不同预处理方式防止超滤膜污染特性及其机理研究

发布时间：2018-08-11 15:23

【摘要】：超滤膜在地表水处理中已得到越来越多的应用，对出水水质的保障得到一致认同。目前，在净水过程中，膜自身会不可避免地受到污染。一方面，膜压差增加，提高了制水成本；另一方面，不可逆污染严重，可能导致更换膜组件。如何在保证水质的前提下降低膜污染，是水处理领域研究的重点和难点。预处理能有效防止膜污染，本研究选取了具代表性、可操作性的预处理方式——混凝、粉末活性炭吸附以及混凝活性炭吸附，考察了采用不同预处理方式及不同投加量，分别与超滤膜联用时的处理效果，重点研究了预处理方式防止膜污染的特性及其机理。论文所得结论如下：（1）采用超滤膜直接过滤不同浊度的原水，在原水浊度的试验范围内，研究发现对有机物的去除主要集中在非溶解性有机物上，对溶解性有机物的去除较少。与错流相比，死端流方式直接处理不同浊度的原水，通量能改善明显。过滤第八个周期时，错流方式的通量能恢复到68.2%，而死端流通量却能恢复到91.5%。同时，直接过滤浊度较高与浊度较低原水相比较，前者过滤初期，通过反冲洗，通量能得到完全的恢复；后者尽管初期反冲洗后的通量能得到恢复，但随着过滤的进行，呈明显下降的趋势。其原因，可能是死端流方式下，且较高浊度，可形成较厚且松散的滤饼层，从而吸附拦截了部分易污染的物质，特别是胶体和细小悬浮物。直接过滤原水的0.45μm微滤膜滤后液，发现小于1kDa的有机物易被膜孔吸附。虽然直接过滤较高浊度原水，能在膜表面形成有效的松散滤饼层，但经过长期运行，胶体、非溶解性有机物残留在膜表面,同时小于1k Da的有机物也随着过滤进行，长期积累在膜孔，水力冲洗难以恢复，是造成膜污染的主要物质。（2）采用粉末活性炭预处理方式，试验发现，溶解性有机物的去除率随粉末活性炭投加量增大而提高。粉末活性炭吸附的DOC主要集中在小于10kDa的分子量区间，而被吸附的UV254主要集中在30kDa~1kDa的分子量区间，，其中10kDa~1kDa区间的UV254去除最多。粉末活性炭与超滤联用处理水样时，原水浊度较低时，粉末活性炭预处理对初始通量基本没有提高，但膜通量有明显改善。反之，原水浊度较高时，能提高初始通量，但不能有效改善膜通量。同时，粉末活性炭投加量相同情况下，含非溶解性物质较少的进膜水对超滤膜表面的亲水改性更加明显。进一步说明，胶体是造成膜污染的主要物质。（3）采用混凝预处理方式，试验发现，混凝去除的DOC主要集中在大于30kDa和小于1kDa的分子量区间，同时投加粉末活性炭与混凝剂（先投加粉末活性炭）的工况下，此时混凝对DOC在大于30kDa、10kDa~1kDa和小于1kDa的分子量区间的去除量基本相同，与单独投加混凝剂的情况相似，混凝后超滤主要增加了分子量小于1kDa的DOC去除率；在30kDa~10kDa、10kDa~1kDa和小于1kDa的分子量区间UV254的去除率基本相同。混凝与膜联用，可有效减缓膜通量的下降趋势。同时，混凝预处理对膜通量的影响与原水浊度之间存在一定的相关性，原水浊度较小时，混凝预处理能更加有效的减缓膜通量的衰减。而且混凝液对膜比通量的改善明显优于上清液。单独投加Fecl3混凝剂的工况下，混凝预处理提高了膜通量的恢复程度，但膜通量的下降趋势并未的得到明显的改善；粉末活性炭与混凝剂投加时，两者的投加顺序对膜通量也有影响，先投加粉末活性炭的工况下，膜通量衰减更加缓慢。（4）对原水直接膜过滤、混凝-膜、粉末活性炭-膜、混凝粉末活性炭-膜这几种工况比较，去除非溶解性物质的干扰后，直接过滤原水0.45μm膜滤后液时的膜比通量恢复最好，对膜的亲水改性现象最为明显，其次依次是单独投加混凝剂工况、粉末活性炭吸附工况。得出，非溶解性有机物较小分子有机物，更易导致膜污染。综上所述，考虑出水水质的前提下，黄河上游兰州段原水，采用“预沉—粉末活性炭吸附—混凝”预处理工艺能有效防止超滤膜污染。
[Abstract]:Ultrafiltration membranes have been used more and more in surface water treatment, and the assurance of effluent quality has been unanimously recognized. At present, the membrane itself will inevitably be polluted in the process of water purification. On the one hand, the increase of membrane pressure difference increases the cost of water production; on the other hand, irreversible pollution may lead to the replacement of membrane components. Pretreatment can effectively prevent membrane fouling. In this study, coagulation, adsorption of powdered activated carbon and adsorption of coagulated activated carbon were selected as representative and operable pretreatment methods. Different pretreatment methods and dosages were investigated. The characteristics and mechanism of pretreatment to prevent membrane fouling were studied. The conclusions are as follows:
(1) Ultrafiltration membrane was used to directly filter raw water with different turbidity. Within the experimental range of raw water turbidity, it was found that the removal of organic matter was mainly concentrated on non-dissolved organic matter, and the removal of dissolved organic matter was less. At the same time, compared with the raw water with lower turbidity, the flux of the former can be completely recovered by backwashing at the initial stage of filtration, while the flux of the latter can be recovered after backwashing at the initial stage, but as the filtration proceeds, the flux of the former can be completely recovered. The reason may be due to the formation of a thicker and looser cake layer under dead-end flow and higher turbidity, which can absorb and intercept some easily polluted substances, especially colloids and small suspended substances. When raw water with higher turbidity is filtered, an effective loose cake layer can be formed on the membrane surface. However, after long-term operation, colloids and non-soluble organic matter remain on the membrane surface. At the same time, organic matter less than 1 K Da is also filtered. It accumulates in the membrane pores for a long time and is difficult to be recovered by hydraulic flushing.
(2) Pretreatment with powdered activated carbon (PAC) showed that the removal rate of dissolved organic compounds increased with the increase of PAC dosage. The DOC adsorbed by PAC mainly concentrated in the molecular weight range of less than 10 kDa, while the absorbed UV254 mainly concentrated in the molecular weight range of 30 kDa~1 kDa, and the removal rate of UV254 in the range of 10 kDa~1 kDa. When the turbidity of raw water is low, the initial flux of PAC pretreatment is not increased, but the membrane flux is obviously improved. On the contrary, when the turbidity of raw water is high, the initial flux can be increased, but the membrane flux can not be effectively improved. The hydrophilic modification of the membrane surface was more obvious in the water with less soluble substances, which further indicated that colloid was the main substance causing membrane fouling.
(3) By coagulation pretreatment, it was found that the DOC removal by coagulation was mainly concentrated in the molecular weight range of more than 30 kDa and less than 1 kDa, and the removal of DOC by coagulation was basically the same in the range of more than 30 kDa, 10 kDa~1 kDa and less than 1 kDa when powdered activated carbon and coagulant were added. Similar to coagulant alone, after coagulation, the removal rate of DOC with molecular weight less than 1 kDa was increased, and the removal rate of UV254 with molecular weight less than 1 kDa, 10 kDa and 10 kDa was almost the same. Coagulation combined with membrane could effectively slow down the decline of membrane flux. There is a certain correlation between turbidity of raw water, coagulation pretreatment can more effectively slow down the decline of membrane flux when the turbidity of raw water is small, and the improvement of membrane specific flux by coagulant is obviously better than supernatant. The membrane flux was also affected by the order of adding PAC and coagulant. When PAC was added first, the membrane flux decayed more slowly.
(4) Compared with the direct membrane filtration of raw water, coagulation-membrane, powdered activated carbon-membrane and coagulated powder activated carbon-membrane, the membrane specific flux recovered best when the non-soluble substances were removed, and the hydrophilic modification of membrane was the most obvious, followed by the coagulant alone. The results show that the membrane fouling is more likely to occur when the non-soluble organic matter is smaller than the molecular organic matter.
In conclusion, considering the effluent quality, the pretreatment process of "pre-sedimentation-powder activated carbon adsorption-coagulation" can effectively prevent ultrafiltration membrane fouling in Lanzhou section of the upper Yellow River.
【学位授予单位】：兰州交通大学
【学位级别】：硕士
【学位授予年份】：2013
【分类号】：TU991.2

【参考文献】