Reverse osmosis and nanofiltration opportunities to concentrate multicomponent mixtures of volatile fatty acids

The application of reverse osmosis (RO) and nanofiltration (NF) to concentrate and/or fractionate volatile fatty acids (VFAs, occurring in acidogenic effluents) was systematically assessed for the first time using three commercial spiral wound modules. A bench scale plant was used to test the polyamide membrane cartridges, namely: AG1812-34D, DK1812-34D and NFS-3B-1812F. The effects of main operational parameters (pH, temperature, applied pressure) were firstly studied in total recirculation mode, using different total and relative VFA compositions. Thereafter, batch concentration tests were carried out (at constant applied pressure, ca. 30 bar) to fully characterize the RO and NF modules. AG1812-34D performed as an RO module and it allowed concentrating target solutions almost 3 times without relevant VFA losses (<4%) in the permeate side. All tested parameters were observed to affect concentration performances. However, typical pH levels occurring in acidogenic effluents allowed to achieve fully satisfactory VFA rejections. DK1812-34D and NFS-3B-1812F performed as NF modules and could represent suitable solutions for VFA fractionation. Separation factors significantly increased by pH rise; in particular, high acetic and propionic acid separation factors over butyric acid were achieved at pH 9, with DK1812-34D module.