Research On Organic Fouling Mechanism Of Reverse Osmosis Membrane And Its Penetration Backwash Control

Membrane fouling is one of the most serious obstacle to restrict its widespread application. One of the critical issues is organic fouling, which not only reduces the permeate flux of a membrane and requires more frequent cleaning, but also shortens its life .Fouling of RO membrane by polysaccharides(alginate) was systematically investigated. Commercial antiscalant and backwash were adopted to alleviate the organic fouling of RO membrane. In addition, Fenton method was used to oxidize alginate so as to alleviate the fouling of RO membrane.Pilot study demonstrated that alginate could form compact gel layer on membrane surface, which leads to a severe flux decline. Especially, permeate flux decreased dramatically in the presence of calcium ions. This behavior can be attributed to calcium bridges between adjacent organic matter and the formation of organic macromolecule complexation. In addition, the effects of chemical composition (foulant concentration, ionic strength and solution pH) and temperature on the membrane fouling mechanisms were investigated. Combined fouling by both alginate and humic acid were studied, in the low salinity conditions, with the increase of sodium alginate, flux decline increased; in seawater conditions, with the increase of humic acid, flux decline increased.Antiscalant and the way of backwash are mainly used in the field of inorganic scaling control of RO technology. We innovatively use the two methods to decrease the organic fouling, and it turns out to be effective. We define the Inhibition Efficiency to evaluate how different parameters affect the fouling, and then speculate the mechanism of how the two methods alleviate the organic fouling.The introduction of antiscalant to feed water was found very effective for relieving organic-fouled RO membrane in the presence of calcium ions. Antiscalant relieves organic fouling by the following three ways. Firstly, it is proposed that Ca²⁺ can be stably dispersed in solution by chelation between Ca²⁺ and antiscalant , which could decrease the quantity of macromolecule complexation. Secondly, antiscalant, organic foulant and calcium ions could form a kind of hydrophilic compound, which could be removed by the shear easily. Thirdly, antiscalant is also a kind of very excellent disperser to organic matters.The way of backwash was also found effective for lessening organic-fouled RO membrane in the presence of calcium ions. This is mainly because there can be a powerful driving force to lift and sweept foulants on the membrane surface . The PASP-Ca-SA molecules formed still have relatively high hydrophilia, and hence can be removed by the shear easily. Moreover, while the antiscalant and the way of backwash are combined, the organic fouling is inhibited efficiently. The presence of PASP-Ca-SA complex molecules leads to a thinner foulant layer, and the following backwash process can disperse the foulant in the layer. Then most of the foulant is removed by the shear.In addition, Fenton method was used to oxidize alginate. The experiment result showed that the sodium alginate may be decomposed into small molecules, which could not have complex reaction with Ca²⁺.