An alternative membrane treatment process to produce low-salt and high-nutrient recycled water suitable for irrigation purposes

The purpose of this work is to assess the flexibility and effectiveness of using nanofiltration (NF) and reverse osmosis (RO) to produce recycled water that is suitable for irrigation purposes from treated water with high salinity. An intermediate NF stage was added to the conventional microfiltration (MF)–RO process to extract the useful nutrient ions from the stream rather than allowing them to be rejected during the RO stage. A secondary effluent from Altona wastewater treatment plant (Victoria, Australia) was used as a feed to the proposed integrated membrane. The rejection of ions by the NF element was examined using two commercial NF membranes (KOCH and NF270) with and without polyacrylic acid (PAA) as chelating agent. The addition of PAA increased the divalent ion rejection by the KOCH membrane, but it did not significantly affect the rejection of ions by the NF270 element. The extent of the nutrient enrichment with KOCH membranes was found to increase with the addition of PAA (from 31% to 68%), but with NF270 membranes, a smaller increase was observed (from 24% to 39%). The application of PAA didn't provide significant improvements in the rejections especially when considering the further treatment requirement as a result of the huge amount of PAA that will end up in the product water. In addition, the KOCH membrane was capable of achieving the required rejections with less energy demand than the NF270 membrane. The experimental results for the NF and RO element rejections obtained from this study will assist in modelling the proposed NF–RO membrane configuration and calculating the final water composition using different numbers of NF and RO elements.

Research highlights
► NF–RO treatment of wastewater is a feasible alternative to RO-only treatment.
► NF–RO treatment can produce recycled water rich in nutrients ions.
► The produced recycled water is suitable for irrigation purposes.
► NF membrane can recover all phosphorous present in the wastewater.
► Divalent ions rejections can be increased by the addition of polyacrylic acid.