CHANGES IN ANOXIC DENITRIFICATION RATE DUE TO PREFERMENTATION OF A SEPTIC, PHOSPHORUS LIMITED, WASTEWATER

A preliminary bench scale study of parallel UCT biological nutrient removal (BNR) systems showed improvement in anoxic denitrification rates due to prefermentation of a septic, phosphorus limited, wastewater. Net phosphorus (P) removals due to Enhanced Biological Phosphorus Removal (EBPR) were only improved marginally by prefermentation in spite of significant increases in anaerobic phosphorus release, PHA formation, and higher anoxic and aerobic uptakes. This was probably due to the high volatile fatty acids (VFAs):TP ratio in the raw influent relative to the VFA requirements for EBPR, since enough VFAs were already present for P removal prior to prefermentation. An additional assessment of prefermentation using parallel UCT systems with step feed of 50% of the influent to the anoxic zone was completed. This second phase quantified the effect of prefermentation in a step feed scenario which prioritized prefermentation use to enhance denitrification rather than EBPR. While specific denitrification rates in the anoxic zone were significantly improved by prefermentation high denitrification in the clarifiers and aerobic zones (simultaneous denitrification) made definitive conclusions concerning the potential improvements in total system nitrogen removal questionable. The prefermented system always showed superior zone settling velocity (ZSV) and sludge volume index (SVI) values and the improvement became increasingly statistically significant when the prefermenter was performing well.