The effects of dry versus wet season on the performance of a wastewater treatment plant in North West Province, South Africa

The study was conducted at a wastewater treatment plant (WWTP) in the North West Province of South Africa (SA), to investigate the effect of seasonal variations of rainfall and temperature on the fate of Escherichia coli (E. coli) and on chemical parameters. Both seasons showed variations in terms of rainfall levels and temperature. The average temperatures measured at the final effluent were 14 and 22°C for the dry and wet season, while the rainfall averages ranged between 0.0 and 69.0 mm and 16.0 and 258.9 mm for the dry and wet season, respectively. The impact of rainfall within the two seasons presented a variation in the plant inflow rate of 34 000 and 48 000 m³ during the dry and wet season, respectively. Higher E. coli concentrations were detected before and after chlorination in the wet season (1.86 x 10⁵ and 8.40 x 10¹ MPN/100 mL) than in the dry season (2.26 x 10⁴ and 5.10 x 10¹ MPN/100 mL). The recorded values for the chemical parameters in the dry and wet season were within the following ranges: ammonia (0.27 and 3.68 mg/L), chemical oxygen demand (COD) (29.53 and 22.10 mg/L), nitrate (9.21 and 2.40 mg/L) and ortho-phosphate (0.46 and 0.39 mg/L). Though the detections of these indicator parameters were affected differently by the seasonal variations, it is important to note that the efficiency of the WWTP in reducing these indicator parameters proved to be consistent across all seasons, except in the case of ammonia and nitrate. The majority of the studied parameters showed effective compliance when measured against SA regulatory standards (general limits) in both the dry and wet season, with the exception of ammonia during the wet season.