In wastewater treatment plants a pretreatment is carried out to remove stones, sand, and solids. The contaminants that the water still contains, such as fats and oils, are then removed. It is then clarified by a biological process and disinfected with chlorine. In this way, water suitable for irrigation or for industrial use is obtained. If you were in charge of a wastewater treatment plant: 

a) What method of separation of mixtures would you use in the pretreatment of wastewater? Why?

b) What methods of separating mixtures would you use to remove grease and oils from wastewater? Justify your answer.