Use of volatile fatty acids from an acid-phase digester for denitrification

This study investigates the ability of naturally-produced volatile fatty acids (VFAs) to act as a carbon source for the removal of nitrate-Nitrogen concentrations in the range of 20 to 200 mg/L. The VFAs were generated from an anaerobic digester treating a 1:1 mixture of starch-rich industrial and municipal wastewater. The experiments were carried out at an ambient temperature of 22+/-2 degrees C using 600 mL batch reactors containing VFA-rich effluent from the digester and prepared primary sludge. Nitrates added in the form of a 1 M potassium nitrate solution were observed to completely disappear in conjunction with significant recovery of pH and alkalinity (i.e. biological denitrification). In all cases, denitrification followed zero-order kinetics with an average rate constant of 2.67 mg/[Lh] and a mean specific denitrification rate of 0.0111 g NOx-N/g volatile suspended solids (VSS) per day. The denitrifier population had a preference for acetic acid which occurred in the largest quantity and was exhausted the most rapidly. The next two most abundant VFAs were butyric acid and propionic acid, which were consumed only after acetate concentrations began to decline. The third "choice" of the denitrifiers was valeric acid, thus it appears that denitrifying bacteria have a sequential preference for VFAs. In addition, they prefer VFAs over other more complex soluble organic carbon forms.