Using MF-NF-RO train to produce low salt and high nutrient value recycled water for agricultural irrigation L. Zou, P. Sanciolo and G. Leslie ABSTRACT L. Zou P. Sanciolo Institute for Sustainability and Innovation, Victoria University, Melbourne, Australia Email: linda.zou@vu.edu.au; peter.sanciolo@vu.edu.au G. Leslie UNESCO Centre for Membrane Science and Technology, University of New South Wales, Sydney, Australia Email: g.leslie@unsw.edu.au 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. Key words | irrigation, polyacrylic acid, recycled water, selective nanofiltration membrane, sodium absorption ratio INTRODUCTION Conventional RO treatment systems commonly produce high purity water but strip valuable elements from the water and these results in recycled product water that is not well matched to the irrigation needs. For agricultural irrigation application, it is important to remove the excessive sodium ions to avoid soil and plant salinity as well as maintaining an adequate nutrient content. This research 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 nutrients and other ions, and lower the sodium absorption ratio (SAR) to minimise soil salinity. The recovery of nutrient and useful ions from the wastewater made NF- RO treated recycled water an attractive option for agricul- tural irrigation. The technology also has the potential to save energy due to the lower energy requirements of NF elements over RO elements. A MF-NF-RO pilot plant has been designed and built at Victoria University. The role of the NF element(s) in the proposed integrated MF-NF-RO system is to enrich the product water with respect to divalent ions (e.g. Ca 2 þ , Mg 2 þ ). NF elements do this by rejecting divalent ions to a greater extent than monovalent ions. The result of this rejection is an NF concentrate stream which is more concentrated than the feed water with respect to both monovalent and divalent ions, but which has an increased ratio of divalent ions and nutrient ions to monovalent ions (Baldini & Vezzani 2003). The NF concentrate is then diluted with the permeate from the RO elements to bring the product water overall sodium content, conductivity and sodium absorption ratio closer to the ideal requirements for irrigation. The treatment process diagram is shown in Figure 1. Nanofiltration pre-treatment has been reported by researcher to use for water softening and desalination purpose (Raitenbach et al. 2000; doi: 10.2166/wst.2008.534 1837 Q IWA Publishing 2008 Water Science & Technology—WST | 58.9 | 2008