| Summary: | 碩士 === 朝陽科技大學 === 環境工程與管理系 === 107 === NF technology is currently implemented in various fields such as wastewater reuse, softening of hard water, desalination, concentration of pharmaceuticals, and purification of drinking water. Due to the ability of NF membranes to effectively remove divalent and multivalent cations, they are often applied as an advanced treatment technology in the tertiary treatment process of wastewater plants.
Concentration polarization (CP) occurs when pollutants over-accumulate on the surface of NF membranes after a period of time, which results in declined flux, decreased permeate quality, and increased transmembrane pressure (TMP). Pollutants differ according to the characteristics of different wastewater streams, and are often characterized as inorganic scalants and organic foulants. Due to cost considerations of membranes, physical or chemical cleaning of membranes is required. If appropriate cleaning procedures were not taken, membranes shall suffer from damage and their lifespan will be shortened.
In this research, simulated inorganic and organic wastewater were used, respectively, to induce scaling and fouling of NF membranes, while various ultrasound-assisted cleaning combinations were performed. The Taguchi method was adopted to determine the optimum cleaning combination. This research aimed to investigate the effects of cleaning on the membranes’ surfaces, so as to optimize the cleaning mechanism of membranes. The quality indicators used were flux recovery or flux increase; while factors influencing cleaning conditions were concentrations of cleaning chemicals, ultrasound cleaning durations, and ultrasound cleaning frequencies.
Results showed that permeate flux declined with time in the inorganic scaling and organic fouling experiments. Organic fouling was quicker to induce membrane blockage compared to inorganic scaling. For the cleaning of scaled membranes, the optimum combination was determined to be 15 minutes of ultrasound-assisted cleaning at 20 kHz with 0.5 wt % of citric acid solution, which achieved a flux recovery rate of 16.06%; for the cleaning of fouled membranes, the optimum combination was determined to be 15 minutes of ultrasound-assisted cleaning at 20 kHz with 0.5 wt % of sodium hydroxide solution, which achieved a flux recovery rate of 16.48%.
Regarding the results of surface analyses, SEM and EDS results showed that repeated cleaning of inorganically scaled and organically fouled membranes had effectively removed the surface precipitants. FTIR results showed that ultrasound-assisted acidic cleaning of inorganic scalants and alkaline cleaning of organic foulants did not altered the chemical bondings of the NSK98 membrane. In fact, inorganic scaling had increased the membrane’s hydrophilicity, while cleaning had removed most of the accumulated scalants, which resulted in a slightly increased contact angle. AFM results also showed that while inorganic scalants had accumulated on the scaled membrane’s surface, which decreased its overall surface roughness, the cleaning procedure was able to increase its surface roughness as it removed most of the scalants.
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