Exploring Enhanced BPR and Increased Yield with Glycerin at Five Full-Scale BNR Facilities

This paper presents observations and lessons learned from using glycerin for enhanced denitrification at five biological nutrient removal (BNR) plants in Virginia; and documents findings that indicate that (a) extended glycerin use in full-scale facilities results in increased carbon demand per pound of nitrate removed, and (b) glycerin addition improves biological phosphorus removal (BPR) in full-scale facilities. It appears that addition of supplemental carbon in large quantities establishes a feast-famine cycle, which selects for organisms with a high propensity to store carbon and utilize it later for denitritation. It also appears that glycerin addition selects for a specific phosphorus accumulating organism (PAO) that can store glycerin as glycogen or polyhydroxyalkanoate (PHA). As a consequence, ordinary heterotrophic organisms (OHOs) are also inclined to store more carbon to reduce nitrate to nitrite and then nitrogen gas. This competition for carbon therefore drives up the overall need for carbon to achieve the plant's denitrification objective. Potential concepts to mitigate this effect are to feed carbon at multiple points throughout the anoxic zone and pace carbon feed to measured nitrate load.