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. 2016 Apr 29;10(12):2931–2945. doi: 10.1038/ismej.2016.67

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Copyright © 2016 International Society for Microbial Ecology

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/

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A defining feature of many biological wastewater treatment systems is the recycling of microbial biomass, commonly called activated sludge (AS). Recycling AS provides two features: (1) a mechanism for achieving high densities of microorganisms and (2) a mechanism for the ecological selection of organisms based on their growth characteristics and physiology. A common design is to have an anaerobic basin preceding an aerobic basin. Under these conditions, polyphosphate accumulating organisms (PAO) are selected for, enhancing the phosphorus removal capabilities of the system. This configuration is commonly referred to as Enhanced Biological Phosphorus Removal (EBPR). Anaerobic zone: in the absence of a terminal electron acceptor, volatile fatty acids (VFA) are transported into the cell and stored as polyhydroxyalkanoates (PHA) with a concomitant release of P and degradation of glycogen. Aerobic zone: carbon stored as PHA is used to drive growth, cell division, P-uptake and glycogen synthesis. At the end of the Aerobic zone, the activated sludge is settled in a clarifier and removed from the system to be recycled, further processes or disposed. Figure adapted from McMahon and Read (2013).