| In the current ecological environment with widespread micro-polluted water sources, the upgrading of the traditional water purification processes through advanced deep treatment technology becomes an important issue for drinking water safety protection. In recent decade, the nanofiltration (NF) technology for drinking water treatment has been developed rapidly. With this new process, not only various organic compounds can be eliminated effectively, but also the inorganic ions in water can be removed moderately, and the pH value of product water can be kept neutral, thus guarantees the drinking water safety under a wider range of water source conditions.In this paper, optimization design of membrane system,process combination and engineering demonstration of NF technology for drinking water treatment are the core research contents. Based on the analysis of the existing commercial NF membranes, the optimization design mathematical model of NF membrane water treatment application was proposed for the first time, via analogizing and referencing with the separation performance and the system design of ultrafiltration (UF) and reverse osmosis (RO) membranes. By the combination of engineering design, experimental test, theoretical model, and computer software simulation, the major constraint indexes and optimized arrangements of the drinking water NF membrane system design were proposed, and a multiple design arrangement model associated with the characteristics of nanofiltration membranes was put forward finally.Meanwhile, nanofiltration membrane purification process design and application research have been carried out based on the characteristics of the water resources of Qiantang River. As a result, integrated three-membranes purification technology with high recovery, consisted of UF, NF and RO membrane systems, has been developed firstly for the Qiantang River tidal water. Through long-term demonstration of the engineering system, a wealth of comprehensive theoretical and practical basis and experience has been gained for meeting the requirements of similar water deep purification.The following progresses were made:1. From mathematical programming point of view, the mathematical model for nanofiltration membrane system of drinking water treatment optimization design was proposed. Combined with engineering design characteristics of nanofiltration membrane systems, the model was further analyzed and simplified to solvable classic mode. Finally various constraint indexes of optimization model targeting at required retention rate and highest recovery were established, and the variable result of optimized short process membrane arrangement was concluded, also the differences of optimized arrangement patterns under different system constraints were stated. These studies provided a theoretical basis for large-scale application and engineering design of nanofiltration technology.2. Through the combination of experimental test and theoretical derivation, the salt concentration polarization degree across the NF membranes surface was measured for the first time. Consequently, a quantitative limit of 1.2 was given clearly to meet the optimized design goals. Based on the association mode thus founded of the concentration polarization index, the minimum flow rate across the membrane surface, the rejection performance and the recovery of membrane systems, a calculation method of maximum recovery rate for nanofiltration system was established, providing a theoretical basis for membrane fouling prevention.3. Based on the solution-diffusion separation mechanism, an optimized arrangement pattern for short process was proposed to meet the constraint index of the NF membrane system. Accordingly, compared with the RO system, shorter process is needed for large-scale nanofiltration system; when the target recovery rate is more than 80%, a two-stage form can be considered rather than the three-stage pattern of reverse osmosis membranes, thus reduces the complexity of the system design and the pressure loss caused by the length of the process.4. A pilot study on the treatment characteristics and optimization of the nanofiltration membrane for the water source of the Qiantang River was carried out. Through long-term follow-up comparison of product water indexes from two kinds of pre-treatment processes, ultrafiltration and nanofiltration were selected as the major purification techniques. According to the experimental test results of nanofiltration membranes system with ultrafiltration pre-treatment, the critical filtration flux of nanofiltration membranes for Qiantang River water was obtained. The rejection performance of inorganic salts, organic compounds and the pH change rules were also studied, and the results showed that nanofiltration membranes with appropriate rejection performance of salts for different water sources and requirements should be chosen in the drinking water purification technology.5. The study of characteristics of tidal water source in Qiantang River and nanofiltration membranes demonstration project was carried out. Based on follow-up analysis of the Qiantang River water quality from different hydrologic station sample points, combined with previous research achievements, a set of 500 m3/d demonstration plant with UF, NF and RO integrated technique was established finally, and the water recovery rate of the whole device was above 90%. After more than two years of operation, the reliability of the three-membranes integrated process with nanofiltration as the core technique was examined and demonstrated to meet the drinking water safety requirements from a number of perspectives like operation, water quality safeguard and economic efficiency. |
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