Enhanced Biological Phosphorus Removal
Supplemental biological P removal occurs by a process known as enhanced biological phosphorus removal (EBPR). EBPR is a more recent innovation than chemical P removal but is also now common. EBPR is implemented into the activated sludge process and P is removed with waste activated sludge (WAS). The potential for EBPR depends on successful cultivation of phosphate accumulating organisms (PAO) which require three conditions for growth: excess phosphate in the wastewater, alternating oxic and anaerobic conditions, and the availability of a particular type of organic carbon called volatile fatty acids (VFA).
In anaerobic conditions, PAOs can store their food (VFA) before eating it. In a similar way, I might stuff cookies that I cannot eat immediately into my pockets to eat later. People without pockets could be left out. The PAOs are people with pockets in this analogy. The VFA are packed into a storage product called poly-hydroxy alkanoates (PHA) and phosphate is ejected from the PAO into the mixed liquor.
PAOs consume the VFA using oxygen. Under oxic conditions (ORP > 100 mV), or possibly even anoxic conditions, the bacteria oxidize the stored carbon (VFA), replenish the depleted phosphate, and accumulate extra phosphate. The accumulated phosphate can be thought of as an investment. The more phosphate that is accumulated the greater the potential to thrive if the conditions are right. The PAOs become rich driving orthophosphate (OP) to nearly 0 and the particulate P (PP) content of mixed liquor is increased up to 8% or higher. If anaerobic conditions are not encountered or the amount of VFA is not sufficient, the investment in accumulated phosphate does not completely pay off and competing organisms which do not accumulate OP prosper limiting the PP content of the mixed liquor and possibly resulting in OP escaping in treated effluent. PP is removed with WAS.
Design and Operation of EBPR
Treatment configurations for EBPR include an upstream anaerobic zone in which DO and nitrate are very low. The recommended hydraulic retention time (HRT) of the anaerobic zone varies from 30 minutes to 2 hours depending on the characteristics of the wastewater and the design and operation of the treatment system. OP may be increased in the anaerobic zone by 3 times or more as PAOs release it to sequester VFA. Primary release of OP in the presence of VFA is desirable. Secondary release downstream, described below, is not desirable. The anaerobic zone may also need to accommodate fermentation of mixed liquor to generate VFA. It has been demonstrated that ORP needs to be -300 mV or less for fermentation to occur in wastewater.
OP uptake occurs in downstream oxic zones. The prevailing wisdom is that DO must be maintained at 2.0 mg/L or higher at the front end of the oxic zone following an anaerobic zone to achieve the greatest uptake. OP uptake occurs rapidly in the front end and tapers off moving downstream. Recently, this wisdom has been challenged as substantial P removal has been observed in simultaneous nitrification-denitrification facilities operated at relatively low DO concentrations.
EBPR is More Complicated than Chemical P Removal
Practitioners have identified several prerequisites for achieving reliable performance. A consistent and adequate supply of VFA is very critical. Variable and insufficient VFA stress PAOs and allow other organisms (or other metabolisms) which do not result in accumulation of P to flourish. WRRF that are required to also transform or remove nitrogen may need to add an external source of carbon. Simple and inexpensive methods for continuous monitoring of VFA do not presently exist. However, online monitoring of soluble COD and dissolved organic carbon (DOC) is possible with an optical spectral sensor and can be beneficial to optimize the use of wastewater carbon and minimize the use of external carbon.
Integrity of the anaerobic zone is also important. DO or nitrates will interfere with P release. For this reason, the DO in downstream oxic zones should be minimized to prevent carryover of DO with return activated sludge (RAS). Nitrification will increase the concentration of nitrate in the RAS. The purpose of a pre-anoxic zone upstream from the anaerobic zone in some configurations is to denitrify return activated sludge (RAS). In other configurations, denitrified mixed liquor is recirculated upstream to mix with raw wastewater.
Efficient operation of solids separation systems is essential to prevent captured particulate P from escaping in the effluent. The P content of EBPR sludges can be 10% by mass or higher. The effluent PP in 10 mg TSS/L is 1.0 mg/L for a P content of 10%. Continuous monitoring of downstream effluent TSS can provide an early indication of upsets and provide a signal to start polymer addition, for example. Monitoring of upstream MLSS can allow fast calculation of solids loading and a signal to switch to step feed operation.
OP previously sequestered by PAO can be released downstream, increasing effluent TP. Secondary release occurs when mixed liquor becomes anaerobic in deep final settling tank blankets or during sludge treatment. Minimizing sludge blanket depths can prevent secondary release in final settling tanks. P can be removed from sludge handling recycle streams by chemical treatment or by installing a struvite recovery system.
Additional Blog Posts of Interest
The Science of Phosphorus | Blog Series | Blog 1 of 5
The Science of Phosphorus | Blog Series | Blog 2 of 5
The Science of Phosphorus | Blog Series | Blog 3 of 5
The Science of Phosphorus | Blog Series | Blog 4 of 5