A tank oxidises; a train removes
An activated sludge tank with air in it removes organic matter, because the biology that grows there consumes it. It does not, by itself, remove nitrogen or phosphorus in a controlled way. Nitrogen leaves the system only when a nitrate ion is turned into nitrogen gas and escapes to the atmosphere, and phosphorus leaves only when it is bound into a sludge that is then taken out of the plant. Both of those need a zone where oxygen is absent, which is why a nutrient-removal plant is built as a sequence of tanks rather than one.
The A²/O layout — anaerobic, anoxic, aerobic — is the common arrangement for municipal sewage where both nitrogen and phosphorus limits apply. Each zone hosts a different set of conditions, and each set selects for a different reaction, which is the reason its volume is what it is.
The anaerobic zone releases phosphorus
The anaerobic zone is the first tank in the train and the one most often misread. It carries no oxygen and no nitrate, and the biology that thrives there takes up volatile fatty acids and stores them. To do that, these organisms release phosphate from their cells into the water, so this zone raises the phosphate concentration rather than lowering it.
That release is not a fault but the first half of a two-step mechanism. The same organisms take up far more phosphate in the aerobic zone than they released here, and the excess goes into the sludge. Phosphorus is not destroyed at any point; it is concentrated into the wasted sludge, which is how the plant removes it. National practice gives the anaerobic tank a hydraulic residence time of 1–2 h (GB 50014-2021); a much longer time risks the wrong biology taking over.
The anoxic zone turns nitrate into nitrogen gas
The second tank receives the mixed liquor with no free oxygen present, and it is here that nitrogen is actually removed. Denitrifying bacteria use nitrate as their oxygen source, and the nitrate is reduced to nitrogen gas, which leaves the water and joins the atmosphere. The reaction needs an organic carbon source, and the influent supplies it.
Two conditions have to hold at once. There must be no dissolved oxygen, because the bacteria would use it in preference to nitrate and the reduction would stop. And there must be nitrate arriving, which is why the nitrate produced in the aerobic zone is returned to the anoxic zone through the recycle loop. National practice gives the anoxic tank a hydraulic residence time of 0.5–3.0 h (HJ 576-2010), a wider band than the anaerobic tank because the time needed depends on how much nitrate is returned and how much carbon is available.
The aerobic zone finishes the job
The third tank is aerated, and two things happen in it. Ammonia is oxidised to nitrate by nitrifying bacteria, the step that makes the nitrogen removable at all: without nitrification the nitrogen stays as ammonia and no anoxic zone can take it out. And the phosphorus-accumulating organisms take up phosphate from the water into their cells, the uptake step the anaerobic release prepared them for.
The aerobic zone also oxidises the remaining organic matter, so it carries most of the aeration energy of the plant. The mixed liquor leaves this tank for a secondary clarifier, where the sludge settles and is separated. The return sludge keeps the biology in the system, and the waste sludge is what carries the phosphorus out of it.
A/O, A²/O and extended aeration
The A/O train drops the anaerobic tank and keeps the anoxic and aerobic stages. It removes nitrogen and organic matter, and the lower tank count makes it simpler, but without the anaerobic release step phosphorus removal is limited. An A/O or A²/O train with a secondary clarifier and disinfection normally delivers the less demanding grades of GB 18918-2002 — Grade 1B and Grade 2 — and where only organics are controlled the biology can fall back to biological contact oxidation.
Grade 1A, the strictest grade published on the standards chart, is a different problem. Its total phosphorus limit is 0.5 mg/L against 1 mg/L at Grade 1B, and ordinary biological treatment does not reach that reliably; the grade is usually met with a filter or a membrane after the biological stage, or with chemical dosing. Its total nitrogen limit of 15 mg/L, against 20 mg/L at Grade 1B, forces the anoxic zone to be sized with margin rather than treated as an afterthought.
Extended aeration is the third arrangement, and it is the Middle East convention rather than a national-standard one. The Saudi MEWA guide describes it as an aeration time of 18–36 h, a mixed liquor suspended solids of 3,000–6,000 mg/L and a sludge age of 10–30 days. It needs no membrane, but it takes more tank volume than any of the zone-based trains above.
What the zones decide about tank volume
The zone definitions translate into tank volume, because volume is flow multiplied by residence time: V = Q × HRT. The anaerobic tank at 1–2 h and the anoxic tank at 0.5–3.0 h are small next to the aerobic tank, which carries the aeration time and therefore most of the volume. Where phosphorus removal is the main objective, the solids retention time is set at 3.5–7.0 days (GB 50014-2021), and that figure sets how much sludge the plant holds rather than the size of any single tank. The split between the anoxic and aerobic volumes is not fixed by the standard; it is balanced against the nitrogen and phosphorus targets.
A zone question, not a separation question
The zones described here govern what is removed from the water. A separate decision is how the biology is held and how it is separated from the treated water, which is the subject of the comparison between MBR, MBBR and SBR. A plant can run A²/O zones with a membrane instead of a clarifier, or hold the biology as a biofilm. The zone sequence and the separation method are two different choices, and the selection guide takes them in that order.