Agricultural Graywater as a Distinct Graywater Category: Insights from Physiochemical and Biological Characteristics

Greywater reuse is increasingly considered a sustainable strategy for addressing water scarcity, particularly in arid regions. While previous studies have primarily focused on household, laundry, and car wash greywater, agricultural wash water remains poorly characterized despite its potential relevance for irrigation reuse. This study investigated the physicochemical characteristics, ionic composition, nutrients, and microbiological quality of agricultural wash water (AW), and compared them with those of car wash water (CW). In Al-Ahsa, Saudi Arabia, we collected eight samples from farms (AW; n = 4) and car wash stations (CW; n = 4). AW exhibited considerable variability in physicochemical and microbiological characteristics; some parameters overlapped those observed in CW. However, AW showed distinctive features, particularly higher ammonia and fecal coliform concentrations. Ammonia ranged from 0.048 to 2.35 mg/L in AW and 0.017–0.028 mg/L in CW; fecal coliforms reached 2400 MPN/100 mL in all AW samples but 2.2–240 MPN/100 mL in CW. Some AW samples were also characterized by high COD (up to 881 mg/L) and BOD5 (up to 86.3 mg/L) and low DO (as low as 1.48 mg/L). Hierarchical cluster analysis demonstrated partial overlap between AW and CW profiles while revealing substantial within-group variability. This finding indicates treatment requirements should be determined by measured contaminant profile rather than source classification alone. Accordingly, AW is proposed as a distinct source-specific, non-toilet wash water stream within a broader “shades-of-gray” framework. The framework’s classification considers both wastewater origin and measured contamination characteristics to better inform treatment and potential reuse.