Long-term evaluation of a forward osmosis-nanofiltration demonstration plant for wastewater reuse in agriculture

Hybrid forward osmosis (FO) processes such as forward osmosis with membrane bioreactors (FO-MBR), electrodialysis (FO-ED), nanofiltration (FO-NF) or reverse osmosis (FO-RO) present promising technologies for wastewater reuse in agriculture as they meet high effluent quality requirements, especially regarding boron and/or salt content. An FO-NF demonstration plant for this application was built and operated treating 3 m3 h−1 of real wastewater with a salinity of 3-5 mS cm−1 and 1.5 mg L−1 of boron in continuous mode for 480 days. Three draw solutions (DS) were evaluated in different periods of experimentation. Sodium polyacrylate led to reversible fouling on the FO and NF membranes and the permeate was not suitable for irrigation. Magnesium sulphate, used as DS in a second phase, generated severe irreversible fouling on NF membranes and therefore it was discarded. Finally, magnesium chloride showed the best performance, with FO-NF membranes presenting a stable permeability and low membrane fouling during long-term operation. The FO-NF permeate showed high quality for irrigation, achieving a conductivity value of 1 mS cm−1, a boron concentration below 0.4 mg L−1 and an average SAR of 1.98 (mequ L−1)0.5. DS replacement costs were reduced by working with high rejection NF membranes. However, energy consumption costs associated with the NF step make the global process more energy intensive than conventional technology.