Molecular Design of Nanofiltration Membranes for the Recovery of Phosphorus from Urban Wastewater

With the rapid depletion of mineral phosphorus, the recovery of phosphorus from sewage sludge will become increasingly important in the near future. However, the presence of various contaminants such as heavy metals in the sewage sludge only serves to complicate the issue, and the challenge lies in the separation of phosphorus from the heavy metals to produce fertilizers of high quality. Among various available methods, nanofiltration (NF) has been demonstrated to be a feasible and promising option when the sewage sludge undergoes acidic dissolution (pH <2). Since the performance of commercially available thin film composite (TFC) NF membranes reported thus far has great rooms for improvement, the development of highly permeable positively charged NF membranes are recommended. To this aim, a NF membrane which is desirable for phosphorus recovery was fabricated via interfacial polymerization of polyethyleneimine (PEI) and trimesoyl chloride (TMC) on a porous polyethersulfone (PES) membrane substrate. Through an optimization of the interfacial polymerization process which involves varying the molecular weight of PEI and the concentration of TMC, the resultant membrane displays a low molecular weight cut-off (MWCO) of 170 Da with a reasonably high pure water permeability (A) of 6.4 LMH/bar. The newly developed NF membrane can effectively reject a wide variety of heavy metal ions such as Cu, Zn, Pb and Ni (>93%) while demonstrating a low phosphorus rejection of 19.6% at 10 bar using a feed solution of pH 2. Thus, up to 92% of the feed phosphorus may be recovered using this newly developed NF membrane at a permeate recovery of 90%. This is a highly competitive value for the recovery of phosphorus.