Numerical Modeling of Unsaturated Flow in Wastewater Soil Absorption Systems

Abstract

It is common practice in the United States to use wastewater soil absorption systems (WSAS) to treat domestic wastewater. WSAS are expected to provide efficient, long‐term removal of wastewater contaminants prior to ground water recharge. Soil clogging at the infiltrative surface of WSAS occurs due to the accumulation of suspended solids, organic matter, and chemical precipitates during continued wastewater infiltration. This clogging zone (CZ) creates an impedance to flow, restricting the hydraulic conductivity and rate of infiltration. A certain degree of clogging may improve the treatment of wastewater by enhancing purification processes, in part because unsaturated flow is induced and residence times are significantly increased. However, if clogging becomes excessive, the wastewater pond height at the infiltrative surface can rise to a level where system failure occurs. The numerical model HYDRUS‐2D is used to simulate unsaturated flow within WSAS to better understand the effect of CZs on unsaturated flow behavior and hydraulic retention times in sandy and silty soil. The simulations indicate that sand‐based WSAS with mature CZs are characterized by a more widely distributed flow regime and longer hydraulic retention times. The impact of clogging on water flow within the silt is not as substantial. For sand, increasing the hydraulic resistance of the CZ by a factor of three to four requires an increase in the pond height by as much as a factor of five to achieve the same wastewater loading. Because the degree of CZ resistance directly influences the pond height within a system, understanding the influence of the CZ on flow regimes in WSAS is critical in optimizing system design to achieve the desired pollutant‐treatment efficiency and to prolong system life.

Numerical modeling of biological clogging on transport of nitrate in an unsaturated porous media

“…They also developed a one-dimensional finite element model to simulate the flow and solute transport in an unsaturated zone. The effect of biomat layer on the hydraulic properties within the unsaturated zone of soil absorption system was investigated by Beach and McCray (2003). Rockhold et al (2005) examined experimentally and numerically the effect of bacterial growth on the hydraulic properties of unsaturated soils.…”

Effluent flux prediction in variably saturated soil zones within a septic tank—soil absorption trench

“…Results were checked by running the same input parameters in SWIM v1.0 (Ross 1990). Biomat resistances used in the model encompassed a range of values reported in the literature Beach and McCray 2003). The FLUX model, in accordance with assumptions held for Eqn 1, assumed that all flow was steady-state with a unit gradient, and flow occurred in a 1-directional manner, vertically through the biomat zone.…”

“…3) is hypothesised to be an important absorption pathway for effluent under situations of peak loading. There are a limited number of studies modelling unsaturated flow in SAS (Janni et al 1980;Hansen and Mansell 1986;Beach and McCray 2003;Radcliffe et al 2005), but the specific partitioning of biomat zone and nonbiomat zone flow in SAS is not widely reported. A study by Brouwer et al (1979) found flow through the sidewall to be greater than bottom flow in some duplex soils in Victoria.…”

“…McGauhney and Winneberger (1964) reported greater sidewall water flow in sands compared with finer grained soils. Beach and McCray (2003) used HYDRUS-2D to predict unsaturated flow within SAS, and described a strong relationship between the biomat zone hydraulic properties and the steady-state (long-term) infiltration rates within the unsaturated zone. However, the model assumed that all flow occurred through either the trench bottom or trench sidewall biomat layer, thus precluding the opportunity to predict flow dynamics for the remainder of the trench sidewall.…”

Biomat Development in Soil Treatment Units for On-site Wastewater Treatment

“…The area over which the effluent is distributed is a major determinant of overall contaminant attenuation and the achievement of treatment objectives. Modeling studies in the literature have largely assumed biomat hydraulic properties are given, and used numerical simulation packages such as HYDRUS2D (Šimůnek et al 2011) and its reactive transport modules (CW2D (Langergraber and Šimůnek 2005) and CWM1 (Langergraber et al 2009)) to investigate varying hydraulic régimes and compare open and gravel IS architectures in a 2D plane perpendicular to the trench (Huntzinger Beach and McCray 2003;Radcliffe et al 2005;Finch et al 2008;Beal et al 2008;Wunsch et al 2009). Our approach differs in proposing a minimal analytical model to describe the biomat development along the IS by coupling the flow and transport with biomass growth, which we then investigate through analytical approximations.…”