Influence of Partial Denitrification and Mixotrophic Growth of NOB on Microbial Distribution in Aerobic Granular Sludge

In aerobic granular sludge (AGS), the growth of nitrite oxidizing bacteria (NOB) can be uncoupled from the nitrite supply of ammonia oxidizing bacteria (AOB). Besides, unlike for conventional activated sludge, <i>Nitrobacter</i> was found to be the dominant NOB and not <i>Nitrospira</i>. To explain these experimental observations, two possible pathways have been put forward in literature. The first one involves the availability of additional nitrite from partial denitrification (nitrite-loop) and the second one consists of mixotrophic growth of <i>Nitrobacter</i> in the presence of acetate (ping-pong). In this contribution, mathematical models were set up to assess the possibility of these pathways to explain the reported observations. Simulation results revealed that both pathways influenced the nitrifier distribution in the granules. The nitrite-loop pathway led to an elevated NOB/AOB ratio, while mixotrophic growth of <i>Nitrobacter</i> guaranteed their predominance among the NOB population. Besides, mixotrophic growth of <i>Nitrobacter</i> could lead to NO emission from AGS. An increasing temperature and/or a decreasing oxygen concentration led to an elevated NOB/AOB ratio and increased NO emissions