Example $($H$.$W$.)$

An industry has decided to treat its process wastewater in a biotower with a plastic

modular medium $(n=0\mathrm{.}6).$ The flow rate of the wastewater is $1000\frac{m^3}{d}$ with a

$BO{D}_5$ of $500m\frac{g}{l}$ and an average temperature of $18C.$ The treatability constant $k$

is $0\mathrm{.}04mi{n}^{-1}$ for the system at $20C.$ Depth of the medium is $5\mathrm{.}5m.$ The desired

effluent $BO{D}_5$ is $15m\frac{g}{L}.$ Calculate the following:

a$.$ The area of biotower required, without any recycle.

b$.$ The organic loading rate for the biotower without recycle.

c$.$ The area of biotower required when direct recirculation ratio is $3$:$2.$

d$.$ The organic loading rate for the biotower with recycle.

e$.$ Which of the above designs seems better to you and why?