煤化工废水反渗透处理系统的运行效果及膜污染分析

Study of the operating efficiency and membrane fouling of reverse osmosis process in coal chemical wastewater treatment

针对煤化工废水反渗透(RO)膜处理系统夏季严重污堵的问题,本研究以实际某煤化工污水处理厂一级两段式RO系统为考察对象,分析了系统运行效能及膜污染特征.研究发现,生化段稳定地实现了有机物的削减,离子交换树脂进一步保障了产水的脱盐率,促进实现高水回收率,是煤化工废水零排放的重要保障.但对膜系统而言,脱盐及有机物去除的主要负荷集中在RO过程,一段/二段RO脱盐率分别为94.16%和96.16%,COD去除率分别为68.12%和87.4%;相对进水,一段RO有机物浓缩了9倍,二段RO盐浓缩了5倍.因此,两段膜过程都出现显著的膜污染,一段RO以有机-微生物-硅污染为主,形成致密的污染层,由进口到出口逐渐增厚,主要为蛋白质、多糖、腐殖酸;二段RO以Ca、Mg等的无机结垢为主,污染层结构相对松散,由进口到出口片状结晶逐渐增大.因此,预处理工艺的稳定运行及对污染物的去除以减轻RO过程的污染负荷是膜污染控制的关键;同时,针对RO系统中膜污染分布特征,制定杀菌、阻垢和化学清洗等膜污染控制策略以防止形成微生物抗性及"清洗剂抗性"具有重要意义.

Membrane fouling is a key issue of reverse osmosis(RO) in treating coal chemical industry wastewater. Therefore, a full-scale two-stage RO process was systematically analyzed in terms of operation efficiency and the membrane fouling in a coal chemical wastewater treatment plant in China. It was found that the A/O process had stable pretreatment performance for organic matters(OMs) removal;the ion exchange processes guaranteed the removal of ions, which promoted the higher water reuse rate. They ware key processes for zero liquid discharge(ZLD) of coal chemical wastewater. However, most of the OMs and salt removal load were happened in RO in membrane process, which offer salt rejection and COD removal rate of 94.16%, 96.16% and 68.12%, 87.4% in the first-stage RO and second-stage RO, respectively. Meanwhile, compared to the feedwater, the OMs were 9 times concentrated in the first-stage RO, whereas the salts were 5 times concentrated in the second-stage RO, respectively. Therefore, membrane fouling appeared in both two stage membrane processes. Organic fouling, biofouling, combined with Si fouling were the main mechanism for the first-stage RO with a formation of dense foulants layer, which became gradually thicker from inlet to outlet. The main organic components were identified as protein, polysaccharide and humic acid. However, mineral scaling was the main mechanism for the second-stage RO process with a main component of Ca and Mg. The foulants layer was relatively loose, and crystal size increased along with the flow direction. Therefore, the selection of pretreatment process and its stable performance was essential to alleviate the pollutants load of RO. And it is recommended to improve the sterilization, scale inhibition and chemical cleaning strategies in consider of the distribution of fouling in RO to avoid the appearance of microbial resistance and cleaning tolerance.