Abstract: Effective operation and maintenance (O&M) of decentralized wastewater treatment plants (WWTPs) play a crucial role in protecting public health and the environment, particularly in rapidly growing cities. This study presents the first thorough, city-wide assessment of O&M practices across all 37 domestic WWTPs in Duhok City. Employing mixed methods, including a questionnaire, on-site observations, and a structured scoring framework, we evaluated ten critical domains: environmental compliance & pollutant removal efficiency (COD, BOD, TSS, TP, TN), reactor parameter compliance (pH, DO, HRT, MLSS), maintenance management, sludge handling, staffing and competency, plant monitoring and management, and energy and cost efficiency. Our findings reveal widespread deficiencies: only 4 facilities consistently fulfill Iraqi effluent regulations for organic loads, and just one reliably manages sludge; operator training and preventative maintenance are almost absent. In contrast, energy consumption and cost-effectiveness scored highly, with modern MBR and RBC technologies outperforming conventional SBR units in energy performance. Overall, only 3 WWTPs in Duhok attained an O&M score above 50%, which indicates that only about 21% of the city’s domestic wastewater is treated to the required standard, while nearly 26,918 m³ of wastewater is discharged untreated on a daily basis. The novelty of this study lies in its thorough scope, integrating quantitative performance metrics with qualitative insights to diagnose systemic O&M gaps and provide a practical benchmarking tool. These results highlight an urgent need for targeted training, standardized O&M protocols, and enhanced regulatory oversight to improve decentralized treatment performance in Duhok and similar urbanizing regions.