Abstract
The release of antimicrobials into the environment is a matter of important concern and constitutes a potential threatfor humans’ health as well as terrestrial and aquatic ecosystems. Antimicrobials are partially removed during conventional primary and secondary wastewater treatment and as a result they are often detected in the aquatic environment.Since 2000,they have been detected at ppb or pptevels in a variety of environmental media across the globe. Amongst different plant-based systems used for wastewater treatment, ponds with the duckweed Lemna minor have been appliedsuccessfullyin several countries for the removal of organic matter, nutrients and heavy metals. As this macrophyte is characterized by the high protein or starch content, during the last decade, several studies are also available investigating its cultivation for animals’ feedstock or biofuels’ production. On the other hand, limited information is available for the ability of such systems to remove organic micropoll ...
The release of antimicrobials into the environment is a matter of important concern and constitutes a potential threatfor humans’ health as well as terrestrial and aquatic ecosystems. Antimicrobials are partially removed during conventional primary and secondary wastewater treatment and as a result they are often detected in the aquatic environment.Since 2000,they have been detected at ppb or pptevels in a variety of environmental media across the globe. Amongst different plant-based systems used for wastewater treatment, ponds with the duckweed Lemna minor have been appliedsuccessfullyin several countries for the removal of organic matter, nutrients and heavy metals. As this macrophyte is characterized by the high protein or starch content, during the last decade, several studies are also available investigating its cultivation for animals’ feedstock or biofuels’ production. On the other hand, limited information is available for the ability of such systems to remove organic micropollutants and especially antimicrobial compounds. The main goals of the current PhD Thesiswere: a) to estimate the potential environmental risks associated with human consumption of antimicrobials in Greece, b) to study the growth and characteristics of L. minor in human urine and municipal wastewaterandc) to investigate the removal efficiency of antimicrobials in L. minor systemsas well as the role of different abiotic and biotic mechanisms on their elimination. In the first part of the current study, consumption data were collected for the 24 most often used antimicrobials in Greece for years 2008-2010 and their Predicted Environmental Concentrations (PECs) in raw and treated wastewater were calculated using mass balances. The ecotoxicological risk was estimated by calculating the ratio of PEC to Predicted No Effect Concentration (PNEC) for three categories of aquatic organisms (algae, daphnids and fish). Based on acute toxicity data for algae, an ecological threat seems possible for 7 out of 24 target antimicrobials in raw and treated wastewater, while no significant risk was estimated for daphnids and fish. For Greek rivers where low and medium dilution of wastewater occurs, a moderate to high risk is expected due to the existence of individual antimicrobials such as amoxicillin, clarithromycin, ciprofloxacin, azithromycin, erythromycin and levofloxacin in discharged treated wastewater.In the second part of the PhD Thesis, experiments were conducted to study the cultivation of duckweedL. minor in human urine(HU) and the role of different parameters such as urine type, dilution factor, temperature, existence of macro- and microelements on growth rate was investigated. The simultaneous removal of nutrients and selected antimicrobials was also studied in experiments with HU and treated domestic wastewater, while the starch and protein content of the produced biomass was determined. Higher growth rates were observed at 24 oC, using HU stored for 1 d and with dilution factor equal to 1:200. In experiments with HU and wastewater, the removal of COD, total phosphorus and total nitrogen exceeded 80%, 90% and 50%, respectively, while ciprofloxacin and sulfamethoxazole were eliminated by more than 80%. The main removal mechanism for the former antimicrobial wasphotodegradation, while plant uptake and biodegradation seem to be of significant importance for the latter. Crude protein content reached 31.6% in experiments with HU and biomass harvesting, while starch content was enhanced when duckweed was transferred to water for 21 d, reaching 47.1%.In the third part of the PhD Thesis, the use of duckweed-based wastewater treatment systems for producing biomass with high crude protein and starch content was investigated. Three lab-scale systems were used; System 1 was planted with L. minor, System 2 with L.gibba and System 3 with the combination of the two duckweeds. The studied duckweeds were cultivated using secondary treated wastewater as substrate (Phase A), in the presence of excess NH4-N (Phase B) and using water with no nutrients (Phase C). All systems achieved average NH4-N removal higher that 90%. The specific duckweeds growth rates and the specific duckweeds growth rates normalized to the area ranged between 0.14 d−1 and 8.9 g m−2 d−1 (System 1) to 0.19 d−1 and 14.9 g m−2 d−1(System 3). The addition of NH4-N resulted to an important increase of biomass protein content, reaching 44.4% in System 3, 41.9% in System 2 and 39.4% in System 1. The transfer of biomass in water containing no nutrients resulted to the gradual increment of the starch content up to the end of the experiment. The highest starch content was achieved for the combination of the two duckweeds (46.1%), followed by L. gibba (44.9%) and L. minor (43.9%).In the last experimental part of the current PhD Thesis, the fate of four antimicrobials (cefadroxil, CFD; metronidazole, METRO; trimethoprim, TRI; sulfamethoxazole, SMX) was studied in L. minor systems and the role of different mechanisms on their removal was evaluated. All micropollutants were significantly removed in batch experiments with active L. minor and the order from the highest to lower removal was CFD >METRO > SMX > TRI till the end of experiment. Calculation of kinetic constants for hydrolysis, photodegradation, sorption to biomass and plant uptake revealed significant differences depending on the compound and the studied mechanism.For METRO, TRI and SMX the kinetic constants of plant uptake were by far higher comparing to those of the other mechanisms. The transformation productsofantimicrobialswere identified using UHPLC-QToF-MS. Two were the main degradation pathways for TRI; hydroxylation takes place during both phyto- and photodegradation, while demethylation occurs only in absence of L. minor. The operation of a continuous-flow duckweed system showed METRO and TRI removal equal to 71±11% and 61±8%, respectively.The plant uptake and biodegradation were the major mechanisms for METRO removal while the most important mechanism for TRI was plant uptake.
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