Abstract
Global supply of commercial reverse osmosis (RO) membranes is growing exponentially due to rapid population growth, industrialization, and urbanization. The continuous demand for enormous quantity of drinking water has brought about process improvements and technological advancements in membrane preparation. The transformation of used RO membranes into nanofiltration (NF) and ultrafiltration membranes by opening up the pores using chemical treatment by inexpensive oxidizing agents could be one of the cost-effective options. The present study investigates the chemical oxidation of the indigenously synthesized RO membrane using aqueous sodium hypochlorite (NaOCl). The performance of the membrane was evaluated by conducting experiments under varying operating conditions of operating time, feed pressure, and total dissolved solids (TDS) in raw water for calculation of flux and salt rejection (%). From an initial flux of 25.2 L/m2 h and TDS rejection of 97.5% for original RO membrane, the values reached 80 L/m2 h and 25.5%, which is in NF range, after a reaction time of 780 min with 4000 ppm concentration of NaOCl oxidizing agent. Further extension of treatment time to 900 min enhanced the flux to 130 L/m2 h with salt rejection lowering to 5.67%. Membrane cleaning was performed efficiently using an advanced technique in which chlorine dioxide (ClO2) was used in combination with citric acid. This combination ensured rapid cleaning with restoration of water flux and % salt rejection. The process was scaled up to pilot plant level using RO membranes modified to NF range of pore size. Permeate water enriched with minerals was further packed using an indigenously designed semi-automatic bottling unit. The studies revealed that the indigenously developed RO membranes are easy to alter into high-performance NF membranes. Overall, the process for production of packaged drinking water was cost-effective, easy to operate, and environmentally friendly.
Highlights
Rapid urbanization and population growth in the globe have caused large-scale exploitation of ground and surface water
The present study investigates the economic strategy for easy alteration of tight reverse osmosis (RO) membrane into high-performance NF membranes for secondary application using NaOCl as a low-cost oxidizing agent
The intensity of the peaks of amides I and II was found to decrease from the original RO (Fig. 5a) to the chemically modified RO membrane (Fig. 5b), which indicates the degradation of the amide group after chlorination by NaOCl
Summary
Rapid urbanization and population growth in the globe have caused large-scale exploitation of ground and surface water. Most of the production of commercial membrane in the form of large flat sheets without substantial defects is extremely difficult, especially if additives such as graphene oxide are used and it needs tremendous efforts to bring these membranes to market at low cost. Mass transfer through the RO membrane occurs by solution-diffusion mechanism as shown, wherein the hydrostatic pressure causes the flow of preferentially interacting and smaller water molecules from the concentrated to the diluted side, while the solutes are retained by the membrane due to lack of any affinity. In the PA-TFC membrane, the non-porous PA layer acts as a catalyst to absorb the water molecules at the upstream side followed by diffusion through the thickness and desorption at the downstream side by potential chemical difference across the membrane. The persisting limitation of the present generation PA-TFC-based RO membranes is their limited resistance to chemical attack
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