A) In a circular stepped cascade aerator, it is found the dissolved oxygen concentration changes from o mg/L to 4.5 mg/L in a single step. The saturation dissolved oxygen concentration at the experimental condition 7.5 mg/L. To achieve an efficiency of 86%, determine the number of steps required. = B) Chlorine gas used for disinfection combines with water to form hypochlorous acid (HOCI). The HOClionizes to form hypochlorite (OC) in a reversible reaction : HOCI H+ + OCl (k= 2.7 x 10-8 at 20C), the equilibrium of which is governed by pH. The sum of HOCI and OCl is known as free chlorine residual and HOCI is the more effective disinfectant. Find out pH value for which 90% fraction of HOCI in the free chlorine residual is available. (6+6)