A facultative pond system for sewage treatment consists of two ponds in series. The hydraulic retention time $\text{(HRT)}$ of each pond is $6$ days. The total $\text{BOD 5}$ reduction through the entire pond system is $90 \%$. If the ponds are considered to be completely mixed, then the rate constant describing the $\mathrm{BOD}_5$ removal is _______ day $^{-1}$ $\text{(rounded off to two decimal points)}$. Assume that the rate constant is same for both the ponds.
