$75000$ m$3/$d raw water with $4$ mg$/$L dissolved oxygen and a temperature of $20$o C is passed over two $(2)$ cascade aerators each having $3$ steps with height of $0\mathrm{.}5$ m$.$ The influent enters to each aerator having $300$ m$3$ water through a $500$ mm diameter pipe. Henry$$s constant for O$2$ at $200$C$=4\mathrm{.}11$x$104$ atm $,$ density of water $=998$ kg$/$m$3$ a$)$ What is the oxygen concentration in the effluent of cascade aerators? b$)$ Calculate

the necessary surface area of the cascade aerator. Decide length and width of each cascade aerator.