Toxicity of Industrial Wastewater Acting Singly or in Joint-Ratios on Clarias gariepinus

Freshwater ecosystems were the most vulnerable to pollution due to their easy accessibility for disposal of wastewaters leading to deterioration of water quality. Ona river, Ibadan, Nigeria is part of a dense network of inland watercourses emptying into the Lagos lagoon and receives industrial effluents from a food and beverage industry. The single and joint-action toxicity of effluents from these industries mixed in predetermined ratios of 75:25, 50:50 and 25:75% (v/v) (food:beverage); 100% (food) and 100% (beverage) respectively were tested against Clarias gariepinus fingerlings in a series of static bioassay to determine the pollution implications of interacting components of effluents and their effect on fish survival. Physico-chemical parameters of exposure concentrations and effluent samples were analyzed using standard methods. Effluent samples were further analyzed for heavy metals and interactions of composite mixtures were evaluated with models such as the Synergistic Ratio (SR), concentration-addition Relative Toxic Units (RTU) and isobologram (pictorial isobole) for joint-action evaluations. The 96 h LC₅₀ values were 23.00, 31.00, 71.79 and 33.50% (food) while no LC₅₀ value was derived for the 100% beverage effluent. Toxicity evaluation of effluent mixtures revealed decreasing toxicity in the order 75:25% (food: beverage); 50:50% (food: beverage); 100% (food) and 25:75% (food: beverage). Effluent samples and exposure concentrations were characterized by acidic pH and low DO (except for control exposures) while other effluent physico-chemical analysis revealed that Pb, Cr and Fe concentrations exceeded NESREA and WHO permissible limit for surface waters and protection of the aquatic ecosystem. The SR and isobologram models both depicted the food-beverage mixture of 75:25 and 50:50% as synergistic and the 25:75% mixture as antagonistic. The concentration-addition model depicted all mixture ratios as synergistic. The observed antagonistic interactions of 25:75% (food: beverage) ratio of synchronized effluent discharge by these industries towards setting safe limits for environmental discharge could be explored.