Problem $4.-$ The average flow rate of a small river during the dry season is $100L*S^{-1}.$ The $BO{D}_5$ of the effluent of a wastewater treatment plant is usually equal or below $20mg{O}_2*L^{-1}.$ The limit value of receiving water for the $BD{O}_5$ is of $4mg{O}_2*L^{-1}.$ How many inhabitants can this treatment plant serve, while ensuring that river water quality standards are met?

Suppose that the $BO{D}_5$ of the upriver water is null and that this river is not the source of drinking water for the inhabitants of this village. The wastewater generation rate is $400L*ha{b}^{-1}{d}^{-1}.$