This study examined the pre-oxidation of ammonium using a nanofiltration (NF) hollow fiber membrane module to provide an alternative to conventional partial nitrification preceding Anammox. A permeability study showed that NF membranes were suitable for use in the pre-oxidation of ammonium due to high ammonium fluxes across the membrane (low rejection of ammonium), while most COD (in the form of glucose) was retained inside the bulk phase