活性污泥吸/脫附和生物降解磺胺劑類抗生素之研究

Abstract

抗生素在世界上被廣泛用來治療人類及動物的疾病。抗生素經食用後大部份仍會以原化學成份或其代謝物經由尿液及糞便而排出體外，經由污水下水道傳送至都市污水處理廠。到目前為止，磺胺劑類抗生素和活性污泥處理程序之間的行為反應、生物降解、生物吸附和脫附等移除機制很少被量化和探討。因此，3種常出現在都市污水處理廠進流水、放流水和活性污泥的磺胺劑類抗生素包括sulfamethoxazole (SMX)、sulfamonomethoxine (SMM)和sulfadimethoxine (SDM)被選擇用來研究抗生素和活性污泥之間的生物吸附、脫附和生物降解。研究目標在定義磺胺劑類抗生素在液相和固相之間的分佈情形、隨著時間濃度變化趨勢和上述移除機制對移除抗生素所作的貢獻，同時也針對抗生素在都市污水處理廠污泥的分佈情形進行調查。 抗生素在都市污水處理廠污泥的分佈情形調查結果顯示，penicillins類抗生素遠低於儀器定量極限，這可能是因為penicillins類抗生素在水中易水解所造成。Sulfonamides、macrolides和tetracyclines類抗生素在污泥的濃度值介於2.56－56.6 μg/kg。 在添加疊氮化鈉生物活性抑制劑探討生物降解、生物吸附和脫附的實驗結果顯示，活性污泥主要是藉由生物吸附和生物降解機制來移除抗生素。在一開始的12小時可能是由於反應系統內生物易分解的基質和抗生素氧化產生競爭，使得生物降解作用被抑制。去活性污泥對於磺胺劑類抗生素吸附親合力依序為SDM>SMM>SMX。在反應系統混合溶液pH為6.8時，帶負電磺胺劑類抗生素是主要的優勢物種，使得被活性污泥的吸附程度變低。當常數n值接近1，Freundlich等溫曲線轉變為線性等溫曲線，相當適合用來描述等溫吸脫附行為。SMX、SMM和SDM分配係數(Kd)分別為28.6 ± 1.9、55.7 ± 2.2和110.0 ± 4.6 mL/g。被污泥吸附的磺胺劑類抗生素是可逆的，脫附完成後固相污泥SMX、SMM和SDM佔原添加濃度100 µg/L的比例分別為0.9%、1.6%和5.2%。 除了添加抑制劑之外，進一步運用超音波溶劑萃取方法來萃取和量化固相活性污泥磺胺劑類抗生素濃度，探討和解釋生物降解和生物吸附。實驗結果顯示三種磺胺劑類抗生素在和活性污泥接觸11 – 13天後就會完全被移除。吸附平衡在實驗一開始的幾個小時就達到，緊接著1–3天為遲滯期，之後生物降解完全啟動，在接著10天做為主要移除機制。藉由量測液相濃度隨著時間變化趨勢和依據生物降解與生物吸附所推導動力模式所作的線性回歸分析，得到在活性污泥濃度為2.56 g/L 時，SDM、SMX和SMM零階速率常數分別為8.1、7.9和7.7 µg/L/d。 抗生素存在於都市污水處理廠污泥，被污泥吸附的抗生素是可逆，如果沒有進一步的處理程序來移除污泥的抗生素，那麼它將會進入自然環境，當污泥再利用作為土壤改良劑時，應注意抗生素在環境流佈可能引起的環境效應。經由實驗所求得的反應動力參數指出，傳統都市污水處理廠所提供水力停留時間(6 h)，只足夠活性污泥處理程序從廢水移除將近2 µg/L的磺胺劑類抗生素。

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.