Anaerobic/aerobic operation for nitrogen and phosphorus removal

In activated-sludge treatment, an anaerobic/aerobic reactor sequence supports both nitrogen and phosphorus removal. The anaerobic section creates an active environment for denitrifying bacteria and for phosphorus-accumulating bacteria; the aerobic section drives nitrification. For nitrogen removal, nitrifying bacteria must thrive in the activated sludge, so ammonia nitrogen is oxidized to nitrate in the aerobic zone. The secondary clarifier effluent containing nitrate then mixes with sewage in the anaerobic zone, where anaerobic microorganisms use the oxygen bound in nitrate to break down organic matter, releasing nitrogen as gas. The key is promoting full nitrification — high MLSS, sufficient air volume and temperature help; nitrification slows markedly in winter. For phosphorus removal, under strictly anaerobic conditions (no oxygen, no nitrate) phosphorus-accumulating bacteria release inorganic phosphorus to obtain energy; in the following aerobic tank they take up more phosphorus than they released, removing it from the water. The same anaerobic/aerobic operation can remove both nitrogen and phosphorus — but the two mechanisms require different conditions: anaerobic (no oxygen, nitrate present) for denitrification, and strictly anaerobic (no oxygen, no nitrate) for phosphorus release.