Groundwater contains $3\mathrm{.}5m\frac{g}{L}Fe(II)$ and $1m\frac{g}{L}N{H}_4^{+}.$ After this groundwater is aerated with sprayers, it is passed through a rapid sand filter and is aerated again with a cascade aerator. Experiments have shown that the sprayers reduce the $O_2$ deficiency from $90\%$ to $5\%,$ while the cascade aerator increases the $O_2$ content from zero to $50\%$ of the saturation value. Assuming that $Fe(II)$ and $N{H}_4^{+}$ are fully oxidized, find the $O_2$ content of the treated water. Assume that $Cs=9m\frac{g}{L}.$

Hint:

$D_0=0\mathrm{.}90C_s=>C_0=0\mathrm{.}10C_s$

$D=0\mathrm{.}05C_s=>C=0\mathrm{.}95C_s$

$($For sprayers