Anaerobic Ammonia Oxidation (Anammox) has become an important topic in environmental microbiology and engineering in the last 15 years. The application of Anammox in wastewater treatment provides many beneficial advantages over traditional nitrogen removal processes, particularly in treating ammonium-rich waste streams.

In this study, the Anammox process was applied to a fed-batch reactor to treat raw digester filtrate from a local treatment plant. During initial treatment, the filtrate was diluted and an external nitrite source was supplemented. After reaching stable removal, a partial-nitritation (PN) reactor was started-up and fed with the same raw filtrate (undiluted). The effluent from the PN reactor was then fed directly to the Anammox (in place of diluted filtrate). A very long solids retention time (SRT) of 200 days was maintained throughout the study via manual wasting and decanting in order to produce very little sludge and still maintain efficient nitrogen removal. Sequence analysis and fluorescence in-situ hybridization (FISH) were performed on the biomass communities from both reactors. Automated ribosomal intergenic spacer analysis (ARISA) was also conducted on the Anammox biomass throughout the study period.

The reactor operated at a moderate loading rate (average 0.33±0.03 with a max of 0.4 g N (L day)^-1) comparable with many other fed-batch reactors in literature. It also achieved significant N removal (average of 82±4%) and specific removal rates (average 0.28±0.05 with max of 0.35 g N (g VSS day)^-1) likewise comparable with similar studies despite maintaining a very long SRT. Sequence analysis and FISH showed that K. stuttgartiensis dominated the enriched Anammox community (approximately 65% of the biomass) along with several unidentified, but seemingly enriched, potential Anammox strains. ARISA analysis of the Anammox community showed no noticeable shift in the community profile despite the change in feed composition during the study period. It has been found in other studies that the species K. stuttgartiensis is capable of dissimilatory nitrate reduction to ammonium (DNRA), which would give it a selective advantage in conditions created by maintaining a long SRT.

Ammonia oxidizing bacteria (AOBs) of the N. europaea lineage dominated the community in the PN reactor, agreeing with literature showing that lineage to dominate in oxygen-limited, ammonium-rich conditions.