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 of the
P-removal after the transient but sharp decrease of Ca. Accumulibacter observed during the
transition from simple to complex monomeric wastewater and likely due to a bacteriophage
attack.
The assembly of PacBio sequences associated to a binning based on composition and cov-
erage of Illumina sequences produced nearly complete draft genomes attributed to poorly
characterized taxa, thus providing information on their potential metabolism and a template
for future metatranscriptomic ana