Which parameter must be present for bacteria to effectively treat wastewater in secondary treatment?

• A. Organic carbon
• B. Oxygen
• C. Chlorine
• D. Nitrogen

Answer: (B)

In secondary wastewater treatment, the organisms doing the work are aerobic bacteria that oxidize organic matter. They rely on oxygen to power their metabolism; as they respire, they transfer electrons to oxygen, breaking down pollutants more rapidly. Providing oxygen through aeration is what lets the activated sludge or other aerobic systems function effectively. Without oxygen, these bacteria can’t carry out the intended oxidation, so removal of organics stalls and the treatment process falters. Organic carbon is the food for the microbes, but it won’t drive the process without the oxygen they need. Chlorine would kill the bacteria, and nitrogen is important as a nutrient—but neither is the condition that enables the secondary treatment to work. Oxygen is the essential parameter.