Modeling Nitrogen Losses under Rapid Infiltration Basins

Rapid Infiltration Basin System (RIBS) is one of the major land treatment techniques used for wastewater treatment and reuse of recovered treated wastewater. In this system, wastewater that is treated using primary, secondary, or advanced treatment techniques is applied at high rates to shallow basins constructed in permeable deposits of soil or sand, with further treatment occurring in soil and the vadose zone before the water recharges groundwater. Because the influent wastewater is usually enriched in nitrogen (N) compounds, there is particular concern that RIBS may contaminant groundwater or nearby surface waters if not designed and operated properly. In most of the new sequenced batch reactor (SBR) wastewater treatment plants, N is found in the form of nitrate in the discharged wastewater, so denitrification (DNF) is the main reaction in N removal. The absence of molecular oxygen is one of the required conditions for DNF. During RIBS operation, application of wastewater is cyclic and typically consists of a flooding period followed by days or weeks of drying. Key operational parameters include the ratio of wetting to drying time and the hydraulic loading rate, which affect water saturation and air content in the vadose zone and as a result have an impact on DNF. Wastewater is typically distributed at a limited number of discharge points in RIBS and basins are not usually completely flooded which result in non-homogeneous distribution of wastewater and unusual surface water flow patterns. For this reason, we couple overland flow within RIBS with subsurface flow to investigate the influence of non-uniform application of wastewater on DNF. No modeling effort has been done for understanding this aspect of RIBS performance previously. TOUGH2/ iTOUGH2, a general-purpose numerical simulation program for multi-phase fluid flow in porous media, is used for modeling fluid movement. Water saturation is used as a surrogate parameter to evaluate oxygen limitations in the pore water and a dimensionless parameter, Fs, is used to represent degree of DNF for different operating conditions. TOUGHREACT is also used for comprehensive modeling of N fate and transport. Flow and transport simulations indicate that discharging wastewater in less permeable soils results in more water spreading over the basin and larger saturated area under the basin, which is more favorable for DNF resulting in higher nitrogen removal than in more permeable basin soils. Moreover, smaller ratios of wetting to drying time, i.e., shorter but more intense flooding periods, result in higher vadose zone water saturations and as a result greater DNF. Using a coupled surface-subsurface model to predict DNF is very important particularly when basin flooding occurs at high rates for short periods.