In extended aeration systems, the oxygen demand per pound of BOD removed is approximately how many pounds of O2?

• A. 1.1
• B. 3.0
• C. 1.5
• D. 2.0

Answer: (C)

Oxygen required to biologically oxidize BOD is needed in excess of the mere stoichiometric amount because microorganisms use O2 not only to oxidize the organic matter but also for growth and maintenance. In extended aeration systems, the practical oxygen demand to remove one pound of BOD is about 1.5 pounds of O2. This figure reflects the typical balance of oxidation plus biomass synthesis in these systems, making it the best match among the options. Values like 1.1 are generally too low for practical design, while 2.0 and 3.0 exceed the common expectation for extended aeration BOD removal.