{ "background": "Access to safe drinking water remains a critical challenge in many regions, with existing water treatment systems often operating below design specifications. There is a recognised need for robust, field-based diagnostic methodologies to evaluate the operational efficiency of such infrastructure, particularly in resource-constrained settings.", "purpose and objectives": "This case study aimed to develop and implement a randomised field trial protocol to diagnostically evaluate the functional efficiency of communal water treatment systems. The objective was to quantify efficiency gains and identify key engineering parameters influencing performance under real-world operating conditions.", "methodology": "A stratified randomised field trial was conducted across multiple communal water treatment facilities. System efficiency was measured via inlet and outlet turbidity (NTU) and faecal coliform counts. A mixed-effects regression model was fitted to the data: $Y{ij} = \\beta0 + \\beta1 X{1ij} + uj + \\epsilon{ij}$, where $Y{ij}$ is log-transformed effluent quality for facility $j$, $X{1ij}$ denotes operational cycle, $uj$ is the facility random effect, and $\\epsilon{ij}$ is residual error. Robust standard errors were calculated.", "findings": "The trial identified significant performance heterogeneity. A key concrete result is that systems receiving the diagnostic intervention demonstrated a mean reduction in outlet turbidity of 42% (95% CI: 34% to 49%) compared to control facilities. The statistical model indicated that operational cycle length and filter media condition were the most significant predictors of efficiency loss.", "conclusion": "The randomised field trial provides a viable and rigorous methodological framework for the diagnostic evaluation of water treatment efficiency in field settings. It moves beyond design-capacity assessment to quantify actual performance gains attributable to systematic operational evaluation.", "recommendations": "Implement routine diagnostic field evaluations using a randomised trial design to prioritise maintenance interventions. Engineering practice should integrate such operational performance data into asset management plans. Further research should adapt the protocol for different treatment technologies.", "