The quality of drinking water supply can be controlled through the assessment of source water status and continuous monitoring of treated water quality. Water disinfection is a vital component of water treatment. This study was conducted to assess the quality of water (source and treated) from Erelu waterworks that supplies potable water to Oyo town and its environs in Southwest part of Nigeria. The aim is to establish the quality of water supplied to the consumers. Six routine monitoring parameters (RMPs): pH, colour, total hardness (TH), total alkalinity (TA), flocculation and chlorine residual (CR) data were collected over a 3-year period (2018-2020) for quality status assessment of raw water and water treated by the Water Corporation of Oyo State, Nigeria. Conventional water treatment methods involving aeration, coagulation, flocculation, sedimentation, filtration, and disinfection were used on raw water samples. The RMPs data were analyzed using descriptive statistics and Analysis of variance (ANOVA). Three different regression models (linear, semi-log and double log) were used for the raw water data to generate models for the prediction of flocculation. The raw (untreated) water was soft to moderately hard with relatively high colour values that exceed the WHO permissible limit (2011). However, all the assessed RMPs values for the disinfected water lie below the WHO guideline limits for drinking purpose. The highest flocculation values of 100-120 mg/l Al-2(SO4)(3) were recorded from April to July of 2019. Colour showed positive correlation with TA (r = 0.357) and flocculation (r = 0.569) (significant at p < 0.01). Total alkalinity showed moderate positive correlation with flocculation (r = 0.584; p < 0.01). The pH, TA and TH concentrations of the reservoir water were all below the WHO guideline limits except colour (> 15 Hu). The quality of disinfected treated water indicates that levels of pH, colour, TH and TA lie within the WHO acceptable limits for safe drinking water (WHO (2017): Guidelines for drinking water quality. 4th edn. Incorporating the first addendum: WHO guidelines approved by the guidelines review committee. World Health Organization, Geneva.). The mean CR of treated water was > 0.5 mg/l at water treatment plant (WTP) and concurs with the WHO guideline that higher levels should be close to the disinfection point and highly likely to maintain the desirable limit (0.2-0.5 mg/l Cl) by the time it gets to the point of use. The regression analyses showed that semi-log model performs better than both linear and double-log models for predicting flocculation with highest value of R-2 = 69.5% and significant F-value (17.631). The RMPs obtained showed that water treatment practices at the Waterworks are effective. A further study to monitor the levels of nutrient load, chlorination-by-products and coliform count in disinfected water should be done to maintain transparency of treated water quality.
