Direct recycling of discarded reverse osmosis membranes for domestic wastewater treatment with a focus on water reuse

The recycling of discarded membranes (end-of-life) represents a relevant alternative for sustainability of reverse osmosis (RO) desalination plants in the context of circular economy. This work evaluated the feasibility of using discarded commercial RO membranes in the treatment of domestic secondary wastewater to obtain water with a certain standard quality. Crossflow filtration tests were conducted to evaluate desalination and wastewater filtration performance at different operating pressures on RO membranes discarded from desalination plans at different working positions (primary M1; secondary M2). The standard manufacturer desalination tests showed a superior performance on M1 membranes, in terms of rejection (∼25 LMH, 97%), compared to M2 (∼33 LMH, 50%); both having a lower performance than a standard membrane (38 LMH± 15%; 99.6%). The failure is sufficient for discarding due to loss of lifespan. Moreover, in wastewater filtration tests using the secondary clarifier outlet effluent from a WWTP at different working pressures, both types of membranes were shown to be effective, with degrees of performance highly dependent on the working pressure. Thus, the operating values of permeate flux/salt rejection were between 56 and 59 LMH/ 96–97% for 600-psi: 33–34 LMH/ 94–96% for 300-psi and in the range of 10–11 LMH/ 90–94% for 80-psi test. Surface characterization of the membrane showed a pressure-related increase in fouling and bacterial adhesion post-filtration. Finally, the operating performance was verified in M1 wastewater filtration at 300 psi over long times (14 h), yielding stable and promising values (∼27 LMH; 96%). The permeate obtained has a low concentration of fecal coliforms (<2 MPN/100 mL, 99.99% removal) and meets local standards for irrigation and drinking water in terms of conductivity, phosphorus and nitrogen concentration in treated water.