## 1 Introduction
### Research Contribution  
Despite the extensive amount of research in the field, to the best of our knowledge, there is no evidence of case studies observed for the application and implementation of preventive maintenance grouping strategies in sewage treatment plants. However, recent studies reveal a positive impact on the application of maintenance policies in the industry, establishing a high correlation between investment in preventive maintenance, savings in energy consumption, and increases in global efficiency indexes [8]. In this way, the research gap reveals two fundamental aspects that this study seeks to exploit: (1) The absence of case studies that address the application of maintenance grouping strategies in the water treatment industry; (2) and besides, the need for adaptive, flexible, and fast implementation strategies of PM plans in the wastewater facilities in response to efficiency, productivity, and quality of service requirements. In summary, this research proposes a new strategy to improve cost-efficiency and asset availability in wastewater treatment plants, with a focus on the implementation of the grouping framework during the maintenance planning and scheduling stages.  
The framework presented below considers the extension of the formulation originally presented in [PERSON] et al. [46], addressing the application of new criteria to improve the applicability and adherence of the strategy in industrial environments, through the incorporation of time-window tolerances for the creation of opportunistic grouping schemes and non-negligible PM execution times. Following the research line, the framework proposed in this article seeks to improve the availability level of complex production systems, reducing the number of system interruptions, while minimizing maintenance costs through the distribution of stoppage costs for PM activities.  
The article is structured as follows: Section 2 presents the established Methodology for the application of the opportunistic grouping strategy; Section 3 presents the results obtained under the application of the strategy; Section 4 discusses the results and their impact on the analyzed case study; Section 5 establishes recommendations, limitations, and future research in the field.  
**Research gaps found in the literature:**  
* Absence of case studies addressing formulation and implementation of maintenance grouping strategies in the wastewater treatment industry;
* New and tailored strategies for an adaptive, flexible, and fast implementation of PM plans in the wastewater facilities, seeking to improve cost-efficiency and quality requirements.  
**Main contributions of the study:**
* Presentation of a novel case study for the implementation of preventive maintenance execution strategy in wastewater and sludge treatment facilities;
* Framework formulation and computational optimization of a grouping strategy for preventive maintenance activities, seeking to minimize the number of planned interruptions, fixed maintenance costs, and system downtime;
* Adaptation of the grouping strategy to the case study under analysis, in response to the requirements of maintenance cost reduction, productivity, and quality of service required on wastewater facilities;
* In-depth discussion of results and recommendations with a focus on managerial insights.