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

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 × 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.

Do antibiotics have environmental side-effects? Impact of synthetic antibiotics on biogeochemical processes

“…Studies on the effects of cocktails or mixtures of antibiotics on biogeochemical processes are even scarcer but of growing concern (Ingvorsen et al 2003;Schmidt et al 2012;Wunder et al 2013). The positive synergistic effect of an antibiotic mixture on the model organism Vibrio Fischeri has been demonstrated by Backhaus et al (2000).…”

“…In most studies (25 out of 31), the antibiotic was added once or several times (Campos et al 2001;Fernandez et al 2009;Rico et al 2014;Schmidt et al 2012). Only three studies continuously supplied the antibiotic (Wunder et al 2013Roose-Amsaleg et al 2013).…”

“…Among the studies showing an effect, only three tested both environmentally relevant and therapeutic concentrations (Rico et al 2014;Toth et al 2011;Schmidt et al 2012). Rico et al (2014) carried out a chronic exposure of tropical freshwater to a fluoroquinolone (enrofloxacin) in a microcosm.…”

“…The two other studies tested therapeutic concentrations of antibiotic mixtures and observed expected inhibition. Schmidt et al (2012) explored the effect of a mixture of ciprofloxacin, gentamycin, sulfamethoxazole, trimethoprim, and vancomycin (0.1-40 mg/L) on chemical oxygen demand and nitrification. The chosen experimental setup for their study simulated conditions of a wastewater treatment plant; nitrification was completely inhibited at 40 mg/L.…”

“…The duration of the exposure of colistin used by Bressan et al (2013) on ammonia-oxidizing bacteria was 5 h. Schmidt et al (2012) already stated that Binhibition of cell division or protein biosynthesis may be observed only after test duration of several days.^The importance of incubation time was also shown in the BSubstrate Induced Respiration (SIR)^test of Thiele-Bruhn and Beck (2005); no effect was observed after 4 h, whereas respiration was inhibited by both oxytetracycline and sulfapyridine after 24 h. Short-term experiments that are shorter than the growth rate of the targeted bacteria, therefore, explore the effect of the antibiotic on enzyme activity. Studies regarding short-term effects (Bressan et al 2013;Costanzo et al 2005;Hou et al 2015;Katipoglu-Yazan et al 2013) can be contrasted to long-term effect studies: from days (Ahmad et al 2014;Alighardashi et al 2009;Conkle and White 2012;Fountoulakis et al 2004;Hou et al 2015;Ingvorsen et al 2003;Thiele-Bruhn and Beck 2005;Yamamura et al 2014), weeks (Campos et al 2001;Cui et al 2014;Klaver and Matthews 1994;Kotzerke et al 2008;Kotzerke et al 2011;Lotti et al 2012;Rico et al 2014;Roose-Amsaleg et al 2013;Toth et al 2011;Underwood et al 2011;Wunder et al 2013;Yan et al 2013), months (Fernandez et al 2009;Hansen et al 1992;Näslund et al 2008;Rosendahl et al 2012), to a year (442 days by Schmidt et al 2012). Conducting experiments for extended time periods is difficult because degradable antibiotics, as well as nutrients, can be quickly consumed unless continuously added …”