Comprehensive Decision-making Framework for a Wastewater Utility Using MCDA (Multi-Criteria Decision Analysis)

Introduction Wastewater utilities have used Water Resource Recovery Facilities (WRRF) to treat municipal wastewater efficiently, protecting both public health and the aquatic ecosystem in receiving rivers and lakes. Utility decision-making has been primarily based on public health considerations, local and regional environmental considerations, and financial considerations. However, the escalation of multiple global challenges, including climate change and resource depletion, have driven wastewater utilities to include broader factors in their decision-making efforts. WRRF are tasked to recover energy and resources such as biogas and fertilizers, reduce energy and chemicals consumption by its treatment process, and reduce greenhouse gas emissions, e.g., 'carbon footprint.' Many WRRFs face diverse social demands such as local employment, water equity, community acceptability, and environmental education. In addition, various stakeholders want to be involved in essential decisions of the facilities and reflect their opinion in the decision-making process. This study develops a new decision-making framework and applies it to the Great Lakes Water Authority (GLWA) WRRF, a 1.7 BGD facility, as a test case to demonstrate the decision-making process. The components of this framework are (1) Improved engagement with stakeholders, (2) Assessment of conflicting tradeoffs, (3) Identification and weighing of evaluation criteria, (4) Efficiency of organizational decision making. The scope of this work is the overall decision-making process, from the initial stage of setting goals to the final stage of ranking alternatives. The study used the evaluation of the GLWA WRRF biosolids treatment operations as a test case for and during the development of the framework. Decision-making approach and framework Decision-making in WRRF is a highly complex process; key decision-makers must coordinate the diverse interests within decision-makers and stakeholders (relevant parties). In the interest of harmonizing these conflicting views, we demonstrate how the decision-making paradigm can be converted from a technical and economic perspective to a more holistic approach, encompassing environmental and social sustainability. Furthermore, to enhance stakeholder buy-in and subsequent support, it is imperative to maintain transparency in the decision-making process, granting stakeholders participation in the decision-making process of WRRF. As a methodology, the comprehensive decision-making framework, which can reflect technical, environmental, economic, and social aspects, was established with MCDA (Multi-Criteria Decision Analysis), as shown in Figure 1. In the decision-making process of WRRF, the goals and boundaries of the decision were defined. The goals to be achieved were clearly stated, and the system boundary for each analysis was outlined. Concurrently, the key decision-makers and diverse stakeholders were identified, and methods were established, and then refined, for their involvement in the decision process. The alternatives which can achieve the decision goals and comply with the legal requirement should be identified through objective assessment. This study proposed the process modeling to assess the technical feasibility of alternatives that can achieve goals and comply with the permits. Screening the alternatives, relevant parties should determine decision criteria, reflecting their preference. The criteria should embrace broad perspectives, including the technical, environmental, economic, social standpoints. The next step is to set up the scaling methods for each criterion. In this stage, the expert group plays a leading role in providing measuring metrics and a framework to assess the alternatives based on facts objectively. This study suggests the scorecard approach to scale the criteria, ranging from 1-5. Weighting is the crucial step in decision-making, entitle the criteria with relevant parties' value. In this step, the relevant parties demonstrate their interests and preferences apparently. This study applies the AHP (Analytic Hierarchy Process) to support to weigh their preference. AHP is the tool to evaluate the parties' preference in pairwise comparison, providing more objective results. Expert group scores objectively the alternatives upon given criteria and scaling methods, cooperating with relevant parties. This study devises a combined LCA and process modeling tool to assess the technical and environmental criteria, delivering more objective and quantitative results (Fig 2). The process modeling provides the expert group with technical assessment, producing input and output parameters of the system. With these parameters, LCA can assess the environmental criteria of the system. After these steps, the alternatives-criteria matrix can be developed and normalized. With this normalized alternative-criteria matrix, the ranking of alternatives can be finalized. This study employs the TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) to search for a more optimal solution. TOPSIS is a tool to seek the best solution, which is the closest to the ideal solution and the farthest to the anti-ideal solution. Project test case: biosolids operations The decision-making framework test case was biosolids process operations of the GLWA WRRF. The GLWA WRRF has primary treatment capacity of 1.7 BGD and secondary treatment capacity of 960 MGD. This case study aims to discover the sustainable and optimal alternative to maximize resource and energy recovery while maintaining the stringent phosphorus effluent limit. Findings and Benefits The alternatives under discussion ranged from anaerobic digestion with phosphorus recovery to HTL (Hydro-Thermal Liquefaction). The SUMO process modeling (Dynamita software) was employed to assess the technical feasibility of alternatives to meet the decision goals and permit compliance. The only feasible options will be examined in subsequent steps. Furthermore, this process modeling can identify the overall mass flow in the GLWA WRRF (phosphorus mass flow is shown in Figure 3). The subsequent LCA utilizes the input and output results and can produce diverse environmental impacts, such as global warming and eutrophication. Table. 1 shows the twelve decision-making criteria under review. Five categories represent the diverse nature of relevant parties. The energy and resource recovery group consist of process energy, fertilizer production of phosphorus, and biosolid transportation. Technical stability and operational risk criteria are grouped into the technical category, mainly assessed with process modeling. The environmental category includes carbon footprint and environmental impacts, which will be evaluated with LCA. Also, the other environmentally emerging issues, such as PFAS (Per- and polyfluoroalkyl substances), microplastics, and air emissions, are selected. The social category can be divided into local job creation, operational health and safety, and acceptability of other wastes. The economic criteria can be dealt with, analyzing life cycle cost. This holistic decision-making framework is expected enhance the facility's sustainability and community compatibility by encouraging more fact-based and inclusive decisions. Specifically, the framework provides the following benefits: (1) Fact-based: Relevant information that the decision should be based on is incorporated, and extraneous information is not given undue weight, (2) Reflects: a variety of relevant perspectives affecting the decision, (3) Clear: The decision to be made is clearly stated. (4) Transparent: The process is clearly defined and documented, (5) Inclusive: The roles and responsibilities of the relevant parties are well defined, with the participation of the relevant parties appropriate to their identified role, (6) Produces a decision: that is supported by the relevant parties, (8) Scalable: The components of the decision process are adjustable to the nature and scope of the decision to be made, (9) Repeatable: The components of the process are clearly defined, (10) Efficient: Support, such as tools, checklists, defined procedures, are provided so that the necessary work can be accomplished as easily as possible. Keywords: Decision-making, MCDA, LCA, process modeling, WRRF, AHP, TOPSIS