Based on loading of $28$ Kg$/$day$/$hectares$,$ what is stabilization pond area $($in hectares$)$ is needed for an average daily flow of $3180$ m$3/$d with $200$ mg$/$L $($g$/$m$3)$ BOD? $(7$ marks$)$

Calculate the minimum water depth for a retention time of $90$ days assuming an evaporation and seepage loss of $3\mathrm{.}5$ mm of water per day.