| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 100 === Antibiotic were used widespread to protect humans and animals from illness and infection, throughout the world. The parent compound or their metabolites will be excreted by human beings and enter waste water treatment plants (WWTPs) through sewer system. To date, little has been quantified for the interactions, biodegradation and sorption, of sulfonamide antibiotics with activated sludge. Hence, three frequently occurring sulfonamide antibiotics including sulfamethoxazole (SMX), sulfadimethoxine (SDM), and sulfamonomethoxine (SMM) were selected to study sorption/desorption and biodegradation of antibiotics by the activated sludge process. Our objectives were to determine the distribution of sulfonamides in the aqueous and solid phases, the concentration changes of the compounds in both phases over time, and the sorption and biodegradation mechanisms contributed by the activated sludge for removal of the compounds. The distribution of antibiotics in municipal sewage sludge was also investigated in this research.
The results of distribution of antibiotics in WWTPs activated and digested sludge showed the concentrations of penicillins in sewage sludge samples were found to be well below the limit of quantification (LOQ). This may be perhaps due penicillins prefer to hydrolyze in the water and are degraded to different products. Average concentrations of sulfamethoxazole, sulfamethazine, sulfamonomethoxine, sulfadimethoxine, erythromycin-H2O, tylosin, tetracycline, oxytetracycline and chlortetracycline in sewage sludge ranged between 2.56 and 56.6 μg/kg of dry weight.
Experimental results in the presence of activated sludge with and without being subjected to NaN3 biocide showed that the antibiotic compounds were removed via sorption and biodegradation by the activated sludge, though biodegradation was inhibited in the first 12 h possibly due to competitive inhibition of xenobiotic oxidation by readily biodegradable substances. The affinity of sulfonamides to sterilized sludge was in the order of SDM>SMM>SMX. The sulfonamides existed predominantly as anions at the study pH of 6.8, which resulted in a low level of adsorption to the activated sludge. The adsorption/desorption isotherms were of a linear form, as well described by the Freundlich isotherm with the n value approximating unity. The linear distribution coefficients (Kd) were determined from batch equilibrium experiments with values of 28.6 ± 1.9, 55.7 ± 2.2, and 110.0 ± 4.6 mL/g for SMX, SMM, and SDM, respectively. SMX, SMM, and SDM desorb reversibly from the activated sludge leaving behind on the solids 0.9%, 1.6%, and 5.2% of the original sorption dose of 100 µg/L.
In addition to apply biocide, an ultrasonic solvent extraction method was employed to quantify and delineate biosorption and biodegradation of three sulfonamide antibiotics in this work. All sulfonamides were removed completely over 11 – 13 d. Sorptive equilibrium was established well within the first few hours, followed by a lag period of 1-3 days before biodegradation was to deplete the antibiotic compounds linearly in the ensuing 10 days. Apparent zeroth-order rate constants were obtained by regression analysis of measured aqueous concentration vs. time profiles to a kinetic model accounting for sorption and biodegradation; they were 8.1, 7.9, and 7.7 µg/L/d for SDM, SMX, and SMM, respectively, at activated sludge concentration of 2.56 g/L.
The results obtained in this study illustrate: 1. the sorbed antibiotics can be introduced into the environment if no further treatments were employed to remove them from the biomass; 2. the existence of antibiotics in municipal sewage sludge and the importance of concern about distribution of antibiotics in the environment when sewage sludge is reutilized in land application; 3. the measured kinetics implied that with typical hydraulic retention time (e.g. 6 h) provided by WWTP the removal of sulfonamide compounds from the wastewater during the activated sludge process would approximate 2 µg/L.
|
|---|
