are critical because of their influence on the whole design process, i.e. they influence many other decisions and hence have a strong impact on future process structure and operation, with a set of possible solutions that result in a similar degree of satisfaction of the design objectives. Decision making for these critical decisions is especially difficult when several design objectives (as detailed in the previous challenge) must be taken into account, due to a lack of support tools for managing the interplay and the apparent ambiguity emerging from the alternatives evaluated in a multicriteria fashion. The ability to look ahead to future design stages might lead to different decisions (Smith 2005). Unfortunately, looking ahead is not possible with the current tools, and instead, decisions are based on incomplete knowledge.&nbsp;</p><p>• Extracting meaningful knowledge during the evaluation of WWTP alternatives. Biological processes in WWTPs present complex relationships between design/operating variables (e.g. anaerobic/anoxic/aerobic retention times, temperature in the anaerobic digesters, flow rates) and process parameters (effluent&nbsp; ammonium, nitrate, etc). Some of these present synergies (interdependences) such as aeration energy and nitrification efficiency, but others are subjected to a clear trade-off (e.g. sludge retention time and nitrification efficiency against risk of bulking). Thus, the result is a hugely complex evaluation matrix consisting of a large number of physicochemical, operational and technical parameters which are often difficult to interpret and drawing meaningful conclusions&nbsp;</p><p>• Including uncertainty during the decision making process. Uncertainty is a central concept when dealing with biological systems like WWTPs that are subject to pronounced natural variations (Grady et al., 1999). Although wastewater models are well characterized, some parameters used during the analysis of the alternatives present uncertainties such as the fractions in which the different compounds arrive at the facility or the effect of either temperature or toxic compounds on the kinetic parameters. Hence, an understanding of these parameters, their inherent uncertainty, the way they are propagated through the model, the effect on the different outcomes and on the whole decision-making process is essential for the correct analysis of a WWTP. The assessment and presentation of the uncertainty is widely recognized as an important part of the analysis of complex water systems (Beck., 1987).</p><p>The challenges listed above demonstrate the complexity associated with the conceptual design of a WWTP. These complexities give rise to a number of important questions. The research work in this thesis attempts to provide with answers to the following questions:&nbsp;</p><p>How can the time-consuming evaluation process of a large number of conceptual design alternatives be reduced?&nbsp;</p><p>How can an optimal solution be found that maximizes the degree of satisfaction of the different objectives included in the evaluation procedure?&nbsp;</p><p>How can the designer be provided with a tool to support the management of the interplay and apparent ambiguity emerging from a multicriteria evaluation of WWTP alternatives?&nbsp;</p><p>How can future desirable (or undesirable) design directions be detected?&nbsp;</p><p>How can tools be found that are efficient at discovering groups of conceptual design alternatives with similar performance and identifying the main features for either a specific or a group of alternatives?&nbsp;</p><p>How the interpretation of the complex interactions amongst multiple criteria be facilitated?&nbsp;</p><p>How can structure be given to the decision making process, and all the knowledge generated during it reused?&nbsp;</p><p>How can the designer be provided with a tool to handle the uncertainty inherent in the early stages of WWTP conceptual design and allow the study of its effect on overall decision making? </p><p> <b>1.3. THESIS STATEMENT&nbsp;</b></p><p>The first hypothesis of this thesis is that the complex problem of the conceptual design of WWTPs can be broken down into a number of simpler steps that follow a predefined order: reaction, separation and recirculation. Such a breakdown facilitates analysis and evaluation of the different design alternatives that are generated without having to obtain a complete solution to a problem when an alternative has shown to be non viable at higher levels of hierarchy.&nbsp;</p><p>The second hypothesis is that each alternative under evaluation can be formulated as a vector of different criteria and represented as an n-dimensional performance score profile. All the features that characterize each alternative can be summarized into a metric (weighted sum) that will give their overall degree of satisfaction according to the defined design objectives and overall process performance.&nbsp;</p><p>The third hypothesis is that a combination of sensitivity analysis, preliminary multi objective optimization and knowledge extraction provides additional information with which to confront the problem of critical decisions. Thus, a better picture of the design space obtained by unravelling future desirable (or undesirable) directions during plant design will be possible.&nbsp;</p><p>The fourth hypothesis is that multivariate statistical techniques can mine the intensive multi-criteria evaluation matrixes and provide aggregate indicators that enhance the understanding of the evaluation procedure. These techniques will unravel the natural association between conceptual design alternatives; design/operating variables and evaluation criteria, thereby highlighting information not available at first glance. <br></p>