Photo-oxygenation for nitritation and the effect of dissolved oxygen concentrations on anaerobic ammonium oxidation

Abstract

Removal of nitrogen from wastewater without using electricity consuming aerators was previously observed in photo-bioreactors with a mixed algal-bacterial biomass. Algammox is the particular process based on algae, ammonium oxidizing organisms and anammox bacteria. In this research the activity of anammox bacteria in such an oxygen-producing environment was tested, as well as the effect of short-duration increase in dissolved oxygen (DO) to values potentially inhibiting anammox activity. Sequencing batch photo-bioreactors were fed with settled domestic wastewater enriched with ammonium (200mgNH4+-N/L) and exposed to light within the photosynthetic active range with intensity of about 500μmol/m2·s. Each cycle consisted of 12h illumination and 12h darkness. A well-settling biomass (10days solids retention time) developed that carried out nitritation, nitrification and anammox. Ammonium removal rate during the light period was 4.5mgN-NH4+/L·h, equal to 858mgN-NH4+/m2·h or 477mgN-NH4+/(mol photons). When the reactors were aerated for 3h to temporarily increase the DO, anammox was inhibited at bulk DO values larger than 0.4–1.0mg/L. For almost oxygen saturated conditions, recovery time was about 9days. Algammox photo-bioreactors are therefore able to overcome short periods of oxygen stress, provided they occur only occasionally.