Biological Phosphorus Removal Intensification and Optimization

Last Modified: Jan 31, 2019


  • Joe Rohrbacher PE – Hazen and Sawyer

Water Resource Recovery Facilities (WRRFs) are under increasing regulatory pressure to reduce effluent total phosphorus (TP) to low levels. Many facilities are now required to remove TP below 0.5 mg/L, either on a mass load allocation or concentration basis. A combination of biological and chemical phosphorus removal with tertiary settling or filtration is typically used to meet these limits. Common challenges to providing reliable biological phosphorus removal (BPR) include limited influent volatile fatty acids (VFAs), solids recycle stream loadings, dissolved oxygen return to unaerated zones, and competition with other organisms that uptake VFA. These challenges often result in greater chemical feed, increasing operational costs and further inhibiting BPR.

Recent advances in BPR understanding provide the opportunity to intensify BPR and reduce reliance on chemical addition. One promising approach is return activated sludge (RAS) fermentation, where a portion of the RAS (typically 5 to 10 percent) is diverted to a dedicated anaerobic zone with a long HRT (up to 48 hours) to ferment the RAS and create volatile fatty acids (VFAs). VFAs created in this zone can provide an effective carbon source for phosphate accumulating organisms (PAOs) and denitrifying bacteria in the anaerobic and anoxic zone. RAS fermentation may also promote growth of a more diverse, robust PAO population that can further enhance biological phosphorus removal. RAS denitrification zones, which are operated at a shorter retention time and typically treat all of the RAS, have also provided significant benefits.

Several facilities in the Mid-Atlantic region have implemented a RAS fermentation or denitrification zone and achieve effluent TN and TP concentrations below 3.0 and 0.3 mg/L respectively, with substantially less chemical addition compared to plants without RAS fermentation. These plants approach RAS fermentation/denitrification differently, and their approaches can be leveraged at existing facilities to optimize nutrient removal.

The presentation will provide case studies of facilities in the Carolinas and Virginia that have leveraged RAS fermentation/denitrification to reduce or eliminate chemical phosphorus removal. Each case study will include the configuration utilized, effluent nitrogen and phosphorus concentrations, and chemical usage. The presentation will conclude with operational and low-cost capital modifications that can be considered to promote more reliable BPR at existing facilities.

For more information, please contact the author at

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