The effect of substrate competition on the metabolism of polyphosphate accumulating organisms (PAOs) M  onica Carvalheira a , Adrian Oehmen a,* , Gilda Carvalho a,b , Maria A.M. Reis a a Requimte/CQFB, Departamento de Quı´mica, Faculdade de Ci^ encias e Tecnologia, Universidade Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal b Instituto de Biologia Experimental e Tecnol ogica (IBET), Av. da Repu ´ blica (EAN), 2784-505 Oeiras, Portugal article info Article history: Received 5 March 2014 Received in revised form 24 June 2014 Accepted 3 July 2014 Available online 10 July 2014 Keywords: Enhanced biological phosphorus removal (EBPR) Polyphosphate accumulating or- ganisms (PAO) Glycogen accumulating organisms (GAO) Volatile fatty acids (VFAs) Substrate competition Metabolic model abstract The type of carbon source present in the wastewater is one factor that affects the competition between polyphosphate accumulating organisms (PAO) and glycogen accumulating organ- isms (GAO) and therefore, the efficiency of the enhanced biological phosphorus removal (EBPR) process. This study investigated the impact of the carbon source composition on the anaerobic and aerobic kinetics of PAOs and the EBPR performance of an 85% PAO enrichment. When both acetate (HAc) and propionate (HPr) were present, propionate was depleted more quickly, with a constant uptake rate of 0.18 ± 0.02 C-mol/(C-mol biomass$h), while the ace- tate uptake rate decreased with an increase in propionate concentration, due to the substrate competition between acetate and propionate. The metabolic model for PAOs was modified to incorporate the anaerobic substrate competition effect. The aerobic rates for phosphorus (P) uptake, glycogen production and polyhydroxyalkanoates (PHA) degradation were within the same range for all tests, indicating that these rates are essentially independent of the acetate and propionate concentration, simplifying the calibration procedure for metabolic models. The metabolic model applied to describe the anaerobic and aerobic activity agreed well with the experimental data of HAc, HPr, P, PHA and biomass growth. The low glycogen con- sumption observed suggest that some reducing equivalents were generated anaerobically through the TCA cycle. The results of this work suggest that the propionate uptake kinetics by PAOs can provide them an advantage over GAOs in EBPR systems, even when the propi- onate fraction of the influent is relatively low. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Enhanced biological phosphorus removal (EBPR) is one of the most economical and sustainable processes used for the removal of phosphorus from wastewater. Phosphorus removal is achieved by recirculating the sludge through anaerobic and aerobic/anoxic conditions to promote the enrichment of activated sludge with polyphosphate * Corresponding author. Tel.: þ351 212948571; fax: þ351 212948550. E-mail address: a.oehmen@fct.unl.pt (A. Oehmen). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 64 (2014) 149 e159 http://dx.doi.org/10.1016/j.watres.2014.07.004 0043-1354/© 2014 Elsevier Ltd. All rights reserved.