Enhanced biological phosphorus removal in dairy wastewater: Mechanistic insights, operational constraints, and research gaps

Phosphorus removal from wastewater is crucial for mitigating eutrophication and enabling resource recovery. Although enhanced biological phosphorus removal (EBPR) is well established in municipal plants, its application to dairy wastewater (DWW) remains challenging. In DWW, usable carbon, volatile fatty acids (VFA), or readily biodegradable chemical oxygen demand (rbCOD) often diverges from total chemical oxygen demand (COD). Nitrate/nitrite (NOx) frequently leaks into the anaerobic selector, and cleaning-in-place (CIP) activities cause pH and alkalinity fluctuations. In addition, fat, oil, and grease (FOG), limited protein hydrolysis, and generally warmer operation further destabilise the selection of phosphorus-accumulating organisms (PAOs). This review synthesises EBPR mechanisms and the roles of PAOs and glycogen-accumulating organisms (GAOs), and then organises key operational factors such as volatile fatty acids/phosphorus (VFA/P) ratio or readily biodegradable chemical oxygen demand/phosphorus (rbCOD/P) ratio, carbon speciation, pH-alkalinity control, dissolved oxygen (DO), temperature, and NOx exclusion/anoxic P uptake into practical windows for dairy contexts. It also summarises dairy-specific constraints and evaluates existing solutions. Closing these gaps will enhance our understanding of EBPR constraints in dairy systems and support the future development and practical application of this treatment method. • Critically reviews EBPR performance under dairy wastewater conditions • Explains PAO–GAO competition driven by dairy-specific carbon and COD/P ratios • Defines operational windows for stable EBPR in dairy treatment systems • Highlights fermentation and lactose conversion as key enablers of PAO activity • Identifies research gaps limiting full-scale EBPR implementation in dairies