Spatiotemporal Patterns and Treatment Efficiency of Heavy Metal and Nutrient Pollutants in Urban Wastewater Systems: A Case Study of Juja and Ruiru, Kenya

Urban wastewater treatment systems play a major role in safeguarding environmental quality and public health. This study investigated the spatiotemporal distribution and treatment efficiency of selected heavy metals (iron, copper, manganese, cadmium) and nutrient pollutants (nitrates and phosphates) in the Juja and Ruiru wastewater treatment plants located in Kiambu County, Kenya. Wastewater samples were collected from the inlet to the outlet points during the dry and rainy seasons. Nutrient concentrations were analysed via UV-visible spectrophotometry, whereas heavy metal concentrations were determined via atomic absorption spectroscopy (AAS). Independent samples t -tests and effect size calculations (Cohen’s d ) were applied to determine statistical significance and practical relevance. Significant spatial and seasonal variations were observed in the pollutant concentrations at the Juja and Ruiru wastewater treatment plants. During the rainy season, Ruiru recorded higher copper levels at the inlet (M = 0.200, SD = 0.079 mg/L) than did Juja (M = 0.024, SD = 0.008 mg/L), whereas in the dry season, Juja had higher outlet levels. Iron concentrations were higher at Juja's dry season inlet (M = 6.953, SD = 0.215 mg/L) than at Ruiru’s (M = 5.739, SD = 0.366 mg/L), whereas Ruiru recorded higher rainy season influent levels. The manganese levels in the dry season were significantly greater at Juja’s inlet (M = 3.094, SD = 0.028 mg/L) and Ruiru’s outlet (M = 1.329, SD = 0.013 mg/L). Phosphate concentrations showed seasonal reversal. In the dry season, Juja’s influent was relatively high (M = 0.930, SD = 0.002 mg/L), whereas Ruiru’s level exceeded Juja’s influent in the rainy season. Juja’s effluent phosphate in the rainy season remained significantly greater. The nitrate concentrations followed a similar trend, with Juja showing higher dry season influent (M = 0.198, SD = 0.001 mg/L), whereas Ruiru’s rainy season influent peaked at 0.441 mg/L. Overall, large effect sizes confirmed that spatial and seasonal factors significantly influenced pollutant dynamics and treatment performance. These findings highlight substantial spatial and seasonal variability in pollutant loads and removal performance. These findings highlight substantial spatial and seasonal variability in pollutant loads and removal performance. The consistently large to extremely large effect sizes underscore the practical importance of location-specific and hydrological factors in influencing wastewater treatment efficiency. Enhanced infrastructure and adaptive management strategies are recommended to improve pollutant removal under varying seasonal conditions.