Long-Term Effects of Antibiotics on the Elimination of Chemical Oxygen Demand, Nitrification, and Viable Bacteria in Laboratory-Scale Wastewater Treatment Plants

Antibiotics and other pharmaceuticals are contaminants of the environment because of their widespread use and incomplete removal by microorganisms during wastewater treatment. The influence of a mixture of ciprofloxacin (CIP), gentamicin (GM), sulfamethoxazole (SMZ)/trimethoprim (TMP), and vancomycin (VA), up to a final concentration of 40?mg/L, on the elimination of chemical oxygen demand (COD), nitrification, and survival of bacteria, as well as the elimination of the antibiotics, was assessed in a long-term study in laboratory treatment plants (LTPs). In the presence of 30?mg/L antibiotics, nitrification of artificial sewage by activated sludge ended at nitrite. Nitrate formation was almost completely inhibited. No nitrification at all was possible in the presence of 40?mg/L antibiotics. The nitrifiers were more sensitive to antibiotics than heterotrophic bacteria. COD elimination in antibiotic-stressed LTPs was not influenced by?≤20?mg/L antibiotics. Addition of 30?mg/L antibiotic mixture decreased COD removal efficiency for a period, but the LTPs recovered. Similar results were obtained with 40?mg/L antibiotic mixture. The total viable count of bacteria was not affected negatively by the antibiotics. It ranged from 2.2?×?10(6) to 8.2?×?10(6) colony-forming units per milliliter (CFU/mL) compared with the control at 1.4?×?10(6)-6.3?×?10(6) CFU/mL. Elimination of the four antibiotics during phases of 2.4-30?mg/L from the liquid was high for GM (70-90?%), much lower for VA, TMP, and CIP (0-50?%), and highly fluctuating for SMZ (0-95?%). The antibiotics were mainly adsorbed to the sludge and not biodegraded.