Enhancing sustainability: Upcycled membrane distillation for water and nutrient recovery from anaerobic membrane bioreactor effluent

Abstract

Addressing the critical issue of recycling end-of-life membranes, this study focuses on the meticulous process of removing contaminants and rejuvenating deteriorated polyvinylidene difluoride (PVDF) membranes for secondary applications. We unveil a successful upcycling approach wherein these membranes are effectively cleaned and recoated, transforming them for utilization in the membrane distillation (MD) process. Citric acid proved to be the most effective cleaning agent, efficiently removing foulants and restoring the membranes’ original color, surface composition, and hydrophobicity. Surface analyses confirmed the removal of metallic foulants, with significant improvements in pore structure and contact angle, indicating effective property restoration. Further analyses revealed that coating PVDF membranes with 5 % carnauba wax and 0.1 % polysulfone resulted in the most hydrophobic surface, characterized by increased prominence of carnauba wax peaks, decreased PVDF peaks, and reduced O 1s peak, enhancing hydrophobicity. Optimal hydrophobicity suitable for MD was achieved with minimal polysulfone addition, producing near-superhydrophobic characteristics with a water contact angle of ∼150° and peak surface roughness of 287 nm. Higher polysulfone content reduced hydrophobicity and surface roughness. The upcycled membrane significantly decreased the crossover of ionic species to the permeate with minimal reduction in water flux, demonstrating superior retention of nutrients as eco-friendly fertilizers from anaerobic digestate. Additionally, acidifying the digestate to pH 5 minimizes NH3 emissions, and MD treatment at 70 °C inactivates pathogens, making the process suitable for water and nutrient recovery, thereby supporting zero liquid discharge and the transition to a circular economy.