Q$2.(6$ Marks$)$

The hapoon relates to a brook as shown in the figure. The ingoon has a surface area of $5\mathrm{.}5ha$ and a depth of $1m$ receiving $0\mathrm{.}2m^3/$s of sewage containing $150mg{L}^{-1}$ of biodegradable contaminnt. At steady tate, the biodegradation reaction rate coefficient is $0\mathrm{.}35d^{-1}.$ The velocity of the brook is $0\mathrm{.}35m/$s and extends for $3\mathrm{.}5km$ assuming the lagoon is well mixed and that there are no losses or gains of water in the lagoon other than the rewage isput. Determine the emnent conceatration at the end of the brook.