Per$-$ and polyfluoroalkyl substances $($PFASs$),$ commonly known as persistent organic pollutants, have prompted health concerns because of their widespread presence in drinking water. In $2023,$ US EPA announced the new drinking water regulation for six PFASs. Perfluorooctanoic acid $($PFOA$)$ is one of the six highly$-$concerned PFASs. The new proposed Maximum Contaminant Level for PFOA is $4$ ng$/$L$,$ while the old standard is $70$ ng$/$L$.$

Suppose you are exploring the potential of a photocatalysis technology to remove PFOA in drinking water to meet the new standard. The lab$-$scale reactor can be seen as a CSTR$.$ The degradation of PFOA in the reactor can be expressed by first$-$order kinetic models, and the first$-$order reaction rate coefficient $($k$)$ is $0\mathrm{.}5$ h$-1.$ Assuming that the concentration of PFOA in the influent is $50$ ng$/$L$,$ please estimate the minimum required hydraulic retention time.