Impact of industrial effluent on the self-purification characteristics and oxygen sag level of Ajali River, South east Nigeria

Pollution and self-purification characteristics of Ajali River stretch impacted by industrial wastewater was studied. Ajali River was sampled from -250 m upstream before the pollution to 2250 m downstream at eight sampling points. The self-purification capacity of the river was assessed in terms of capacity for self-purification per unit length and Fair’s number. De-oxygenation data were fitted to first and second-order models. Sag analyses was carried out using Streeter-Phelps method and some parameters such as saturation DO, minimum dissolve oxygen (DO min ), initial oxygen deficit (D O ), initial ultimate BOD (Lo), critical oxygen deficit (Dc) and maximum permissible BOD Load (L A ) were determined. De-oxygenation rate of the river were 0.149 and 0.152 day -1 for dry and rainy seasons respectively. Re-aeration rate were 0.271 and 0.415 day -1 for dry and rainy seasons respectively. Fairs ratio was estimated to be 1.8 and 2.73 for dry and rainy seasons respectively. Oxygen deficits at the sag point were 3.62 and 2.92 mg/L for dry and rainy seasons. The minimum dissolved oxygen was 4.17 and 5.21 mg/L for the dry and rainy seasons. The maximum permissible BOD limits were 11.460 and 13.597 mg/L for both seasons. Ajali was found to have a self-purification capacity for organic pollutants. It has an initial oxygen deficit before the introduction of effluent. Ajali River recovered from the pollution effect at 1000 m from the point of effluent discharge during the rainy season and 1500 m during the dry season. It was concluded that Ajali River can naturally purify itself at a distance of 1000 m and 1500 m for dry and rainy season respectively. Based on Fair’s ratio, re-aeration was seen to pre-dominate de-oxygenation process and the river falls into the category of large stream of normal velocity possessing moderate re-aeration potential. The results showed that Ajali river possess potential for self-purification especially organic pollutants.