Establishing a co-culture aggregate of N-cycle bacteria to elucidate flocculation in biological wastewater treatment.

Biological flocculation is a complex phenomenon that is often treated as a black box. As a result, flocculation problems are usually remediated without knowledge of the exact causes. We show that it is feasible to exploit a model (N-cycle) consortium with reduced complexity to fundamentally study bioflocculation. Strong nitrifier microcolonies were formed during oxic/anoxic cycles in sequencing batch reactors, using alginate entrapment as a cell retention system. After the release of these aggregates into suspension, macroclusters with flocs of the denitrifier were observed. These results suggest that a living model of a full-scale activated sludge floc can be built through the use of this bottom-up approach. By eliminating shifts in the microbial community, the applied experimental conditions have a more direct effect on the observations. Key Points $\bullet$ Studying flocculation with a model consortium is feasible $\bullet$ Alginate entrapment leads to strong microcolony formation of nitrifiers $\bullet$ FISH by itself is not suitable to study aggregation of a coculture.