{ "background": "Municipal water treatment systems in South Africa face persistent challenges regarding operational efficiency and financial sustainability. There is a recognised lack of robust, field-based methodologies for diagnosing cost-effectiveness that account for the varied local conditions across treatment facilities.", "purpose and objectives": "This study aimed to develop and implement a novel randomised field trial (RFT) framework to diagnose the cost-effectiveness of municipal water treatment systems. The primary objective was to quantify the operational cost per megalitre of potable water produced against key performance indicators.", "methodology": "A stratified randomised field trial was conducted across a representative sample of municipal water treatment works. Facilities were randomly assigned to a diagnostic protocol involving detailed technical audits and cost analysis. Cost-effectiveness was modelled using a generalised linear model: $Ci = \\beta0 + \\beta1Ti + \\beta2Qi + \\epsiloni$, where $Ci$ is cost per megalitre, $Ti$ is treatment technology type, and $Qi$ is raw water quality index. Robust standard errors were used for inference.", "findings": "The RFT identified significant variance in cost-effectiveness, with operational costs per megalitre ranging from ZAR 1,200 to ZAR 3,850. A key finding was that facilities employing advanced membrane filtration were, on average, 22% more cost-effective than conventional coagulation plants when controlling for raw water turbidity (95% CI: 15% to 29%).", "conclusion": "The randomised field trial provides a rigorous methodological framework for diagnosing cost-effectiveness in water treatment, revealing substantial inefficiencies linked to specific treatment technologies.", "recommendations": "Municipalities should adopt targeted diagnostic audits based on the RFT framework to identify and prioritise cost-saving interventions. Policy should support the conditional implementation of advanced treatment technologies where raw water conditions justify the investment.", "key words": "randomised field trial, cost-effectiveness, water treatment, municipal engineering, performance diagnostics", "contribution statement":