Anaerobic metabolic models for phosphorus- and glycogen-accumulating organisms with mixed acetic and propionic acids as carbon sources Chao Zhang a , Yinguang Chen a, *, Andrew A. Randall b , Guowei Gu a a State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road., Shanghai 200092, PR China b Department of Civil and Environmental Engineering, University of Central Florida, Orlando, FL 32816-2450, USA article info Article history: Received 4 March 2008 Received in revised form 15 June 2008 Accepted 17 June 2008 Available online 4 July 2008 Keywords: Anaerobic metabolic model Stoichiometry Phosphorus-accumulating organisms Glycogen-accumulating organisms Acetic acid Propionic acid abstract With acetate or propionate as the sole carbon source, anaerobic metabolic models describ- ing phosphorus- and glycogen-accumulating organisms (PAO and GAO) have been devel- oped in the literature. However, comprehensive models are in need for the description of PAO and GAO behaviors with mixed acetic and propionic acids as carbon sources since they are the two main volatile fatty acids (VFA) that coexist in real wastewater. Two metabolic models were proposed to characterize the anaerobic stoichiometry of PAO and GAO, respectively, and two groups of sequencing batch reactors (i.e. 5 PAO-SBRs and 5 GAO-SBRs) with different propionic to acetic acid ratios were used for the validation of the models. The experimental data indicated that polyhydroxyalkanoates were synthe- sized via random condensation in GAO cells, whereas the semi-selective/semi-random pathway was used for the integration of acetyl-CoA and propionyl-CoA in PAO cells. When the VFA was pure acetic or propionic acid, the proposed PAO (or GAO) model reverted back to the reported acetate or propionate PAO (or GAO) model. Results also showed that the energy required for the transportation of 1 C-mol VFA across the mem- brane of both PAO and GAO cells was independent of the propionate/acetate ratio. ª 2008 Elsevier Ltd. All rights reserved. 1. Introduction Excessive phosphorus discharged from sewage and industrial wastewater is one of the key reasons for eutrophication. The process of enhanced biological phosphorus removal (EBPR) has been accepted widely as one of the most economical and sustainable methods to remove phosphorus from wastewater, in which the activated sludge is operated in an anaerobic/ aerobi configuration for the enrichment of phosphorus- accumulating organisms (PAO). Under anaerobic conditions, PAO take up volatile fatty acids (VFA) and store them as poly-b-hydroxyalkanoates (PHA). The energy for this trans- portation and storage reaction is generally considered to be linked with the hydrolysis of intracellular polyphosphate (poly-P) and glycogen, while glycolysis also supplies the reduc- ing power to maintain the cell redox balance (Mino et al., 1998). During the subsequent aerobic period, the internally stored PHA are oxidized for phosphate uptake and glycogen replenishment as well as cell growth (Arun et al., 1988). When the aerobic phosphorus uptake is greater than the anaerobic phosphorus release, a net phosphorus removal can be achieved by wasting excess sludge. There are, however, instances of poor performance or com- plete failure of phosphorus removal even under favorable con- ditions for EBPR processes (Liu et al., 1994; Satoh et al., 1994; Cech and Hartman, 1990). Another group of microorganisms * Corresponding author. Tel.: þ86 21 6598 1263; fax: þ86 21 6598 6313. E-mail address: yg2chen@yahoo.com (Y. Chen). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.06.025 water research 42 (2008) 3745–3756