In a sewer with long, sluggish-flow and flat grade, what is a likely consequence of biological activity on air quality?

• A. Increased oxygen in air
• B. No effect
• C. Oxygen deficiency in air in manholes, sewers, or wet wells
• D. Reduced solids deposition

Answer: (C)

In systems with long, sluggish flow and a flat grade, microorganisms oxidize the organic material in the wastewater, consuming the available oxygen. Because the flow is slow, there’s limited air exchange and little replenishment of oxygen in the water and the surrounding air pockets. This combination leads to oxygen-deficient air in confined spaces like manholes, sewers, or wet wells. So the most likely consequence is a deficiency of oxygen in the air there, which can pose hazards to workers and affect ventilation. Increased oxygen would be unlikely under these stagnant conditions, and the issue is about air quality rather than solids deposition, so those options don’t fit.