| Brackish water desalination, one of the effective means to obtain fresh water in the reign of northwest of China, eases the worries about the water resource crisis and becomes a strategic choice to solve this problem. The technology of brackish water desalination includes distillation, electrodialysis (ED), and reverse osmosis (RO), and each technique plays a special function for different water quality and different areas. While nanofiltration (NF), as an emerging technology matured in recent decades, has already been widely used in brackish water desalination for the preparation of safe drinking water. As most of the researches are operated with expensive imported NF membranes, the exploitation and application of domestic NF membranes opened up a new field for the production of safe drinking water. This will not only ease the crisis of water shortage, but save the cost of project. With the problem of water shortage being solved, the NF system itself has some defects. Unbefitting operating conditions will restrict the efficiency of the NF unit, then the permeate water will subsequently be affected. The direct drain of cleaning wastewater with many chemical reagents must cause a huge problem of environment. And the remained reagents will doubtless damage the performance of NF membranes and then shorten the service life.Based on the desalination of arsenic-rich brackish water with domestic NF membranes, whether this technique can be used for the questions above will be investigated in this article. The mainly research contents and followed results are as follows:1. The research are conducted with three different kinds of domestic NF membranes (NF-3A, NF-2A and NF-6) to desalinate the arsenic-rich brackish groundwater. The contrast experiments are conducted with a laboratory-scale cross-flow NF test unit and adequate operating conditions are adjusted to study the performance of the three kinds of NF membranes. From the results, it is easy to find that the performance of NF-3A membrane is the best one with NF-2A membrane subsequent, and NF-6 membrane has a relatively weaker desalination property. Many different kinds of operating conditions, such as temperature, applied pressure, pH of the feed water, concentrations of natural organic matters and inorganic salt have been studied to optimize and enhance the efficiency of the NF system.2. Membrane fouling is an unavoidable long-standing problem which has a strong impact on the efficiency of this process. The fouled membranes with different kinds of pollutant are cleaned by traditional chemical cleaning method. For inorganic contaminators in this study, the best cleaning agent is HCl (pH=2); while for organic ones, it is EDTANa2 (3.5 mg/L).3. Traditional chemical cleaning method has two vulnerabilities:the addition of chemical agents will damage the membrane to a certain degree, and the direct discharge of cleaning wastewater will pollute the environment. So on the basis of the above research, a new-style membrane and environment friendly technique has been studied. There is a strong driving force to lift and sweep the foulants from the membrane surface which will be carried over to the brine. At the initial stage of fouling, the main foulants are inorganic matters and they are easier to be cleared than organic matters during the osmosis backwash cleaning process. The optimal conditions in terms of HS (0.150 g/mL NaCl solution) flow rate of 0.121 L/min are selected for the subsequent experiments. With a result that the fouling can be almost fully reversible (more than 99.78% permeate water flux recovery), it needs to be cleaned for about 10 min because of the appearance of HA on membrane surface at the fouling time of 24 h. Then the performance of the membrane will restore to a great extent, the service-life of the membrane will subsequently be prolonged. |
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