An overview of analytical methodologies for the determination of antibiotics in environmental waters

The widespread occurrence of antibiotics as contaminants in the aquatic environment has increased attention in the last years. The concern over the release of antibiotics into the environment is related primarily to the potential for the development of antimicrobial resistance among microorganisms. This article presents an overview of analytical methodologies for the determination of quinolone (Qs) and fluoroquinolone (FQs), macrolide (MLs), tetracycline (TCs), sulfonamide (SAs) antibiotics and trimethoprim (TMP) in different environmental waters. The analysis of these antibiotics has usually been carried out by high-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) or tandem mass spectrometry (MS/MS) and to a lesser extent by ultraviolet (UV) or fluorescence detection (FD). A very important step before LC analysis is sample preparation and extraction leading to elimination of interferences and prevention of matrix effect and preconcentration of target analytes.     

基于SMP破坏准则的饱和砂土弹塑性本构模型

利用土体的塑性流动理论，提出了用于描述饱和砂土在单调荷载作用下的应力一应变反应性质的弹塑性本构模型。土体总的变形由三部分组成：即弹性应变、与体积屈服机制相关的塑性应变和与剪切屈服机制相关的塑性应变，其中与剪切屈服机制相关的塑性应变的得出是基于SMP破坏准则。通过将模型预测的结果与试验结果进行对比，表明该模型能够较为准确地描述饱和砂土在单调加载条件下的反应性质。     

Effects of several different flux enhancing chemicals on filterability and fouling reduction of membrane bioreactor (MBR) mixed liquors

The main objective of this work was to determine the effectiveness of various chemicals on filterability and fouling reduction in MBR mixed liquors. Different lab-scale experiments were conducted with a total of 7 different additives (3 cationic polymers (MPL30, MPE50, KD452), a biopolymer (Chit), a starch (Sta), and 2 metal salts (FeCl₃, PACl)). Initially, batch shaker tests were performed for each additive to determine the optimum dosages in terms of soluble microbial products (SMP) removal. Then, short-term filtration trials and critical flux tests were performed. All tested additives were able to remove SMP, but at different extent; 33, 45, 51, 36, 38, 54, and 56% for MPL30, MPE50, KD452, FeCl₃, PACl, Chit, and Sta, respectively. The cationic polymer KD452 exhibited the best performance in terms of the extent of SMP removal and the required dosage. All tested cationic polymers, starch and chitosan significantly reduced fouling rates and increased permeability values. At their optimum dosages, the cationic polymers MPE50, MPL30 and KD452 provided 96, 80 and 74% reductions in fouling rates, respectively. The enhancements in critical flux achieved by MPL30, MPE50, KD452, FeCl₃, PACl, Chit, and Sta were 38, 46, 38, 14, 14, 0, and 22% in comparison with raw mixed liquor. Cationic polymers increased critical flux values to levels above 50 L m⁻² h⁻¹. SMP removal from MBR mixed liquors and further improvement in filtration performance and fouling control did not always correlate. Overall, based on the lab-scale tests conducted, cationic polymeric additives were found to be favorable over the other additives due to their steady and successful performance in fouling control. The performance of cationic polymers was independent of small variations in dosing, while for other additives over- or under-dosing showed detrimental effects on filterability.     

Impact of cation concentrations on fouling in membrane bioreactors

In this study, the interaction of calcium, magnesium, and sodium as well as impact of monovalent to divalent (M/D) cation ratio and magnesium to calcium (Mg/Ca) ratio in the feed wastewater on membrane fouling in submerged membrane bioreactor (MBR) was investigated. The protein and carbohydrate content of soluble microbial products (SMP) and extracellular polymeric substances (EPS) as well as their relative hydrophobicities was examined. The mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance, as reflected by the modified fouling index (MFI). Based on the findings of this study, the optimum conditions with respect to fouling rate were calcium and sodium concentrations of 36 and 140 mg/L, respectively, M/D of 1:1 and Mg/Ca of 5:1, with all parameters on an equivalent basis. High sodium concentration at high M/D ratio was found to decrease the floc size and increase the fouling rate. At the low M/D ratio of 1:1, introduction of magnesium was beneficial in reducing the fouling rate by increasing the EPS concentration and floc size and decreasing the SMP concentration and relative hydrophobicity in the supernatant. The fouling rate was found to be statistically correlated with the concentrations of Ca, Mg, and Na, with both Ca and Na adversely impacting fouling and Mg alleviating fouling propensity.     

反激开关电源中基于PC817A与TL431配合的环路动态补偿设计

开关电源市场中占很大份额的单端反激开关电源通常采用PC817A与TL431配合来组成控制环路.然而,目前设计这个环路的动态补偿参数,基本上采用试验方法.文中利用开关电源的小信号传递函数,对此环路的动态补偿进行了定性分析和定量计算,通过设计合适的相位裕量来保证开关电源的稳定性.其过程经实验证明具有较好的通用性,在实际应用中取得了很好的效果.     

Quality-of-Service Analysis for Statistical Multiplexing with Gaussian Distributed and Autoregressive Input

We investigate multiplexers in telecommunication systems with a workload process developing equivalent to that of a service system with semi-Markovian input, which includes fluid flow and time slotted systems. Discrete time methods are used to analyze their waiting time and loss rate.Our focus is on the performance evaluation of statistical multiplexing. Traffic flows are modeled by autoregressive processes producing autocorrelated and Gaussian distributed workload increases. The superposition of on-off voice sources approaches autoregressive processes and they also serve as a basic model for video traffic in an appropriate time scale, although video reveals a more complex autocorrelation structure. Performance results are obtained depending on only two parameters, which allow for clear conclusions about the statistical multiplexing gain with regard to bounds on loss rates as demanded in quality-of-service guarantees.     

基于显式有限元方法的两相介质弹塑性动力反应计算分析

针对增量形式的流体饱和两相多孔介质弹塑性波动方程组,运用基于显式逐步积分格式的时域显式有限元方法对该波动方程组进行求解,并应用基于SMP破坏准则的弹塑性动力本构模型描述两相介质的动力反应性质,对两相介质在输入地震波作用下的弹塑性动力反应进行计算和分析,将计算结果与相应的弹性动力反应的计算结果进行对比;对本文应用的弹塑性...     

Shape memory epoxies based on networks with chemical and physical crosslinks

Epoxies are an important family of shape memory polymers (SMP) due to their excellent stability and thermo-mechanical endurance and the high values of shape fixity and shape recovery. Actuators based on these materials can be designed for large tensile elongations (e.g., 75% or higher) or large recovered stresses (e.g., 3 MPa or higher). However, meeting these requirements simultaneously is a difficult task because changes in the crosslink density affect both variables in opposite ways. We show that an SMP based on an epoxy network with both chemical and physical crosslinks could be strained up to 75% in four repeated shape memory cycles with tensile stresses close to 3 MPa. Shape fixity and shape recovery values were close to 98% and 96%, respectively, for everyone of the cycles, without any significant change between the first and subsequent cycles.     

虚拟专用网License管理的一种改进方案

虚拟专用网VPN拥有大量的用户群,其用户数量的License功能管理也显得非常重要.针对虚拟专用网用户数的管理存在缺陷的现状,提出了一种改进的License管理方案.增加一个是否支持公司License控制方案的开关来兼容原有功能;由SMP统一获取所有SCP用户数,并对所有SCP进行统一控制;对servicenumber表和mvpn_servicenumber表用户数量进行加密修改,确保用户数不被非法用户盗取:同时提供了一条MML命令来激活License文件,有效解决从License文件读取用户配置项的数据.最后,分析比较了当前改进方案的优点.     

岩土材料塑性应变增量方向的确定

通过与金属材料的类比,分析了岩土材料的特征面(SMP面)的特性,提出了SMP面上的应力 决定岩土材料塑性应变增量流动方向的观点,并由此推导出计算公式. 通过增加一个材料参 数,还可以考虑存在黏聚力时的情况. 由该公式计算的塑性应变增量方向得到了 试验结果的验证. 该公式也在用变换应力修正的剑桥模型中得到应用,模型预测结果能够合理反映试验数据.