The effect of different organic substrates on the microbial communities of aerobic granular wastewater treatment sludge

Aerobic granular sludge (AGS) is a promising alternative wastewater treatment to the conven- tional activated sludge system, allowing space and energy savings. This process is particularly suited for biological phosphorus removal, avoiding use of coagulant chemicals. Basic under- standing of this process has mainly been obtained in laboratory-scale studies with simple synthetic wastewater containing volatile fatty acids as main carbon source. Yet, the aspect and performance of granular sludge cultivated in such model systems are rather different from those obtained in systems treating real wastewater. In order to make a step toward the comprehension of AGS treating municipal wastewater, two approaches were applied to investigate the impact of the wastewater composition on AGS bacterial communities, settling properties and nutrient removal performance. The first approach was to transform activated sludge performing enhanced biological phosphorus removal into AGS in four parallel lab-scale reactors fed with different wastewater types: simple and complex polymeric synthetic, as well as raw and clarified municipal wastewater. The second approach was to progressively change the composition of the wastewater treated by AGS acclimated to simple synthetic wastewater to complex polymeric wastewater with an intermediary step with complex monomeric wastewater as a control. The whole DNA of biomass corresponding to the three wastewater types was sequenced with PacBio and Illumina technologies. During the two experiments, the bacterial communities, settling properties and nutrient removal performance were monitored. The bacterial communities in AGS treating simple wastewater were drastically different form the ones treating complex wastewater. Several taxa belonging to Actinobacteria and Saccha- ribacteria were largely underrepresented with the simple wastewater. Lower nitrogen removal, lower settling properties and higher proportions of flocs were observed with the polymeric wastewaters compared to the monomeric wastewaters. The lower concentrations of diffusible organic carbon rather than the bacterial community compositions were identified as the cause for these differences. Indeed, genes putatively involved in denitrification and biofilm formation were found in the AGS treating monomeric and polymeric wastewater. Moreover, different denitrification efficiencies and settling properties were observed with AGS having very similar bacterial communities but treating wastewater with a different concentration of organic carbon. The phosphate accumulating organism (PAO) Candidatus (Ca.) Accumulibacter, abundant in most of the AGS samples, was highly diverse and different clade repartitions were found within the AGS treating different wastewater types. The fermentative PAO Tetrasphaera was less diverse and mostly found in the AGS treating complex wastewater. The co-occurrence of two PAO occupying distinct ecological niches likely participated to the quick recovery