Multi-scale modeling of a vegetated submerged bed system with application in organic matter removal and clogging

A multi-scale model for vegetated submerged bed (VSB) wastewater treatment systems based on subsurface flow and reactive transport principles is developed. The model includes substrate diffusion and utilization in biofilm as microscale; reactive transport of substrate, biomass and solids in saturated porous medium as mesoscale; and the resulting effects on a system as macroscale properties of a VSB. The model includes precipitation, evapotranspiration, influent flow and porous media properties for hydraulics, advection and dispersion of substrate, solids, and biomass for transport, and substrate utilization and growth in biofilm and bulk liquid for biological reactions. Biofilm dynamics and solids deposition in a VSB are included to analyze clogging processes. The mathematical model is numerically solved using MATLAB®. The applicability of the model is demonstrated by applying it to a field-scale VSB system to predict COD, solids and biomass removal, solids deposition, and biological clogging for a five-year period. The model is also used for sensitivity analysis of key parameters. The model predicted effluent COD, biomass and total solids with mean absolute errors of 0.098, 0.007 and 0.102 kg/m3, respectively, for a wide variation of influent concentrations. Flow velocity generally increased over the simulation period and ranged from 0.93 to 2.9 m/d. Biofilm thickness changed dynamically between 8 μm and 211 μm and resulted up to 85% reduction in hydraulic conductivity. Bed elevation due to solids deposition increased by 0.04 m and porosity due to biological clogging decreased from 0.26 to 0.16 near the inlet end. Sensitivity analyses showed that the kinetic parameters are more sensitive than transport parameters in predicting COD removal and biological clogging of the system. A long-term simulation, replicating the available data for a 25 year-period predicted up to 88% reduction in hydraulic conductivity but no hydraulic failure of the system. Extrapolation of the long-term simulation results indicates that a failure of the implementation site due to solids deposition may occur after about 45 years of operation