Neco Model Questions Vol1 2017 Chemistry Question 16

A factory discharges treated effluent into a river. The effluent initially contains pollutant X at a concentration of \(120 \text{ mg L}^{-1}\). The treatment plant operates in two successive stages. The first stage removes 40 % of the pollutant, and the second stage removes 30 % of the amount remaining after the first stage. After treatment, the effluent mixes with river water whose flow rate is five times that of the effluent. (a) Calculate the concentration of pollutant X in the river water after mixing. (b) The environmental limit for X is \(10 \text{ mg L}^{-1}\). Does the concentration obtained in part (a) satisfy the limit? (c) If, instead, the river flow were only twice the effluent flow, what minimum overall removal efficiency must a third treatment stage achieve (applied to the concentration after the second stage) so that the final concentration after mixing meets the limit?