Competition of Nitrite Oxidizing Bacteria (NOB) versus Anaerobic Ammonium Oxidizing bacteria (AMX) at different oxygen concentrations in aerobic granular sludge

Abstract

In the present study it is used a sequencing batch reactor (SBR) of 3L. Dissolved oxygen and pH are controlled continuously. One complete cycle consists of an anaerobic feeding period in a plug flow regime from the bottom for 60 min, followed by an aerobic phase of 112 min, a settling phase of 3 min, and an effluent withdrawal phase of 5 min. The project is based on previous research in which anaerobic Anammox dominated granules were subjected to low oxygen concentrations to achieve aerobic granules. Aerobic granules have multistructural layer in which AOB bacteria are located on outer layers, building an inner oxygen protected core for Anammox bacteria (AMX) and providing them at the same time with nitrite to ensure electron acceptor. On the other hand Nitrite Oxidizing Bacteria (NOB) can invade the system if either oxygen concentrations are too high or if ammonium concentrations are too low (Slikers). Earlier results have shown that at low oxygen concentrations the AOB capacity was limited. However higher oxygen concentrations produce NOB growth into the system, leading to an unwanted accumulation of nitrate in the effluent and consequently to a lower Anammox activity and hence lower removal efficiencies. Furthermore It was observed that top biomass contained more white and smaller granules whereas bottom biomass red and big granules. Molecular techniques (FISH and qPCR) then revealed that top biomass contained more NOB while bottom biomass more Anammox. Further research is needed in order to maintain higher oxygen concentrations in the reactor but at the same time achieve to out compete AMX bacteria over NOB.