What Is Biological Phosphorus Removal? A Professional Glossary of Wastewater Terms
Some microorganisms, under aerobic conditions, 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 (metachromatic granules) for endogenous respiration. These bacteria are called polyphosphate-accumulating organisms (PAOs). Under anaerobic conditions PAOs release phosphate (PO4) back into the water. By creating alternating anaerobic, anoxic and aerobic conditions, PAOs first release phosphorus in the phosphorus-containing wastewater, then take up phosphorus excessively under aerobic conditions, and the phosphorus is finally removed with the waste sludge, lowering the phosphorus content of the wastewater.
As noted above, 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 it is a mixed population of acid-producing bacteria (collectively) and PAOs. According to the literature, the polyphosphate-accumulating bacteria isolated from activated sludge are numerous, and 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.
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: (1) lower fatty acids such as formic, acetic and propionic acid; (2) glucose, methanol and ethanol; and (3) butyric, lactic and succinic acid. Under anaerobic conditions PAOs break down stored polyphosphate to produce ATP, and use ATP to take up these three substrates by active transport and synthesise PHB, while releasing PO4 into the environment.
Comeau proposed the acetate uptake theory: 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 (CH3COOH) inside the cell is converted into PHB.
Under aerobic conditions 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, and the excess PO4 is polymerised into a storage product: polyphosphate (metachromatic granules).
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 they grow and multiply in harmony and both goals are met. If only phosphorus removal is required, chemical dosing alone is sufficient.
Phosphorus removal processes include 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 of these processes 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. However, the organisms at the heart of each process, namely phosphorus-removing bacteria, nitrifiers and denitrifiers, have different physiological characteristics, and phosphorus-removing bacteria and denitrifiers compete for carbon source. A separate ecological environment suited to each must therefore be created, which is why so many process configurations have been developed. Even for the same A/O arrangement, the sequencing and operating conditions differ.
For good phosphorus removal, 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.
The phosphorus release and uptake processes of PAOs are shown in the figure:
