Polyelectrolyte multilayer membranes for advanced wastewater treatment

Global water scarcity is expected to increase due to climate change, population growth, and urbanization. Wastewater treatment can help alleviate this problem by enabling water reuse. However, traditional methods, such as conventional activated sludge (CAS), fail to effectively remove organic micropollutants (OMPs). These compounds, present in surface waters at low concentrations, pose significant environmental risks. Nanofiltration (NF) membranes, particularly hollow fiber (HF) membranes, are identified as a promising technology for removing OMPs. With numerous pilot- and full-scale HF NF plants already in operation, these membranes have proven their commercial viability. Here, polyelectrolyte multilayer (PEM) membranes, fabricated using the layer-by-layer (LbL) method, stand out for their chemical stability and offer a good alternative to other NF membrane designs, especially in high-fouling environments.

To enhance OMP removal, the Membrane Concentrate Recirculation to Activated Sludge (MCRAS) process is proposed, combining CAS treatment with NF membranes. This approach is particularly beneficial for OMPs with low biodegradability and high membrane retention. However, salt retention remains a challenge, as it can lead to undesired scaling, making the selection of membrane type a balance between high OMP retention and low salt retention.

Different strategies to lower salt retention in PEM membranes via membrane modification are explored. This includes using diblock copolymers (DBCs) to limit surface charge, which lowers Donnan exclusion without affecting other membrane characteristics. Asymmetric PEM membranes are also explored, offering improved selectivity and permeability. The OMP retention of PEM membranes can be increased by increasing the extent of hydrophobic interactions in the multilayer, which leads to a denser membrane. Overall, PEM membranes hold a great promise for wastewater treatment, contributing to addressing global water scarcity by advancing methods to optimize OMP removal.