A Biochemical Model for Biological Enhanced Phosphorus Removal

Abstract

The limited understanding of the fundamental mechanisms involved in biological enhanced phosphorus (bio-P) removal has hindered the successful development of this technology for wastewater treatment. The major objective of this research (Comeau, 1984) was to propose a model that would explain the bio-P removal phenomena under both anaerobic and aerobic conditions. The model presented here is based on the observations made with bio-P removal processes and on principles of microbial biochemistry. Bio-P bacteria are defined as being responsible for bio-P removal and are proposed to be capable of both po1yphosphate (polyP) and po1y-β-hydroxybutyrate (PHB) storage. Under anaerobic conditions (absence of both free oxygen and nitrate - Fig. 1) available substrates such as acetate (HAc) will be transported across the membrane by facilitated diffusion. Under usual conditions of pH, most of the acetate will be in the anionic form (Ac− ) and one H+of the proton motive force1 will be required for Ac− intracellular transport. Once inside Ac− can then be stored as PHB. It is postulated that the mechanism by which polyP confers an advantage to bio-P bacteria for PHB accumulation is by re-estab1ishing the pmf in order to allow more Ac− to be transported across the membrane and be stored. If the pH gradient of the pmf was not re-estab1ished no more Ac− could be accumulated. The pmf is proposed to be re-estab1ished by polyP which provides a source of phosphate molecules that can be neutrally transported across the plasma membrane. Once outside, phosphate molecules will dissociate and the released protons (H+) will re-establish the pmf and H+ can be used again for acetate transport. The source of energy required to metabolize acetate into acetyl CoA could possibly come from polyP breakdown. For acetyl CoA to be stored as PHB, a source of NADH is required. NADH can be produced anaerobically from acetyl CoA utilization by the tricarboxylic acid (TCA) cycle which functions at a reduced rate under these conditions. Under subsequent aerobic conditions (Fig. 2) bio-P bacteria now have the advantage of a reserve of PHB that can be used for energy production while any available substrate in solution is heavily competed for by other aerobic bacteria. Energy is produced aerobically from PHB via acetyl CoA metabolized in the TCA cycle to produce NADH which is oxidized at the electron-transport chain (ETC) in the presence of oxygen (or nitrate) in order to expel protons and thus establish the pmf. The pmf is used to form ATP which can be converted to polyP for storage. The availability of PHB can result in a high intracellular ATP to ADP ratio. Thus polyP can be accumulated and bio-P bacteria can grow and multiply. Therefore, the proper sequencing of environmental conditions in the waste treatment reactors results in anaerobic exposure to simple substrates such as acetate which are stored with the assistance of polyP. This is followed by aerobic conditions where polyP accumulates and phosphorus is removed from the wastewater. The proposed models were found to provide not only very satisfactory explanations for the observed phenomena related to bio-P removal, but were also found to be of a very valuable predictive power to assess the pertinence of a design or of an operational modification for upgrading a bio-P treatment plant (Comeau, 1984). 1The proton motive force (pmf) is composed of a charge gradient and of a pH gradient across the plasma (inner) membrane of bacteria which is impermeable to ions (notably to H+). The major roles of the pmf are related to energy production (e.g. ATP), substrate transport and cellular movement (Harold, 1977). The sum of the gradients of the pmf should remain constant for a bacteria.