Resource Center for Environmental Technologies (CRTE), CRP Henri Tudor, 66, rue de Luxembourg, 4221 Esch-sur-Alzette, Luxembourg.
Water Res. 2011 Nov 15;45(18):6152-62. doi: 10.1016/j.watres.2011.09.005. Epub 2011 Sep 22.
The effect of mixing regimes and residence time distribution (RTD) on solute transport in wastewater treatment plants (WWTPs) is well understood in environmental engineering. Nevertheless, it is frequently neglected in sampling design and data analysis for the investigation of polar xenobiotic removal efficiencies in WWTPs. Most studies on the latter use 24-h composite samples in influent and effluent. The effluent sampling period is often shifted by the mean hydraulic retention time assuming that this allows a total coverage of the influent load. However, this assumption disregards mixing regime characteristics as well as flow and concentration variability in evaluating xenobiotic removal performances and may consequently lead to biased estimates or even negative elimination efficiencies. The present study aims at developing a modeling approach to estimate xenobiotic removal efficiencies from monitoring data taking the hydraulic RTD in WWTPs into consideration. For this purpose, completely mixed tanks-in-series were applied to address hydraulic mixing regimes in a Luxembourg WWTP. Hydraulic calibration for this WWTP was performed using wastewater conductivity as a tracer. The RTD mixing approach was coupled with first-order biodegradation kinetics for xenobiotics covering three classes of biodegradability during aerobic treatment. Model simulations showed that a daily influent load is distributed over more than one day in the effluent. A 24-h sampling period with an optimal time offset between influent and effluent covers less than the half of the influent load in a dry weather scenario. According to RTD calculations, an optimized sampling strategy covering four consecutive measuring days in the influent would be necessary to estimate the full-scale elimination efficiencies with sufficient accuracy. Daily variations of influent flow and concentrations can substantially affect the reliability of these sampling results. Commonly reported negative removal efficiencies for xenobiotics might therefore be a consequence of biased sampling schemes. In this regard, the present study aims at contributing to bridge the gap between environmental chemistry and engineering practices.
混合状态和停留时间分布（RTD）对污水处理厂（WWTP）中溶质传输的影响在环境工程中得到了很好的理解。然而，在 WWTP 中调查极性外来化合物去除效率的采样设计和数据分析中，经常忽略这一点。大多数关于后者的研究都使用进水和出水的 24 小时混合样品。由于假设这可以完全覆盖进水负荷，因此通常会根据平均水力停留时间来调整出水采样时间。然而，这种假设忽略了混合状态特征以及水流和浓度的变化，从而可能导致有偏差的估计甚至出现负的去除效率。本研究旨在开发一种建模方法，从监测数据中估计外来化合物的去除效率，同时考虑 WWTP 中的水力 RTD。为此，在卢森堡的一个 WWTP 中应用完全混合的串联罐来解决水力混合状态问题。该 WWTP 的水力校准是使用废水电导率作为示踪剂进行的。RTD 混合方法与好氧处理中涵盖三种可生物降解性的类别的外来化合物的一级生物降解动力学相结合。模型模拟表明，每日进水负荷在出水中分布在一天以上。在旱季情况下，24 小时采样期与进水和出水之间的最佳时间偏移仅覆盖不到一半的进水负荷。根据 RTD 计算，为了以足够的精度估计全规模的去除效率，需要在进水口连续测量四天的优化采样策略。进水流量和浓度的日变化会极大地影响这些采样结果的可靠性。因此，通常报道的外来化合物的负去除效率可能是采样方案有偏差的结果。在这方面，本研究旨在为弥合环境化学和工程实践之间的差距做出贡献。
