Bacterial diversity shifts in AGS reactor treating food industry wastewater

Aerobic granular sludge (AGS) is a promising technology for treating industrial wastewater, possessing higher biomass retention and tolerance to toxic substrates than conventional activated sludge systems. AGS presents a diverse microbial community responsible for the simultaneous removal of carbon and nutrients. These communities are protected by extracellular polymeric substances (EPS) that allow for the compact structure of the granules. As a result, bacteria present in the aerobic granules are more resistant to variable wastewater composition, as commonly produced in food industry. The main objective of this work is to study the microbial community dynamics of an AGS reactor treating wastewater from a fish canning plant. The reactor was monitored during 220 days, divided into eight operational phases. COD, NH₄⁺ and PO₄^3- removal were assessed and biomass samples were collected throughout time for microbiome profiling. The reactor presented good COD, PO43- and NH4+ removal during phases I, II and III, but decreased performance during phase IV, when a higher organic load was applied. The removal processes recovered after phase IV until the end of operation. Proteobacteria were dominant in the inoculum (relative abundance of 64.8 %) and dominated almost all reactor phases. Bacteroidetes were second dominant in the inoculum (17.5 %) as well in most reactor phases, being present with higher relative abundance (55.5 %) than Proteobacteria (38.4 %) during phase IV. Within Proteobacteria, Gammaproteobacteria were initially more abundant but Betaproteobacteria predominated after phase IV. For Bacteroidetes, the community dynamics has also changed from phase IV onwards, with Flavobacteriia losing its high relative abundance to Saprospiria and Cytophagia. Several bacterial genera were detected throughout reactor operation, such as Phenylobacterium and Flavobacterium, while other were detected with higher abundance before (Methylocaldum and Plasticicumulans) or after phase IV (Thauera and Paracoccus). The relationship between bacterial community shifts and process performance was assessed. This study increases our knowledge on AGS technology application in real wastewater treatment.