Polyphosphate-Accumulating Organisms Explained: The Microbial Engine Behind Biological Phosphorus Removal

Some microorganisms have a habit that entire treatment plants are built around. Under aerobic conditions, they not only absorb large amounts of phosphate (PO4) to synthesise their own nucleic acids and ATP, but can also take up phosphorus far beyond their metabolic need, against a concentration gradient, storing it as energy-rich polyphosphate granules - the metachromatic granules microscopists know well - for endogenous respiration. These bacteria are called polyphosphate-accumulating organisms, or PAOs.

The trick has a second half. Under anaerobic conditions, PAOs release phosphate (PO4) back into the water. By creating alternating anaerobic, anoxic and aerobic conditions, engineers make PAOs release phosphorus first, then take it up excessively under aerobic conditions, and finally remove the phosphorus with the waste sludge - lowering the phosphorus content of the wastewater without a gram of dosing chemicals.

Who the PAOs Actually Are

PAOs are a group of microorganisms able to absorb phosphate and accumulate it as polyphosphate inside the cell. They can also store poly-beta-hydroxybutyrate (PHB). Almost all known phosphorus-accumulating microorganisms are bacteria. Phosphorus-accumulating activated sludge consists of many aerobic heterotrophs, anaerobic heterotrophs and facultative anaerobes - in essence a mixed population of acid-producing bacteria and PAOs.

The literature on isolated strains is rich: the dominant, strongly accumulating species include more than 60 organisms such as the Acinetobacter-Moraxella group, Pseudomonas, Aeromonas, Flavobacterium and Citrobacter freundii. Nitrifying bacteria also accumulate phosphorus, including Nitrosomonas, Nitrosococcus, Nitrosolobus, Nitrobacter and Nitrococcus.

What Happens in the Anaerobic Zone

Under anaerobic or anoxic conditions, acid-producing bacteria break down macromolecular organics such as proteins, fats and carbohydrates into three types of readily degradable substrate: lower fatty acids such as formic, acetic and propionic acid; glucose, methanol and ethanol; and butyric, lactic and succinic acid. Under anaerobic conditions PAOs break down stored polyphosphate to produce ATP, use that ATP to take up the three substrates by active transport, synthesise PHB, and release PO4 into the environment.

Comeau proposed the acetate uptake theory to explain the mechanism: CH3COO- and H+ outside the plasma membrane combine into a neutral molecule, enter the cell, and are hydrolysed back into CH3COO- and H+ ions. The ATP produced drives H+ out of the cell, rebuilding the proton motive force so that CH3COO- keeps entering. The acetate inside the cell, in the form of CH3COOH, is then converted into PHB.

What Happens in the Aerobic Zone

Under aerobic conditions the direction reverses. PAOs break down intracellular PHB and exogenous substrate, generating a proton motive force that transports PO4 from outside into the cell to synthesise ATP and nucleic acids. The excess PO4 is polymerised into the storage product again - polyphosphate, the metachromatic granules - and this time it leaves the plant inside the wasted sludge.

Orchestrating Phosphorus and Nitrogen Together

To remove phosphorus and nitrogen simultaneously from the same wastewater, the sludge age and hydraulic retention time must be adjusted sensibly to accommodate the physiological requirements of phosphorus-removing organisms, nitrifiers and denitrifiers, so that all three grow and multiply in harmony. If only phosphorus removal is required, chemical dosing alone is sufficient - the biological route earns its complexity when nitrogen targets exist too.

The process catalogue reflects that balancing act: the Bardenpho biological phosphorus removal process, the Phoredox process, the A/O process and its variants, the UCT process, the VIP process, sidestream phosphorus removal such as Phostrip, and sequencing batch reactor (SBR) processes. Each has its own advantages and disadvantages and can be selected according to water quality. All of them remove both phosphorus and nitrogen and are built from anaerobic, anoxic and aerobic stages in different arrangements. The reason so many configurations exist is physiological: phosphorus-removing bacteria and denitrifiers compete for carbon source, so a separate ecological environment suited to each must be created. Even for the same A/O arrangement, the sequencing and operating conditions differ.

The Operating Window That Makes It Work

For good phosphorus removal, the conditions must be held deliberately: NO3 and NO2 must be very low, DO below 0.2 mg/L, redox potential below 150 mV, temperature around 30 C, and pH between 7 and 8. Inside that window, the release-and-uptake cycle of the PAOs runs reliably; outside it, the phosphorus simply stays in the water.

Practical Notes for the Operator

Two implications follow from the biology for day-to-day operation. First, carbon source is the currency of the whole cycle: PAOs need volatile fatty acids in the anaerobic zone to build PHB, and denitrifiers need the same carbon to drive NO3 down to nitrogen gas. When the influent is carbon-poor, the two populations compete, and the usual casualties are both effluent phosphorus and effluent nitrogen. Second, the anaerobic zone must stay genuinely anaerobic - nitrates carried back into it let other bacteria consume the fatty acids first, leaving the PAOs nothing to store and breaking the release step that powers the whole sequence.

Those constraints explain why the process catalogue is so crowded. Every configuration - Bardenpho, Phoredox, UCT, VIP, SBR and their relatives - is essentially a different answer to the same two questions: how to give each population its own ecological niche, and how to keep NO3 away from the anaerobic zone without starving the denitrifiers. Selecting among them is less about finding the best process in the abstract and more about matching the wastewater's carbon, temperature and nitrogen-to-phosphorus ratio to the configuration whose compromises cost least.