Transforming Agricultural Drainage into a Sustainable Water Resource for Agricultural Reuse: Insights from Al-Ahsa Oasis

Agricultural drainage water (ADW) represents a significant yet challenging water resource in arid regions, characterized by high and variable salinity that complicates its reuse. This proof-of-concept field study evaluated whether a mobile reverse osmosis (RO) unit with flexible, operator-adjusted pretreatment can convert variable ADW into irrigation water in Al-Ahsa Oasis, Saudi Arabia. Over 32 operating days in autumn–early winter 2024, the unit treated feed water with a total dissolved solids (TDS) concentration of 6556 ± 591 mg/L (mean ± SD, n = 28), achieving an overall recovery of 66.7% and a permeate TDS of 499 ± 58 mg/L at a specific energy consumption of 0.45 ± 0.22 kWh/m3. Salt passage and recovery showed no trend, indicating stable short-term operation. At design capacity (133.4 m3/day of permeate), the levelized cost of water was 0.55 USD/m3 (Monte Carlo 90% interval 0.52–0.78 USD/m3), including pretreatment chemicals but excluding concentrate management; at the output achieved in the trial (51.3 m3/day), it rose to 1.20 USD/m3, making capacity utilization the main economic lever. Returning the concentrate to the drain reduced the salt load reaching the evaporation lake by only about 5%. Multi-season operation and sustainable concentrate management remain to be demonstrated before large-scale deployment.