Determine the NRC BOD removal efficiency for a two$-$stage trickling$-$filter plant based

on the following: primary clarification with $35\%$BOD reduction, first$-$stage filters loaded

at $80$lb$1000$ft$3$day$,$ intermediate settling, second$-$stage filters sized identical to the first$-$

stage units, an operating recirculation ratio of $1\mathrm{.}0$for all filters, final clarification of the

effluent, and a wastewater temperature of $18$C$.$