Abstract
In this thesis, bench-scale laboratory experiments were used to evaluate the efficacy of an algae-based treatment process to remove ciprofloxacin (CIPX), a highly-prescribed human and veterinary antibiotic for treatment of bacterial infections. As a result of its widespread use, CIPX has been frequently detected in wastewater treatment plant (WWTP) influents and effluents, but it is unknown to what extent its presence affects organisms in receiving waters and natural systems downstream. Previous studies have shown that algae are able to effectively remove estrogens from artificially spiked growth medium. Based on this observation, it is hypothesized that algae may also remove other priority emerging contaminants; e.g., antibiotics, including CIPX. If so, the integrated algae-treatment system could improve WWTP effluent water quality while also providing energy to the WWTP through anaerobic digestion of the algae biomass into methane-derived electricity.
We examined the behavior and fate of CIPX under simulated algae cultivation conditions, using the model freshwater alga, Scenedesmus dimorphus. First, we analyzed biomass production of S. dimorphus during exposure to different concentrations of CIPX (0-5000 ng/L) over 192 hours. Interestingly, a dual effect was observed, whereby CIPX at an artificially high experimental concentration (5000 ng/L) was found to stimulate growth, while CIPX at lower environmentally relevant concentrations (0-600 ng/L) were found to inhibit cumulative algae growth. However, little to no impact on algae growth rate was observed within the first 24-48 hours of experimentation. Since it is anticipated that commercial-scale algae-treatment systems will use hydraulic residence times (HRT) on the order of 24 hours, the presence of CIPX in wastewater effluents is expected to have little to no observable impacts on algae productivity.
Second, we examined removal of CIPX as mediated by S. dimorphus. Appropriate controls were used to assist in allocating apparent removal among several possible removal mechanisms reported in literature: sorption, photolysis (i.e., light-mediated transformation), and algae-mediated biotransformation. Results show an average of 65% removal of CIPX over 8 days, mostly occurring within the first 24 hours. In contrast, negligible CIPX removal was observed in “dark reactors” and “light controls.” These data rule out CIPX disappearance as an experimental artifact, and suggest that photolysis is not a significant contributor to CIPX removal under algae cultivation conditions. Negligible CIPX removal was also observed in foil-wrapped controls comprising autoclave-inactivated (i.e., dead) algae. This suggests that sorption is also not a significant contributor to CIPX removal in this system. As such, CIPX removal under the tested conditions is dominated by biotransformation reactions mediated by active algae biomass. This is consistent with previous investigations of algae-mediated removal of steroid estrogens (17α-estradiol, 17β-estradiol, estrone, and estriol); moreover, it brings to light many interesting questions about how algae-mediated biotransformation of CIPX and other antibiotics could impact the dissemination of antibiotic resistance. Overall, it was determined that the model alga does indeed provide rapid removal of the selected model antibiotic, which increases the appeal of algae-mediated WWTP effluent polishing.
