An oil refinery has a wastewater flow of 18 liters per second, a temperature of 38.6oC and a concentration of oil and grease of 680 mg/L. It is treated at a pressurized-air flotation (DAF) system. An effluent oil and grease concentration of 50 mg/L is required by the pretreatment permit issued by the Puerto Rico Aqueducts and Sewer Authority. An aluminum sulfate dosage of 13.8 ppm is needed to break the emulsions. The required A/S is 0.046 and the overflow rate of the flotation unit is 2.12 GPM/ft2 . The operating pressure is 5.75 atm. Consider that the sludge production is 0.37 mg per mg of alum used and that the fraction of saturation in the retention tank is 0.853. Stating your assumptions, determine the: a. required recycle flow of the flotation system (m3 /hr and GPM). b. surface area (ft2 ) and height (ft) of the flotation unit. c. volume of sludge (GPD) produced if the skimmings are 2.90%, by weight