The Power and Nuance of Biological Phosphorus Removal from Wastewater

Phosphorus (P), an essential nutrient for life, becomes a formidable environmental adversary when discharged excessively into aquatic ecosystems. Found in municipal sewage (from detergents, food waste, human excreta), agricultural runoff (fertilizers), and various industrial effluents (food processing, metal finishing), phosphorus is a primary driver of eutrophication.

The "Phosphate-Accumulating Organism" (PAO) Advantage

The magic of EBPR lies within a specific group of bacteria known as Phosphate-Accumulating Organisms (PAOs) and to a lesser extent, Glycogen-Accumulating Organisms (GAOs – sometimes competitors). These microbes possess a unique metabolic trick far beyond typical bacterial nutrition:

The Anaerobic "Feast": Under anaerobic (oxygen-free) conditions, PAOs break down internally stored carbon reserves, primarily a complex sugar called polyhydroxyalkanoates (PHAs), to generate energy. Crucially, this energy is used to actively uptake volatile fatty acids (VFAs – like acetate, propionate) present in the wastewater from the environment and store them as more PHAs. To balance the negative charge of the incoming VFAs, the PAOs release positively charged orthophosphate (PO₄³⁻) ions stored within their cells back into the wastewater. This step increases soluble P in the anaerobic zone.

The Aerobic/Aerobic "Purge": When the PAOs move into an aerobic or anoxic (oxygen-free but nitrate present) environment, they switch modes. They oxidize their stored PHAs to generate substantial energy. This energy fuels three critical processes:

Key Biological Treatment Configurations

EBPR isn't just a biological phenomenon; it requires carefully engineered reactor sequences to create the alternating anaerobic-aerobic/anoxic conditions PAOs need:

Anaerobic-Oxic (A/O) Process: The simplest and most common EBPR configuration.

Anaerobic Zone: Influent wastewater (providing VFAs) mixes with return activated sludge (RAS - bringing PAOs). VFA uptake, P-release, and PHA storage occur here. Oxygen and nitrate must be absent.

Oxic Zone: Oxygen is supplied. PHA oxidation, growth, glycogen replenishment, and massive P-uptake occur. Nitrification (ammonia to nitrate) also typically happens here.

Clarifier: Separation of P-rich sludge.

Anaerobic-Anoxic-Oxic (A²/O) Process: Adds an anoxic zone between anaerobic and oxic zones primarily for denitrification.

Anaerobic Zone: Same as A/O (VFAs uptake, P-release).

Anoxic Zone: Nitrate-rich mixed liquor (from the oxic zone) is recycled back here. Denitrifying bacteria use organic carbon (often provided internally by PAOs/other microbes or externally if added) to reduce nitrate (NO₃⁻) to nitrogen gas (N₂), removing nitrogen. Some P-uptake can occur here if conditions allow.

Oxic Zone: Oxygen supplied for P-uptake (primary location), nitrification, and final organic carbon oxidation.

Clarifier: Sludge separation. This is the workhorse for combined N and P removal in municipal plants.

University of Cape Town (UCT) & Modified UCT (MUCT): Developed to handle wastewaters with low VFA content or high nitrate in the RAS.