Wastewater typically contains a large amount of nutrients - particularly phosphorus - that can pose a harmful threat to infrastructure and the environment, causing problems such as eutrophication in waterbodies and a buildup of struvite in mechanical systems.
To remove it from wastewater it must be converted to a solid form and then separated from the liquid which can be accomplished chemically or biologically. When both removed and recovered, wastewater treatment plants can improve effluent quality, meet stringent discharge limits, and convert discarded phosphorus into an environmentally friendly fertilizer needed to grow food.
Phosphorus removal: chemical or biological
To remove phosphorus from wastewater, dissolved phosphorus must first be converted into a form that can be separated from the liquid stream. This can be accomplished through chemical or biological phosphorus removal, depending on the treatment objectives, existing infrastructure, wastewater characteristics, and local requirements.
Biological phosphorus removal can offer an efficient approach by using naturally occurring microorganisms to capture phosphorus within the treatment process.
Enhanced biological phosphorus removal (EBPR)
EBPR is the biological uptake of phosphorus by selected microorganisms called phosphorus-accumulating organisms (PAOs). While the actual uptake of phosphorus occurs in an aerobic environment, PAOs must first be conditioned by exposure to volatile fatty acids (VFAs) in an anaerobic environment. PAOs store food under anaerobic conditions and then process the stored food once under aerobic conditions. The preferred food for PAOs is VFAs.
How EBPR works
- Anaerobic phase: PAOs are exposed to volatile fatty acids (VFAs) under anaerobic conditions. The microorganisms take up VFAs and store them as an internal food and energy reserve.
- Aerobic phase: Under aerobic conditions, PAOs metabolize their stored food and take up phosphorus from the wastewater.
- Phosphorus removal: The phosphorus-rich biomass is subsequently separated from the treated water through solids removal, taking phosphorus out of the wastewater stream.
Why VFAs are essential to effective EBPR performance
To reduce the need for carbon or chemical addition, facilities with insufficient influent VFA for effective phosphorus removal can encourage additional VFA production through fermentation. An anaerobic selector provides an optimal environment for the uptake of VFA by PAOs, but PAOs constitute a small subset of the bacterial population in the mixed liquor suspended solids (MLSS). The rest of the bacteria ferment organic compounds to obtain food and energy. These facultative bacteria do not consume VFA; they break down complex soluble organic compounds to form VFA, allowing the PAOs to take up additional VFA and release phosphorus. Therefore, the anaerobic selector or fermentation tank in an EBPR facility simultaneously conditions PAOs and provides an environment for additional fermentation of soluble organics to VFA.
Proven solutions for phosphorus mitigation
Xylem works closely with consulting engineers and municipalities and understands phosphorus removal challenges. Our comprehensive portfolio and experienced professionals help identify the best options for new treatment plants, retrofits, upgrades, or sustainable nutrient recovery applications. We offer technologies and services used in phosphorus removal and recovery, including biological treatment process solutions, solids separation, mixing and fermentation support, and nutrient recovery.
Phosphorus management can help contribute to a future circular economy
