School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore; Singapore Membrane Technology Centre, Nanyang Technological University, Singapore 639798, Singapore.
Water Res. 2014 Jan 1;48:387-95. doi: 10.1016/j.watres.2013.09.051. Epub 2013 Oct 11.
Pressure retarded osmosis (PRO) is an osmotically-driven membrane process that can be used to harvest salinity-gradient power. The PRO performance (both water flux and power density) can be severely limited by membrane fouling. The current study, for the first time, investigates PRO scaling in a bench-scale pressurized system using calcium sulfate dihydrate (gypsum) as a model scalant. In addition to the bulk feed solution (FS) saturation index (SI bulk), gypsum scaling was found to be strongly affected by the draw solution (DS) type and concentration, the applied hydraulic pressure, and the membrane orientation. The commonly recommended active layer facing draw solution (AL-DS) orientation was highly prone to internal scaling. In this orientation, severe internal concentration polarization (ICP) of scaling precursors induced gypsum clogging in membrane support layer even when the FS was undersaturated (e.g., SI bulk = 0.8). At higher SI bulk values, external gypsum crystal deposition occurred in addition to internal scaling. More severe scaling was observed when the DS contained scaling precursors such as Ca(2+) or SO4(2-), suggesting that the reverse diffusion of these precursors into the FS can significantly enhanced gypsum scaling. Increasing applied hydraulic pressure could enhance reverse solute diffusion and thus result in more severe gypsum scaling when the DS contained scaling precursors. A conceptual model, capturing the two important PRO scaling mechanisms (ICP of scaling precursors from FS and reverse diffusion of scaling precursors from the DS), is presented to rationalize the experimental results. Our results provide significant implications for PRO scaling control.
压力延迟渗透（PRO）是一种渗透压驱动的膜过程，可用于收获盐度梯度能。膜污染会严重限制 PRO 的性能（水通量和功率密度）。本研究首次使用二水硫酸钙（石膏）作为模型结垢物，在中试加压系统中研究 PRO 结垢。除了本体进料溶液（FS）饱和度指数（SI bulk）之外，发现石膏结垢强烈受汲取液（DS）类型和浓度、所施加的液压以及膜取向的影响。通常推荐的活性层朝向汲取液（AL-DS）取向极易发生内部结垢。在这种取向中，即使 FS 未过饱和（例如，SI bulk = 0.8），结垢前体的严重内部浓差极化（ICP）也会导致石膏在膜支撑层中堵塞。在较高的 SI bulk 值下，除了内部结垢外，还会发生外部石膏晶体沉积。当 DS 中含有结垢前体（如 Ca(2+) 或 SO4(2-)）时，观察到更严重的结垢，这表明这些前体向 FS 的反向扩散可显著增强石膏结垢。当 DS 中含有结垢前体时，增加施加的液压可以增强反向溶质扩散，从而导致更严重的石膏结垢。提出了一个概念模型，该模型捕捉了两种重要的 PRO 结垢机制（FS 中结垢前体的 ICP 和 DS 中结垢前体的反向扩散），以合理化实验结果。我们的结果对 PRO 结垢控制具有重要意义。
