Development and validation of a flux-based stoichiometric model for enhanced biological phosphorus removal metabolism

Abstract

Enhanced biological phosphorus removal (EBPR) is a wastewater treatment process involving metabolic cycling through several biopolymers (polyphosphate, polyhydroxyalkanoates, and glycogen). This metabolic cycling is induced in microorganisms by treatment systems that alternate between initial carbon-rich, anaerobic environments followed by carbon-poor, aerobic environments. While the appearance and disappearance of these biopolymers has been documented, the intracellular pressures that lead to their synthesis and degradation are not well understood. To understand how carbon, energy, and redox potential are channeled through the metabolic pathways in each treatment process stage, a metabolic flux model that contained a complete set of the pathways involved in biomass synthesis and energy production in bacteria was developed. The model accounts for the energy requirements of macromolecule synthesis and of metabolite transport across the cell membrane. The equations for the 163 reversible and 166 irreversible reactions were solved using linear optimization. Data from a laboratory scale sequencing batch reactor performing EBPR were used as model inputs. Given polyhydroxyalkanoate synthesis and glycogen degradation rates in the anaerobic phase, the model predicted reasonable anaerobic acetate uptake and polyphosphate consumption rates. In the aerobic phase, the polyphosphate and glycogen synthesis rates were used to predict the polyhydroxyalkanoate consumption rate. In addition, the model predicted the ratio of acetate uptake to phosphate release observed experimentally, as well as an inverse relationship between polyhydroxyalkanoate and polyphosphate consumption. The model provides information on the pathways by which the energy-rich molecules ATP, NADH, and NADPH are produced and consumed during the EBPR processes. In doing so, it supports the hypothesis that biopolymer metabolism provides a means for organisms to balance intracellular energy supplies. Moreover, the model suggests pathways at which metabolic regulation should occur and provides a comprehensive account of EBPR metabolism.