Small aggregates can cause nitrite accumulation in one-stage partial nitritation and anammox

Aerobic and anoxic ammonium-oxidizing bacteria (AerAOB and AnAOB) cooperate in partial nitritation/anammox systems to remove ammonium from wastewater. In this process, nitrite accumulation is not desirable, since it decreases the nitrogen removal rate and since it can potentially inhibit the AnAOB. This study addressed several aspects of the microbial aggregate size for three independent types of suspended biomass, designated ‘A’, ‘B’ and ‘C’, for the first time including biomass from pilotand full-scale applications. Firstly, the AerAOB and AnAOB abundance and the activity balance were quantified for the different aggregate sizes, separated in up to six size classes. Secondly, aggregate morphology, size distribution and architecture were examined. In all systems, the AerAOB abundance was highest in small aggregates (25-80% AerAOB; 1-21% AnAOB), while the AnAOB abundance was highest in large aggregates (136% AerAOB; 22-85% AnAOB). A nitrite accumulation rate ratio (narr) was defined as the net aerobic nitrite production rate divided by the anoxic nitrite consumption rate. The smallest ‘A’, ‘B’ and ‘C’ aggregates were nitrite sources (narr > 1.7). Large ‘A’ and ‘C’ aggregates were granules capable of autonomous nitrogen removal (narr 0.61.1) with internal AnAOB zones surrounded by an AerAOB rim. Large ‘B’ aggregates were thin film-like nitrite sinks (narr < 0.5) in which AnAOB were not shielded by an AerAOB layer. Overall, no unique, optimal aggregate size distribution or aggregate architecture was required to obtain good nitrogen removal in the three independent partial nitritation/anammox systems. Yet, selecting for larger aggregates can be an effective measure to prevent nitrite accumulation.