With the continued growth in global water treatment demand, reverse osmosis (RO) membranes play an irreplaceable role in seawater desalination and industrial wastewater treatment. However, as the service life increases, membrane fouling and chemical cleaning lead to performance decline, resulting in a large amount of waste RO membranes, posing significant challenges to the environment and resource recovery. It is estimated that by 2025, the global number of discarded 8-inch RO membrane elements will exceed 2 million. To enable their reuse, waste RO membranes can be treated using oxidative regeneration technologies, which can be classified into four main approaches.
1.Direct Oxidation
The waste RO membranes are directly immersed in an oxidizing solution, where the polyamide (PA) selective layer is degraded and converted into nanofiltration (NF) or ultrafiltration (UF) membranes.
Common oxidizing agents: sodium hypochlorite (NaClO), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂).
Key findings:
NaClO achieves efficient degradation through amide bond cleavage under alkaline conditions (pH>10), and the oxidation intensity (ppm·h) is positively correlated with treatment time.
KMnO₄ shows better oxidation performance under acidic conditions, but its reaction mechanism needs further investigation.
H₂O₂ relies on the Fenton reaction to generate free radicals (OH·) that attack the PA layer and requires iron ion catalysis.
2.Pretreatment + Oxidation
To address membrane surface pollutants (e.g., organics, colloids, inorganic scaling), acid-base cleaning or solvent pre-soaking is used to restore membrane performance prior to oxidation treatment.
Acid-base cleaning: Removes contaminants such as Al³⁺ and Si, preventing over-degradation during oxidation.
Pre-soaking: Soaking in ethanol or isopropanol helps restore membrane pores that collapsed due to drying, enhancing subsequent oxidation efficiency.
3. Synergistic Oxidation
Combining physical or chemical methods to enhance oxidation efficiency:
Ultrasound-assisted: 40 kHz ultrasound combined with KMnO₄ can reduce treatment time from 165 hours to 15 minutes, with a tenfold increase in flux.
Metal ion catalysis: Ca²⁺, Fe²⁺, etc., can accelerate the oxidation reaction of NaClO and lower the required oxidation intensity.
Photo-Fenton process: UV light combined with NaClO rapidly degrades the PA layer via free radical attack.
4. Oxidation + Post-treatment
After oxidation, performance can be further improved through modification or reconstruction of the selective layer:
Surface coating: Hydrophilic materials such as tannic acid (TA) and sericin are used to repair the PA layer, enhancing antifouling properties.
Interfacial polymerization: Reconstructing NF or UF selective layers on the degraded substrate to achieve performance upgrades.
Practical Applications
Drinking water treatment: Ultrafiltration membranes regenerated by NaClO oxidation have been applied to water supply in remote areas, with effluent meeting drinking water standards.
Landfill leachate treatment: Regenerated membranes have demonstrated stable flux and low-pressure operation during a 27-month pilot study.
Industrial wastewater reuse: Oxidatively regenerated NF membranes achieve a 96% dye rejection rate and significantly improve salt separation efficiency.
Although substantial progress has been made in practical applications, technical bottlenecks remain, such as complex processes and significant influence of initial pollution types on regenerated membrane performance.
The oxidative recycling of waste RO membranes not only enables water resource recycling but also offers new ideas for the green transition of the water treatment industry. With the continuous improvement of oxidation technology and the incorporation of smart solutions, it is helping to drive sustainable development even further.
