Effect of Rainfall Variability on Water Treatment Costs at the Gihira Water Treatment Plant, Rubavu District, Rwanda

Rainfall variability poses a growing challenge to surface water treatment plants by altering sediment loads and degrading raw water quality, thereby increasing operational costs and threatening financial sustainability. This study assessed the effects of rainfall variability on water treatment costs at the Gihira Water Treatment Plant (WTP) in Rubavu District, Rwanda, covering the period 2019–2024. Three specific objectives were pursued: (i) examining the influence of rainfall variability on key raw water quality parameters such as turbidity, total suspended solids (TSS), and total dissolved solids (TDS); (ii) analysing the relationship between these parameters and operational costs; and (iii) evaluating the effect of rainfall variability on water production capacity and unit production costs. A descriptive and analytical case study design was adopted, using secondary annual data on rainfall, raw water quality, water production volumes, and operational costs sourced from WASAC plant records and the Busogo Meteorological Station. Data was analysed using descriptive statistics, Pearson correlation, simple linear regression, and independent samples t-tests. Results reveal that rainfall variability strongly increases turbidity (r = 0.85) and TSS (r = 0.64), while TDS exhibits a weak and inconsistent response (r = −0.27). Elevated turbidity and suspended solids significantly drive chemical treatment costs, whereas energy costs are primarily governed by operational rather than water quality factors. Although total production volumes remain relatively stable across seasons, wet seasons incur substantially higher total (RWF 28,079,802) and unit (98.67 RWF/m³) costs compared to dry seasons (RWF 17,906,761; 60.59 RWF/m³), a difference that is statistically significant. The study concludes that rainfall-induced sediment inflows are a dominant driver of water treatment cost escalation at Gihira WTP and recommends strengthening catchment management, adopting rainfall-responsive operational planning, and integrating source water protection to sustainably reduce treatment costs.