Secondary biological treatment consists of _______, either mixed in suspension in a basin or attached to a media of some type where the _____ material is broken down and consumed. Most secondary treatment processes require _____ for the bacteria.

• A. solids, inorganic, oxygen
• B. microorganisms, organic, oxygen
• C. bacteria, inorganic, carbon dioxide
• D. microbes, inorganic, nitrogen

Answer: (B)

Secondary biological treatment relies on microorganisms to oxidize organic matter. In this stage, these microbes are either kept in suspension in a basin or attached to a surface as a biofilm, where they feed on the organic material present in the wastewater. Oxygen is required because most of these bacteria use aerobic respiration to metabolize that organic matter, turning it into energy, carbon dioxide, and water. That makes the correct pairing microorganisms, organic, and oxygen the best fit: the organisms perform the breakdown, the material being broken down is organic matter, and oxygen provides the necessary support for the bacteria to do their job.

The other options don’t fit because solids are not the primary substrate targeted in typical secondary treatment—the focus is on dissolved or suspended organic matter, not mainly solids. Inorganic material isn’t the main food source for the bacteria in this phase, and carbon dioxide is a product rather than the substance being consumed. Nitrogen isn’t the primary requirement described for driving the biological breakdown in standard secondary treatment.