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A performance and energy evaluation of a dye drawn forward osmosis (FO) system for the textile industry
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Continuous growth in the world population has raised significant fears with regards to the sustainability of energy and water resources. Globally, water is an indispensable resource as it is essential for the sustenance of human, animal and plant life. Water is essential for all forms of life and plays a pivotal role in economic growth. The textile industry is one of the greatest consumers of water, it is, therefore, necessary to effectively treat the large amounts of wastewater before discharge to the environment. It is estimated that annually, more than 700,000-tonnes of textile wastewater is produced by the dyeing industry. Textile wastewater is generally characterised by electrolytes, suspended solids, mineral oils and multiple textile dyes, and has therefore been classified as one of the most polluting wastewaters. These dyes are toxic and, in most cases, are not biodegradable. The presence of very small amounts (i.e. < 1 ppm) of dyes in water has aesthetic impacts and is thus undesirable. It is, therefore, necessary to treat textile wastewater before discharging. Currently, membrane technology is widely used for wastewater treatment, as well as water purification. Forward osmosis (FO) is a promising technology for both these applications. FO is characterised by the flow of water through a semipermeable membrane from a feed solution (FS) characterised by the low solute concentration or low osmotic pressure (OP) to a draw solution (DS) characterised by the high solute concentration or high OP, due to the OP gradient across the membrane. The FO process eliminates the need for high hydraulic pressure, as required in traditional membrane technologies, and also has low fouling tendencies. Furthermore, FO has the advantage of lower energy requirements and membrane replacement costs. However, there are still many disadvantages such as reverse solute flux (RSF), membrane fouling, and concentration polarisation (CP) amongst others that still need to be addressed. Therefore, more research needs to be done in light of these limitations to better understand and mitigate these limitations to increase the effectiveness and efficiency of the FO process. This study aimed to evaluate a dye-driven FO system for the reclamation of water from textile wastewater and synthetic brackish water (BW5) by investigating the effects of membrane orientation, system flowrate, change in DS, and membrane fouling on the FO systems performance and energy consumption. The FS used was BW5 with sodium chloride (NaCl) content of 5 g/L whereas Reactive Black 5 (i.e. a reactive dye) and Maxilon Blue GRL (i.e. a basic dye) dyes were used as a DS, respectively. The membrane utilised was a cellulose triacetate (CTA) membrane and was tested in FO mode and pressure retarded osmosis (PRO) mode whilst the system flowrate was adjusted to 400, 500 and 600 mL/min, respectively. Experiments were performed using a bench-scale FO setup which comprised of an FO membrane cell, a double-head variable speed peristaltic pump, a digital scale, two reservoirs for the FS and DS, respectively, a digital multiparameter meter and a digital electrical multimeter to measure system energy consumption. Each experiment comprised of six steps: baseline 1 (membrane control), main experiment (dye-driven FO experiment), baseline 2 (membrane control repeat), membrane cleaning, membrane integrity (membrane damage dye identification) and membrane cleaning (preparation for next experiment). The baseline 1 and baseline 2 experiments operated for 3 h whilst each membrane cleaning procedure operated for 30 min. The main experiments operated for 5 h in the FO mode and 4 h in PRO mode whilst the membrane integrity experiments operated until a minimum of 10 mL water was recovered. Results showed that the PRO mode achieved both higher forward flux (𝐽𝑤) (i.e. 8.87, 8.71 and 9.13 L/m2.h for flowrates of 400, 500 and 600 ml/min) and water recovery (𝑅𝑒) rates compared to FO mode (i.e. 6.60, 6.88 and 7.58 L/m2.h for flowrates of 400, 500 and 600 ml/min). The variation of flowrates had little to no influence on the 𝐽𝑤, 𝐽𝑠 and 𝑅𝑒 of the system. The system consumed less energy in PRO mode (i.e. 381 kWh/m3 average consumption for all three flowrates) than FO mode (i.e. 417 kWh/m3 average consumption for all three flowrates). It was also observed that at a higher DS 𝑂𝑃, the system consumed less energy. Therefore, selecting an optimum initial 𝑂𝑃 is essential for a FO process to minimise the pumping energy. Furthermore, a change in DS from Reactive Black 5 dye to Maxilon Blue GRL dye had no significant impact on the system performance and energy consumption. In this study, no significant membrane fouling was observed, however, minute traces of fouling in the form of foreign functional groups could be observed in the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) spectrums of the used membranes. Additionally, the observation of negligible changes in baseline 2 (membrane control) Re and Jw results suggested the possible occurrence of membrane fouling during the main experiment (dye-driven FO system).