Using MF-NF-RO train to produce low salt and high nutrient value recycled water for agricultural irrigation

The project investigated and demonstrated that using a selective nanofiltration (NF) membrane and a reverse osmosis (RO) membrane in sequence can produce recycled water with useful levels of nutrient ions, and lower the sodium absorption ratio (SAR) to minimise soil salinity. The recovery of nutrient and useful ions from the wastewater makes NF-RO treated recycled water an attractive option for agricultural irrigation. It was found that the addition of polyacrylic acid (PAA) significantly increased the rejection of divalent ions by SR2 NF element, and kept the sodium rejection largely unchanged. This effect enhanced the enrichment of divalent ions in the NF rejects, and allowed sodium ions to pass to the RO stage. The product water included NF reject and the RO permeate. Overall, the MF-NF-RO train with a PAA aided NF pre-treatment can produce a product water fit for agriculture irrigation purpose, and reduce the scaling on the RO membrane due to calcium ions which are removed by NF, leading to the higher RO recovery. This approach achieved overall higher water production with less waste for disposal.