Analysis of water quality by comprehensive pollution index (CPI) and self-purification capacity of Shinta River, Ethiopia

Shinta River is one of the tributaries of the Megech River basin and is used for various purposes. However, the river is affected by Dashen Brewery effluents and municipal waste. The general objective of the study was to evaluate water quality using Comprehensive Pollution Index and determine the self-purification capacity of Shinta River using the Streeter–Phelps model. The water samples were collected for four consecutive months at seven monitoring stations from November 2021 to February 2022. The river water quality parameters were analyzed as per standard methods for the examination of water and wastewater. The suitability of the river water quality for irrigation, drinking, and aquatic life was evaluated using a comprehensive pollution index. The self-purification capacity of the river was determined using a Streeter–Phelps model. The comprehensive pollution index (CPI) analysis results demonstrated that the river water quality was found in sub-cleanness grade II (0.21–0.4) which is suitable for irrigation purposes. The CPI for drinking quality and aquatic life showed that the river quality varied from Grade III–IV (0.42–3.42) and (0.82–5.05), respectively, which is slight pollution to severe pollution for drinking purposes and aquatic life. As per the Streeter–Phelps model, the Shinta River's minimum oxygen sag curves DO in mg/L ranged from 1.21 to 0.86 after traveling 4.43 km and taking in a range of 1.04–2.17 h from sample point one in the studied four consecutive months, which were lower than the DO requirement for aquatic life. The self-purification capacity factor (f) values were greater than unity, indicating that reaeration predominates over deoxygenation implying that the river is naturally purified, but less than unity, implying that the river is polluted rather than purified. The river was highly polluted by Dashen Brewery industry effluents and municipal untreated wastewater that was beyond the river's natural capacity to purify it. This has affected human health and aquatic life. Therefore, to manage properly and treat before discharge into river effluents of Dashen Brewery and municipal wastewater as well as provide wastewater treatment plant.