Removal of tetracyclines in wastewater : accumulation and distribution of chlortetracycline in bulk water and biomass compartments in activated sludge

Abstract

A study was conducted to examine the removal of chlortetracycline and its distribution and accumulation in three compartments; bulk water, extracellular polymeric substance (EPS) and the microbial cells in activated sludge. Also the effect of different environmental conditions on the distribution and accumulation in the three compartments was investigated. Effluent samples collected from a municipal activated sludge treatment system were set up in bath experiments to test the distribution and accumulation of chlortetracycline under aerobic and anoxic conditions for 14 days. In addition, the impact of the activity of the microbial community on the amassing of the antibiotic in the biomass was examined. The effect of divalent cations on import and accumulation of chlortetracycline was tested. Sorption in believed to be the main removal pathway in wastewater treatment systems for tetracyclines in general and chlortetracycline in particular. In this study that notion was confirmed, and it was found that the removal via sorption under anoxic condition (43.2%) is almost double of that under aerobic conditions (27.0%). The amount of what accumulated in the cells compared to that sorbed in the EPS is twice as much in the former and triple as much in the latter. These findings suggest that changes in the structure and charge of the EPS could be the reason of higher accumulation in the polymeric substance. The impact of microbial activity on the sorption and distribution of the chlortetracycline in the three compartments showed almost a similar behaviour to that under aerobic and anoxic conditions. It was clear that the more viable the microbial community, the less the antibiotic accumulated in the [sic] both biomass compartments; the EPS and microbial cells. Biomass with inhibited respiration accrued 90% of the initial concentration; where as the active microbial community was more resistant and only 24.2% of the initial concentration accumulated within the cells. The findings suggest that the antibiotic makes its way to the cells thus bypassing the EPS, and is trapped in the EPS as it is pumped out of the cells in an energy dependent mechanism. The presence of ethylenediaminetetraacetic acid (EDTA) which is a strong chelator had no import effect. Nevertheless it did indicate that the accumulation in the EPS could be attributed to the presence of cations since there was a high negative correlation (-0.98) between the disappearance of the antibiotic from the EPS compartment and the EDTA concentration used in incubation.