PHOSPHORUS ACCUMULATING ORGANISMS UTILIZATION OF VOLATILE FATTY ACIDS PRODUCED BY FERMENTATION OF ANAEROBIC MIXED LIQUOR

Demonstration testing was conducted to evaluate a side stream mixed liquor fermentation process for internal generation of volatile fatty acids (VFAs) in a biological nutrient removal (BNR) treatment train. The trials were performed at the McDowell Creek Wastewater Treatment Facility (WWTF) in Huntersville, NC, where the current method of VFA supplementation is addition of acetic acid or waste sugar water to the University of Cape Town (UCT) process influent. A portion of the anaerobic zone mixed liquor was pumped to an upflow sludge blanket (USB) reactor to accomplish fermentation. Initially, a 2.5-day solids retention time (SRT) was tested, followed by trials at 7 and 12-day SRTs. A 110 gpm flow rate into the fermentation basin maintained a 13.6 hour hydraulic retention time (HRT) in all trials. Gas chromatography tests revealed that the fermentate contained a ratio of acetic to propionic acid of approximately 65:35, which is well-suited for BNR microbial populations. This indicated that mixed liquor is a viable VFA source. The VFA production averaged about 147, 206, and 94 lb/d, for the 2.5-day, 7-day and 12-day SRTs, respectively. Although the 2.5-day SRT did not generate the highest amount of VFAs, it is likely sufficient for biological phosphorus removal if VFA elutriation is efficient. The lower VFA levels at the 12-day SRT in comparison to the 7-day SRT trial may reflect the onset of methanogenesis.