38. Biological oxygen demand (BOD) is a measure of the relative oxygen depletion effect of a waste contaminant and is widely used to assess the amount of pollution in a water source. The BOD in the effluent ( in mg/L) of a rock filter without recirculation is given by:

where is the influent BOD (mg/L), D is the depth of the filter (m), and Q is the hydraulic flow rate [L/(m2-day)]. Assuming L/(m2-day) plot the effluent BOD as a function of the depth of the filter ( m)

for mg/L. Make the three plots in one figure and estimate the depth of filter required for each of these cases to obtain drinkable water. Label the axes and display a legend.