EXAMPLE $4-9$ Before entering an underground utility vault to do repairs, a work crew analyzed the gas in the vault and found that it contained $29mg*m^{-3}$ of hydrogen sulfide. Because the allowable exposure level is $14mg*m^{-3},$ the work crew began ventilating the vault with a blower. If the volume of the vault is $160m^3$ and the flow rate of contaminant$-$free air is $10m^3*mi{n}^{-1},$ how long will it take to lower the hydrogen sulfide level to a level that will allow the work crew to enter? Assume the manhole behaves as a CMFR and that hydrogen sulfide is nonreactive in the time period considered.

a$.$ Convert the allowable exposure level given in the example $(14m\frac{g}{m^3})$ to $pp{m}_v,$ assuming the temperature and pressure in the vault are $28C$ and $\frac{1}{b}ar,$ respectively. For this question, don't neglect the difference between $1$ bar $(100\mathrm{.}000$kPa and $1$ atmosphere $(101\mathrm{.}325$kPa$).$

b$.$ What type of microbial activity might be responsible for the generation of hydrogen sulfide $(H_2(S))$ in the vault?

c$.$ What physicochemical process $($among the types of equilibria we have studied$)$ can explain the presence of $H_{2}S$ in both the wastes and the atmosphere in the vault?

d$.$ Given the processes that are the answers to parts a and $b,$ what would your recommendation be regarding the flushing to reduce $H_{2}S$ to the allowable level after that allowable level is reached? Should flushing continue or can it be stopped? Briefly explain the logic you're basing your recommendation on$.$