Practice

Raw wastewater passes through primary settling tanks and then an activated-sludge process, and the discharge permit limits the final effluent to $S_e=30\,\text{mg/L}$ BOD$_5$. The primary unit removes 35% of the BOD and the secondary unit removes 85% of the BOD reaching it. The highest raw-wastewater BOD$_5$ (mg/L) the plant can receive and still meet the limit is most nearly:

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