BIOMASS CAPACITY FOR ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL AS A FUNCTION OF VFA CULTIVATION

This paper presents an overview of the last 10 years of work at the University of Central Florida (UCF) on propionic and other C3+ VFAs (volatile fatty acids) on Enhanced Biological Phosphorus Removal (EBPR), including never before published and reanalyzed data. In addition subsequent work at the University of Queensland which provided a critical verification and extension in this area is reviewed to give an overall picture of the state of our knowledge. The accumulated results are consistent with the hypothesis, first put forward by Hood (1998), that propionic acid results in a selective advantage for PAOs over their competitors (e.g. GAOs). In addition it may be that other C3+ VFAs (i.e. isovaleric acid) share this property. However propionic acid has the most practical implications since it is present in most wastewaters in measurable quantity, and can in some septic or prefermented wastewaters approach or even exceed the concentration of acetic acid. It may be that acetic acid is not the best VFA for EBPR, and that some of the unexplained variability in EBPR process performance is due to changes in VFA speciation. Also data seems to imply that EBPR superior to 100% acetic acid is achieved with a mixture of propionate and acetate rather than 100% propionic acid. Ratios at a 1:1 molar ratio of propionic to acetic acid (about 55%:45% by weight) or slightly greater than 1:1 produced superior results relative to mixtures dominated by acetic acid or 100% acetic acid in long term cultivation. However, for a given biomass acetic acid was superior to propionic acid, so superior EBPR was almost certainly related to biomass properties (presumably selection of the PAO phenotype) resulting from long term cultivation. Data from the University of Queensland directly verified a higher PAO/GAO ratio as propionate fraction of influent VFA increased (Pijuan et al., 2004).