THE EFFECTS OF PROPIONIC VERSUS ACETIC ACID CONTENT OF DOMESTIC SEWAGE ON ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL

Enhanced Biological Phosphorus Removal was found to be superior for a Sequencing Batch Reactor (SBR) treating septic domestic wastewater with a propionic/acetic acid ratio of 2.12 (Propionic SBR) compared to a parallel and identical system with a propionic/acetic acid ratio of 0.145 (Acetic SBR). The Propionic SBR biomass removed 10.65 mg phosphorus (P)/gVSS (s=0.68) compared to 8.20 mgP/gVSS (s=0.93) for the Acetic SBR. However the maximum Propionic SBR PHA content was 49.3 mgPHA/gVSS with a 3HV/3HB ratio of 2.69. This was much lower than the Acetic SBR PHA content of 76.4 mgPHA/gVSS with a 3HV/3HB ratio of 0.449. The stoichiometric relationship between PHA and P was significantly different for the two populations studied. This was in spite of the fact that acetic acid gave superior results relative to propionic acid in batch experiments using Propionic SBR biomass. It is probable that while propionic acid was less efficient for EBPR than acetic acid in pure biochemical terms, it was even less efficient for organisms uninvolved in EBPR, giving it a stronger selective pressure favoring EBPR than acetic acid. The selective pressure of the VFAs was more significant than biochemical processing.