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Q3. Answer all sections (a) The two-reservoir system shown in the figure below is fed by a stream with flow rate of 5000 mº day' and dissolved organic matter (a non-conservative pollutant) concentration of 25 mg/L. The rate of decay of this organic matter is 0.20 day". The volume of the first pond is 20,000 mº and the second is 13,000 m² Stream First reservoir Second reservoir (i) Draw a sketch of the control volume and mass balance for each reservoir with indication of all the information about volume, flow rate and pollutant concentration and justify all the hypothesis made for this. [5 marks) (ii) Calculate the concentration of dissolved organic matter leaving each pond. [5 marks] (b) A river with 200 mg/L of total suspended solids (a conservative substance) and an upstream flow of 1500 mº/min receives an agricultural discharge of 18000 m/h carrying 2000 ppm of total suspended solids. The suspended solids quickly become uniformly distributed in the river. A municipality just downstream withdraws water and mixes it with enough pure water (no solids) from another source to deliver water having no more than 250 ppm solids to its customers. (i) Draw a sketch of the control volume and mass balance with indication of all the information about volume, flow rate and pollutant concentration. [5 marks] (ii) What should be the mixture ratio of pure water to river water? [5 marks] (ii) What should be the mixture ratio of pure water to river water? [5 marks] Total [25 marks]