9.4 A river has been contaminated with approximately 150µg/L of atrazine.It has been proposed to pass part of the river through a marsh in which the atrazine would be degraded

by microorganisms. The degradation of atrazine follows the rate equation -r_{\text {antrazine }}=k C_{B M} C where k is the rate constant, CBM is the biomass concentration, and C is the atrazine concentration. The marsh is rectangular and the flow of the water through it can be modeled as plug flow. The rate constant is experimentally determined to be 2 x 105 L/mg volatile suspended solids(VSS)/hr under steady state conditions at a biomass concentration of 60mg VSS/L. In order to meet environmental standards, the concentration of atrazine exiting the marsh must be less than 3µg/L. The entering flowrate is 2m3/hr. (a) Determine the marsh volume necessary to meet the environmental standards. If the average depth of the marsh is 0.25 m and the width is 100 m, is the necessary length reasonable? (b) If the entering flow rate increases to 3m3/hr during a high precipitation event, what will be the expected exit concentration of atrazine?What would you recommend doing to prevent this concentration from being too high? What variables other than flow rate might cause a high exit concentration of atrazine?