Washing Bay effluent contains several organic and inorganic pollutants that require effective treatment techniques. The new trend of utilizing plant-based coagulants for treating washing bay effluent can help overcome drawbacks associated with chemical coagulants such as large sludge volumes, health risks, and high costs. Hence, this study evaluates the comparative performance of C. arietinum and alum coagulants in continuous feed bench‑scale experiments for treating washing bay effluents. Sampled effluent from an automobile washing station in Borg El-Arab City, Egypt, contained mild alkali pH (8.1 ± 0.03) with proportions of turbidity (159 ± 7.78 NTU), oil & grease (59.4 ± 1.52 mg/L), surfactants (24.60 ± 1.25 mg/L), and COD (216.67 ± 5.77 mg/L) above discharge limits. Hence, its direct discharge into waterways can degrade water quality, aquatic habitats, and biodiversity. Continuous-flow experimental results showed an enhanced pollutant removal efficiency after coagulation/flocculation/sedimentation (C/F/S) treatment stages. The average removal efficiency reached 86.1±2.6% for turbidity, 65.4±3.4% for surfactants, 82.4±1.6% for oil-grease, 61.3±2.8% for chemical oxygen demand (COD). On the other hand, alum achieved average removal percentages of 96.2±1.8%, 60.5±1.5%, 85.5±1.5%, and 80.6±2.7% for turbidity, surfactants, oil-grease, and COD, respectively. Variations in pH and alkalinity were higher for alum than for C. arietinum. Also, the sludge generated with alum was three times greater than for C. arietinum. A primary cost evaluation for treating 1 m³ showed that alum was slightly cheaper than C. arietinum powder. Finally, the multi-criteria decision analysis (MCDA) was performed using Simple Additive Weighting (SAW) method to determine a sustainable coagulant material based on the established criteria. Results showed that C. arietinum produced a higher utility value (0.803) than alum (0.532). Generally, C. arietinum bio-coagulant could be a preferable alternative to alum in the C/F/S treatment of washing bay effluent based on the selected criteria and weights.