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

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Abstract
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 × 106 to 8.2 × 106 colony-forming units per milliliter (CFU/mL) compared with the control at 1.4 × 106–6.3 × 106 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.

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