Environment & Water Principle Animation:AAO Wastewater Treatment Lab

Municipal sewage carries six to ten times the allowed COD, ammonia and phosphorus. Both nitrogen and phosphorus removal feed on the organics in the sewage; if carbon is short they compete. Try the low-carbon influent.

The anaerobic tank is mixed but not aerated. PAOs take up readily degradable organics and store them as PHA, paying by breaking down internal polyphosphate and releasing phosphate—so phosphorus here is higher than in the influent. Nitrate in the return sludge grabs some of the organics first, leaving less for the PAOs.

Nitrate is made in the aerobic tank, so mixed liquor is pumped from its end back to the anoxic tank, where denitrifiers use organics to reduce it to nitrogen gas without oxygen. More recycle brings back more nitrate, but beyond 200–300% carbon runs short and recycled oxygen interferes, so total nitrogen stops falling.

Nitrifiers grow slowly, dislike cold and need oxygen: below about 1 mg/L DO or near 10 °C, ammonia tends to break through. PAOs use the PHA stored earlier to take up more phosphorus than they released; the phosphorus finally leaves with the waste sludge.

Activated sludge settles in the clarifier and clear water overflows. Most settled sludge returns to the anaerobic tank; the excess is wasted, and the phosphorus-laden PAOs leave the system with it. Top right checks Class 1A: COD 50, ammonia 5 (8 in winter), TN 15, TP 0.5 mg/L.