Polishing low-biodegradable and saline industrial effluent in a full-scale horizontal subsurface flow constructed wetland: evaluation of bio-treatability and predictive power of kinetic models

This study evaluates the bio-treatability performance and kinetic models of full-scale horizontal subsurface flow constructed wetland used for the tertiary treatment of composite industrial effluent characterized by high-salt content ranging from 5830 to 10,400 µS/cm and biochemical oxygen demand (BOD5): chemical oxygen demand (COD) ratio below 0.2. The wetland vegetated with Phragmites australis was operated in a semi-arid climate under an average hydraulic loading rate of 63 mm/d. The results of a 4-year operation calculated based on the concentration of pollutants showed that the average removal efficiency of COD, BOD5, and total suspended solids (TSS) were 17.5, 5.1, and 11.2%, respectively. The system reduced up to 6.5 ± 0.7% of electrical conductivity presenting poor phyto-desalination potential without considering the contribution of evapotranspiration in water balance in contrast to satisfying performance for heavy metals reduction. The comparison of the kinetics of organic matter removal obtained by the first-order and Monod models paired with continuous stirred-tank reactor and plug flow regime showed that Monod-plug flow model provided the best fit with the constants of 2.01 g COD/m2·d and 0.3014 g BOD5/m2·d with the best correlation coefficient of 0.610 and 0.968 between the predicted and measured concentrations, respectively. The low kinetic rates indicate that the process is capable of effluent polishing instead of purification due to the presence of organic compounds recalcitrant to biodegradation and a high level of salinity.