Dynamic assessment of the floc morphology, bacterial diversity and integron content of an activated sludge reactor processing hospital effluent

The treatment of hospital effluents (HE) is a major concern, as they are suspected of
disseminating drugs and antibiotic resistance determinants in the environment. In order to
assess HE influence on wastewater treatment plant biomass, lab scale conventional activated
sludge systems (CAS) were continuously fed with real HE or urban effluent as a control. To
gain insights into the main hurdles linked to HE treatment, we conducted a multiparameter
study using classical physicochemical characterization, phase contrast and confocal laser 35
scaning microscopy, and molecular biology (i.e. pyrosequencing) tools. HE caused erosion of
floc structure and the production of extracellular polymeric substances attributed to the
development of floc forming bacteria. Adaptation of the sludge bacterial community to the
HE characteristics, thus maintaining the purification performance of the biomass, was
observed. Finally, the comparative metagenomic analysis of the CAS showed that HE 40
treatment resulted in an increase of class 1 resistance integrons (RIs) and the introduction of
Pseudomonas spp. into the bacterial community. HE treatment did not reduce the CAS
process performance; nevertheless it increases the risk of dissemination into the environment
of bacterial species and genetic determinants (RIs) involved in antibiotic resistance
acquisition.