Question: Chlorination is used for disinfection of the final effluent of a waste water treatment plant. The EPA requires that a certain chlorine residual be maintained. To meet this requirement, the free chlorine residual is measured at the beginning of the chlorine contact basin, as shown in Fig. An aqueous solution of sodium hypochlorite is added to the filter effluent to maintain the free chlorine residual at the contact basin. The amount of sodium hypochlorite required is directly proportional to the flow rate of the effluent. The wastewater plant has two parallel filter effluent streams, which are combined in the chlorine contact basin. Sodium hypochlorite is added to each stream on the basis of the free chlorine residual in the basin.

(a) Design a control scheme to control the chlorine residual at the beginning of the basin.

(b) As a result of a number of reactions occurring in the contact basin, the chlorine residual exiting the basin is not equal to the chlorine residual entering the basin (the one being measured). It happens that the EPA is interested in the exiting chlorine residual. Thus a second analyzer is added at the effluent of the contact basin. Design a control scheme to control the effluent chlorine residual.