Liquid effluent $(104$ m$^3/$day$)$ from a food processing unit is to be treated by the activated sludge process at $30\backslash$deg C from an initial $($BOD$)5$ days of $650$ mg$/$l to a final $($BOD$)5$ days of $25$ mg$/$l$.$ Bench$-$scale studies at $20\backslash$deg C and mixed$-$liquor biomass concentration of $3000$ mg$/$l gave BOD removal rate coefficient of $14($day$)^-1.$ Estimate the retention time and size of the unit. $($temperature coefficient$)=1\mathrm{.}02.$