Nitrogen removal in a modified anaerobic baffled reactor (ABR): 1, denitrification

In order to achieve nitrogen removal within a single reactor unit, an eight compartment anaerobic baffled reactor (ABR) was modified to accommodate an in-situ aerobic stage in the penultimate compartment, which allowed ammonia oxidation to nitrite and nitrate via nitrification. In theory, the nitrite/nitrate may be recycled to the inlet of the ABR and be reduced via denitrification. This paper deals with the anoxic denitrification of a nitrate feed, while a later paper examines nitrification in the aerobic stage. Denitrification occurred almost exclusively in the front two compartments of the anaerobic baffled reactor, with rates of 0.335 (82% reduction) and 0.085 kg NO3/kg VSS.d (96% reduction) in compartments 1 and 2, respectively. Denitrification had several positive effects on overall reactor performance, and this was due to the following factors: the utilisation of an oxidisable electron donor in the form of the feed COD; increased system pH at the reactor inlet thus improving environmental conditions; a high hydrogen demand during dissimilatory nitrate reduction to ammonium, therefore improving conditions for syntrophic bacteria; and, the generation of ammonium from dissimilatory reduction which provided slowly growing bacteria at the front of the ABR with a reduced nitrogen source. The overall effect of these influences was improved hydrogenotrophic methanogenesis. Denitrification also increased the residual COD in the first two compartments, and this was attributed to faster growth/decay rates of denitrifying bacteria. Nitrate reduction also influenced the ratio of volatile fatty acids produced and catabolised, with a significant reduction in propionate and butyrate, while acetate levels increased.