Water Online

September 2014

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BNR 101: White Paper 18 E BPR is performed by polyphosphate-accumulating organisms (PAO) in a two-step process. During the first anaerobic stage (lack of oxygen, nitrate, and nitrite), PAO are able to accumulate volatile fatty acids (VFA) as internal reserve polymers (poly-hydroxyal- kanoates [PHA]). The energy required is obtained through the hydrolysis of intracellular polyphosphate reserves, there- by releasing phosphate to the medium. Anaerobic organic matter uptake by PAO is a competitive advantage over other microorganisms unable to perform this uptake in the absence of an electron acceptor (oxygen, nitrate, or nitrite). Subsequently, under aerobic (oxygen) or anoxic condi- tions (presence of nitrate and/or nitrite), PAO degrade the accumulated PHA, obtaining enough energy for growth and cell maintenance. Part of the energy obtained is used to uptake and accumulate phosphate in the form of polyphos- phate chains, which are used as energy sources (i.e., which may be used under posterior anaerobic conditions). The net result of this process is the uptake of phosphorus (P) from wastewater, since the P taken up under aerobic and/or anoxic conditions is higher than the previously released under anaerobic conditions. P-removal is achieved due to the purge of these microorganisms after the aerobic phase, when the intracellular P levels are maximal. When the electron acceptor is nitrate or nitrite instead of oxygen, a fraction of PAO called denitrifying PAO (DPAO) is able to uptake P linked to denitrification. PAO are now classified into two types according to their different denitrifying capabilities (Flowers et al, 2009). One type (named IA or nitrate-DPAO) is able to couple nitrate and nitrite reduction with P uptake, while the other (named IIA or nitrite-DPAO) can only use nitrite instead of oxygen. Most of the research conducted on DPAO metabolism was initially focused on using nitrate as an electron acceptor, while the use of nitrite as a potential electron acceptor has been a recurrent research topic in recent years. The Nitrogen Issue The implementation of simultaneous biological removal of organic matter (chemical oxygen demand [COD]), nitrogen, and phosphorus in wastewater treatment plants (WWTPs) allows a sustainable and economic removal of nutrients from wastewater. However, over the years, it has been observed that the combination of these processes as in the conventional anaerobic/anoxic/aerobic (A 2 /O) configuration (see Figure 1) results, sometimes, in the failure of EBPR due to interactions occurring between the metabolism of PAO and nitrate recycle from the settler to the anaerobic reactor through the external recycle. The presence of nitrate in the reactor that should theo- retically be anaerobic is one of the most widely reported causes of EBPR failure in WWTPs (Henze et al, 2008) and, despite its importance, the real reasons for this failure were not fully understood yet. The prevailing hypothesis assumed that, in the presence of nitrate or other inter- mediate species of denitrification, organic matter was Can Nitrogen Removal And Enhanced Biological Phosphorus Removal Coexist? Enhanced biological phosphorus removal (EBPR) is considered the most economical and sustainable way to remove phosphorus from municipal wastewaters, but its integration with nitrogen removal puts EBPR at risk for failure. Recent research helps resolve the conflict. By Dr. Juan A. Baeza, Dr. Javier Guerrero, Dr. Albert Guisasola Figure 1. Scheme of WWTP configurations under study (top: anaerobic/anoxic/ aerobic (A 2 /O) configuration; bottom: MLE anoxic/aerobic configuration). Red arrows show the main nitrate inlets to the anaerobic phase. wateronline.com ■ Water Online The Magazine

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