Anoxic conditions are defined as:

• A. Fully oxygenated
• B. Oxygen deficient less than 0.5 ppm
• C. Low pH
• D. High salinity

Answer: (B)

Anoxic means there is effectively no free dissolved oxygen available for aerobic respiration. In wastewater treatment, this is achieved by reducing aeration so the dissolved oxygen drops to very low levels, typically below about 0.5 mg/L (ppm). At these low concentrations, aerobic microbes can’t function, and processes like denitrification can occur, using nitrate as the electron acceptor instead of oxygen. This helps remove nitrogen from the water. The other conditions don’t define anoxia: being fully oxygenated is the opposite, low pH relates to acidity, and high salinity concerns salt content, not oxygen availability. Remember that in water, dissolved oxygen is measured in mg/L, which is the same as ppm for practical purposes.