Time-dependent retardation model for chemical oxygen demand removal in a subsurface-flow constructed wetland for winery wastewater treatment

The relative success of chemical oxygen demand (COD) removal models to desc ribe measured rates of COD removal in a pilot-scale constructed wetland des igned for treatment of high-strength winery wastewater are evaluated using retention times determined from tracer studies. Not surprisingly, two-param eter residual and retardation models better fit the measured removal data t han single-parameter, first-order decay models for wastewater at average CO D loadings up to nearly 5000 mg/L. The residual and retardation models yiel ded nearly equivalent fits to the measured data. However, the retardation m odel had more consistent parameters for COD removal data across different d epth levels in the constructed wetland and at different loadings, and a sli ghtly smaller sum of least-squared errors. The retardation model seems to b e appropriate for constructed wetland design because it allows a steady dec rease in COD with increased treatment time rather than a constant residual COD (C*) value. From the least-squares optimization procedure used to estim ate model parameters (a volumetric rate constant, K-v, range of 3 to 12 d(- 1)), nonrealistic, or physically meaningless, large C* values (C* range of 23 to 450 mg COD/L) that were dependent on COD loading were obtained, poten tially underestimating the constructed wetland system's actual winery waste water treatment potential. The optimal parameters for the retardation model applied to the pilot-scale constructed wetland ranged from 9 to 12 d(-1) f or the initial degradation rate constant, K-o, and 2 to 5 d(-1) for the tim e-based retardation coefficient, b. These values should be verified for ful l-scale field systems based on field measurements currently underway.