An industrial wastewater treatment plant consists of a primary clarifier followed by a biological tower (trickling filter). the soluble bod concentration leaving the primary clarifier is 250 mg/l, and the corresponding flow rate is 1.0 million gals per day (mgd). the biological tower consists of a high-density, crossflow packing media with dimensions of 480 ft2 by 20 ft high. it has a specific area of 42.0 ft2 /ft3 , n = 0.5, and k20 = 0.0035 (gpm/ft2 ) 0.5. the recirculation ratio, r = 1.0; the water temperature is 15°c. calculate the soluble bod concentration in the biological tower effluent.