Biological Salinity Reduction in Agricultural Drainage Water Using Chlorella vulgaris: Treatment Performance Across an Irrigation-Relevant Salinity Gradient and Implications for Sustainable Water Reuse

Agricultural drainage water (ADW) is an underutilized water resource in water-scarce regions, but elevated salinity limits its reuse. Laboratory batch experiments evaluated Chlorella vulgaris for salinity reduction using ADW as both the cultivation medium and treatment matrix, with NaCl amendment used to establish a controlled initial EC gradient (≈1050–13,800 µS cm−1). Despite the increasing salinity, ADW supported growth without nutrient addition at all salinities, and specific growth rates (0.17–0.21 d−1) did not differ significantly among treatments. Maximum chloride removal reached 35.2% at EC ≈ 6200 µS cm−1 and maximum EC reduction reached 27.1% at EC ≈ 10,400 µS cm−1, both within 3–5 days. From an irrigation perspective, treatment reduced EC by 18.8–27.1%, but none of the treatments moved into a less restrictive FAO salinity category, indicating that highly saline ADW would require additional treatment or blending for reuse. Overall, these results identify C. vulgaris as a potential biological pretreatment that could contribute to more sustainable agricultural water management by supporting the reuse of saline drainage water. However, because the salinity gradient was established by NaCl amendment under laboratory batch conditions, performance under naturally varying ADW, continuous-flow operation, and field conditions remains unverified.