Permeate quality assessment and concentrate recovery in wastewater treatment by membrane processes and reuse for irrigation purposes

The reuse of treated wastewater in irrigation is fundamental in the management of water resources. The aims of this study were the performance assessment of NF/RO processes for reclamation of MBR-treated wastewater, the concentrate recovery and permeate quality assessment for irrigation. Wastewater processed by MBRs in “ITOB” wastewater treatment plant, located in Izmir, Turkey, was used in NF/RO filtration tests in a pilot-scale plant containing BW30 (RO), NF90 (NF) and NF270 (NF) membranes. Experiments were conducted using single membrane modules (in batch mode and concentrate recirculation flow configuration) and with two membranes connected in series. MBR effluent and NF/RO permeate quality were compared with the standard values found in guidelines (FAO, WHO and USEPA) and regulations (Turkish and Italian regulations). Permeate flux was higher for NF270 membrane while the lowest was for BW30 membrane. Salt rejections of BW30 membrane were significantly higher than those obtained with NF270 membrane, while with NF90 membrane, permeate fluxes were slightly lower than those of NF270 membrane with salt rejections close to those of an RO membrane. With concentrate recirculation flow configuration, the concentrate recovery was increased by increasing the concentrate recirculation. However, the operation has to be stopped when the feed solution concentration can compromise the performances of the system and shorten the membrane life. In the system with two membranes in series, high water recovery was achieved. BW30-NF270 system showed higher permeate flux, however, its quality is comparable to this one of NF270 membranes. NF90 and BW30 permeates meet the standards for irrigation, and negative effects on soil permeability can be minimized by mixing them with low amounts of MBR effluent. NF270 permeates contain excessive amount of nitrogen, and NaCl. MBR effluent is not suitable for irrigation, due to excessive salinity, NaCl, TDS and nitrogen content.