Conceptual Models and Simulations for Biological Clogging in Unsaturated Soils

Abstract

Biological clogging in unsaturated soils is an important concern in the design of biofilters that are used to treat wastewater in rural areas. Several conceptual models have been developed to simulate biological clogging in saturated flow systems but limited research has been performed to develop similar conceptual models in unsaturated soils. This study developed three conceptual models for biological clogging in unsaturated soils. The model formulations varied from microscale to macroscale and from analytically derived to empirical equations. They were all formulated based on the approaches proposed by Burdine and Mualem to estimate the relative permeability based on the effective water saturation and the soil moisture curve. A one-dimensional unsaturated flow and transport code was developed, which incorporates Monod kinetics to simulate the biodegradation of an organic substrate. The three conceptual models that were developed relate the relative permeability to the microbial growth term in the unsaturated flow equation. The models were implemented in a numerical model to illustrate the impact of microbial growth on the biological clogging of unsaturated soils. Also the effect of continuous loading versus pulse loading was simulated to illustrate the difference between the loading scenarios on the clogging process within biofilters.

Computational pore network modeling of the influence of biofilm permeability on bioclogging in porous media

“…For parameter set 1, the difference is only by a factor of 5 (two-dimensional networks), or 9 (three-dimensional networks), but for parameter set 3, the difference is by more than two orders of magnitude. This observation has relevance to the conclusion reached by Vandevivere et al (1995) that bioclogging models based on the biofilm approach could reproduce the hydraulic conductivity losses versus biomass volume relationships found in experiments with coarse-textured materials (Cooke et al, 2001(Cooke et al, , 2005a(Cooke et al, , 2005bMostafa and Van Geel, 2007) but could not reproduce that in medium-or fine-textured materials. Clearly, in retrospect, this deficiency may have been caused by an unrealistic assumption about the permeability of the biofilms.…”

“…Thullner et al (2002) reached a similar conclusion. Possibly as a result of this constraint of biofilms, biofilm-based models of the bioclogging of porous media have tended in recent years to restrict their scope to materials engineered to have initially high hydraulic conductivities, such as gravel-packed columns (Cooke et al, 2001(Cooke et al, , 2005a(Cooke et al, , 2005b or septic tank systems (Mostafa and Van Geel, 2007).…”

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

“…Rosenzweig et al (2009) examined significance of biofilm in controlling the hydraulic properties of unsaturated soils. Mostafa and Van Geel (2011) validated the conceptual models proposed by Mostafa and Van Geel (2007) by conducting soil column experiments. Since nitrate transformation processes occur in the presence of bacteria, while the investigation on movement of nitrogen species in presence of bacterial clogging is scant, there is a critical need to account the effect of biological clogging for comprehensive modeling of nitrogen species.…”

“…Yarwood (2006) monitored the interaction of microbial growth on water flow and solute transport in unsaturated porous media. Mostafa and Van Geel (2007) proposed three different conceptual models that directly correlate the relative permeability of unsaturated soils to the microbial growth. Soleimani et al (2009) implemented a twodimensional unsaturated flow and transport model along with substrate degradation and microbial growth to simulate the clogging process in unsaturated media.…”

Applying Process-Based Models for Subsurface Flow Treatment Wetlands: Recent Developments and Challenges

“…Bacteria density and accumulated inert solids also had an effect on simulated effluent concentrations [50] and on effluent pollutant concentrations; again, only without considering the effect of organic matter accumulation on water flow. Additionally, the effect of a pore size reduction on the hydraulic conductivity is described using the biological clogging model proposed by Mostafa and Van Geel [51]. Using this approach, a realistic behaviour of HF wetlands, including overland flow and re-infiltration of water in the unused filter material could be simulated [52,53].…”