EXAMPLE $12\mathrm{.}2$

A combined sewer catchment serves a population of $50,000$ and has an impervious area $(A_i)$ of $18ha.$ Determine the overflow setting required upstream of the main outfall sewer using the following approaches:

$6$ DWF

Formula A

River water quality adjacent to the overflow limited to a $BO{D}_5$ of $10m\frac{g}{L}$ for the $1-$year return period, $20-$minute duration event.

Additional information:

Wastewater flow $=250\frac{L}{h}d.d$

Rainfall intensity for the $1-$year, $20-$minute event, $i=20m\frac{m}{h}$

River flow upstream of CSO $=1\frac{m^3}{s},$ with $BO{D}_5$ of $2m\frac{g}{L}$

Overflow $BO{D}_5=500m\frac{g}{L}$

pe infiltration and industrial flows are negligible.