Answer Happy

3.2. A lake with a single inflow stream has the following characteristics: Mean depth 5 m Surface area = || X 106 m² Residence time = 4.6 yr An industrial plant presently discharges malathion (W = 2000 x 106 g yr!) to the lake. In addition the inflowing stream also contains malathion (in 15 mg L-'). Note that the volumetric rate of inflow and outflow are equal. Assuming that a first-order decay reaction can be used to characterize malathion decay (k = 0.1 yr-). (a) Write a mass-balance equation for malathion for this system. (b) If the lake is at steady-state, compute the in-lake malathion concentration. (c) If the lake is at steady-state, what industrial plant loading rate must be maintained to lower the lake's concentration to 30 ppm? Express your result as a percent re- duction (d) Evaluate each of the following engineering options to determine which is the most effective for lowering the steady-state concentration: (i) Reduce the present loading rate of the industrial plant by building a waste treatment facility that will remove 50% of the malathion from the plant's ef- fluent (ii) Double the lake's depth by dredging. (iii) Double the lake's outflow rate by diverting malathion-free water from a nearby unpolluted stream into the lake. (e) What other factors would need to be considered aside from lowering concentra- tion) when making a decision in (d) in the real world"? (1) Determine the 95% response times for each of the options in part (d).