Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Denitrifying phosphorus removal: Linking the process performance with the microbial community structure Gilda Carvalho a,b , Paulo C. Lemos a , Adrian Oehmen a , Maria A.M. Reis a,Ã a REQUIMTE/CQFB, Chemistry Department, Faculdade de Cie ˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b IBET/ITQB, Apt.12, 2781-901 Oeiras, Portugal article info Article history: Received 24 January 2007 Received in revised form 26 June 2007 Accepted 29 June 2007 Available online 31 July 2007 Keywords: Denitrifying P removal Acetate Propionate Accumulibacter Nitrate-DPAO Nitrite-DPAO abstract This study investigated the link between the process performance of two denitrifying phosphorus (P) removal systems and their microbial community structure. Two sequencing batch reactors (SBRs) were operated with either acetate or propionate as the sole carbon source, and were gradually acclimatised from anaerobic–aerobic to anaerobic–anoxic conditions. It was found that the propionate SBR was able to sustain denitrifying P removal after acclimatisation, while the enhanced biological phosphorus removal (EBPR) activity in the acetate reactor collapsed after the aerobic phase was eliminated. The results suggested that the anoxic glycogen production rate in the acetate SBR was insufficient to support the anaerobic glycogen demand for acetate uptake. The chemical transformations in each SBR suggested that different types of polyphosphate-accumulating organisms (PAOs) were present in each system, possessing different affinities for nitrate. Microbial characterisa- tion with fluorescence in situ hybridisation (FISH) revealed that Accumulibacter was the dominant organism in each reactor, although different cell morphotypes were observed. A coccus morphotype was predominant in the acetate SBR while the propionate SBR was enriched in a rod morphotype. It is hypothesised that the coccus morphotype corresponds to an Accumulibacter strain that is unable to use nitrate as electron acceptor but is able to use oxygen, and possibly nitrite. The rod morphotype is proposed to be a PAO able to use nitrate, nitrite and oxygen. This hypothesis is in agreement with literature studies focussed on the identity of denitrifying PAOs (DPAOs), as well as a recent metagenomic study on Accumulibacter. & 2007 Elsevier Ltd. All rights reserved. 1. Introduction Enhanced biological phosphorus removal (EBPR) is a cost- effective and environmentally friendly technology to remove phosphorus from wastewater. In this process, the polypho- sphate-accumulating organisms (PAOs) are able to store phosphorus through sequential anaerobic–aerobic condi- tions. Carbon sources, particularly volatile fatty acids (VFA), are taken up anaerobically and stored as poly-b-hydroxyalk- anoates (PHA) through the release of phosphorus (P) and degradation of glycogen. A higher amount of phosphorus (P) is then taken up when an electron acceptor is supplied (normally oxygen, i.e., aerobic conditions) through PHA oxidation, which is accompanied by biomass growth and the regeneration of glycogen. Alternatively, nitrate or nitrite (NO x ) can be used as electron acceptors (i.e. anoxic condi- tions) instead of oxygen, which is advantageous because both N and P are removed in the same process (Kerrn Jespersen and Henze, 1993; Kuba et al., 1993). Moreover, when compared to conventional EBPR, simultaneous denitrification and P ARTICLE IN PRESS 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.06.065 Ã Corresponding author. Tel./fax: +351 212948385. E-mail address: amr@dq.fct.unl.pt (M.A.M. Reis). WATER RESEARCH 41 (2007) 4383– 4396