Thermodynamics of synthesis and separation performance of Interfacially polymerized “loose” reverse osmosis membrane: Benchmarking for greywater treatment

Abstract

• Interfacially polymerized hollow fiber “Loose Reverse Osmosis” LRO membranes. • LRO capable of treating greywater and removal of TDS. • Energetic performance of LRO close to thermodynamic limits of separation. • LRO potential to be positioned as a new class of membranes for low TDS streams. Greywater recycling is a promising alternative to purify a relatively clean stream at a lower energy penalty. In the present work, greywater treatment through “loose” thin-film composite (TFC) reverse osmosis (RO) membranes have been attempted. Polymer interactions between cellulose acetate (CA) and additives like polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) have been studied along with their miscibilities in n-methyl pyrrolidone (NMP) solvent. The compatibility was understood through dilute solution viscometry measurements, and ternary phase diagrams, the miscibilities/immiscibilities of the CA-PVP, CA-PEG blends in NMP was explored. It was found that CA-PVP yielded less stable solutions than CA-PEG. This was reflected in their membrane morphologies, permeability, and porosities. Thus, two membranes were chosen, one of the highest permeability and one of the lowest permeability and underwent interfacial polymerization to yield “Loose” TFC – RO membranes. These were subjected to synthetic TDS (Total Dissolved Solids) levels (500–1000 ppm) and real-life greywater streams. The energetics of separation at bench scale against the theoretical thermodynamic analysis of free energy was calculated. For synthetic streams, it was observed that experimental energy of separation was found closer to the theoretical minimum. Thus, it was concluded that with proper pre-treatment strategies, real-life greywater could be treated in an energy-efficient way.