Determination of characteristics and drinking water quality index in Mzuzu City, Northern Malawi

Abstract

An assessment of characteristics and chemical water quality index (WQI) of water supplied by the Northern Region water Board (NRWB) in Mzuzu City was carried out in order to ascertain the quality of water for domestic purposes. The WQI offers a single number that expresses overall water quality for a water sample based on several water quality parameters. In this study raw water and 72 tap water samples were collected monthly between March and September, 2011 and analyzed for major ions, pH, total dissolved solids (TDSs), electrical conductivity (EC), turbidity, total hardness (TH), suspended solids (SSs) and alkalinity using standard methods. The quality and accuracy of the chemical data was assessed by checking electrical balances. The calculated electrical balance errors were found to be less than ±10%, which meant the results were reliable. Based on the Sawyer and McCarty TH classification, 100% of the samples were soft waters (TH<150mg/L). Nitrates, which registered medium or average WQ-rating of 69.77 and WQ-rating range of 52.06–86.94, were observed to have significantly affected the overall water quality index of the treated water since the rest of the parameters registered good–excellent WQ-ratings (average WQ-rating: 80.21–97.87). The pH, which is used to determine suitability of water for various purposes, ranged between 6.40 and 6.90 and registered a good water quality rating (WQ rating range: 72.73–87.02) for both raw and treated water. Raw water registered an overall medium water quality rating of 62.67%. Overall, 91.67% of the samples registered a good water quality rating (WQI range: 80.28–88.80%) and 8.33% registered a very good water quality rating (WQI=90.07%). The results suggested substantial water treatment by the NRWB since the treated water is protected with some negligible degree of impairment that rarely departs from desirable levels of domestic water quality. It is recommended that the WQI should be adopted as a tool to monitor and establish trends in quality of water supplied by the NRWB since it is a composite index that turns complex water quality data into an aggregate rating that reflects the combined influence on the overall water quality as opposed to the univariate water quality assessment approaches such as the Malawi Bureau of Standards.