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

Abstract

We provide a simple mathematical model of the bioremediation of contaminated wastewater leaching into the subsoil below a septic tank percolation system. The model comprises a description of the percolation system's flows, together with equations describing the growth of biomass and the uptake of an organic contaminant concentration. By first rendering the model dimensionless, it can be partially solved, to provide simple insights into the processes which control the efficacy of the system. In particular, we provide quantitative insight into the effect of a near surface biomat on subsoil permeability; this can lead to trench ponding, and thus propagation of effluent further down the trench. Using the computed vadose zone flow field, the model can be simply extended to include reactive transport of other contaminants of interest.

The influence of pre-treatment on biomat development in soil treatment units

“…Depending on substrate availability and mass transfer processes, biomat growth will cause increased resistance to water flow (Rosenzweig et al, 2014) and, thus, contribute to the eventual distribution of effluent along the entire length of STU trenches (Beach et al, 2005;McKinley and Siegrist, 2011;Siegrist and Boyle, 1987;Winstanley and Fowler, 2013). As soil microorganisms are regularly challenged by changing moisture conditions potentially reducing substrate diffusion, restricting motility and experiencing desiccation stress in the soil environment, a mature biomat may serve as a protective barrier to ensure efficient soil treatment and variably saturated condition in the underlying soil (Geza et al, 2013;McKinley and Siegrist, 2011;Potts, 1994;Roberson and Firestone, 1992).…”

“…Once complete clogging is reached, ponding may occur if the long-term acceptance rate (LTAR) is below the hydraulic loading rate (Beal et al, 2006;Winstanley and Fowler, 2013). Where possible, a second, non-linear, model was fitted to the data to obtain an estimate for the biomat length (𝑙 𝑏𝑖𝑜 ) using a steady-state biomat length (𝑙 𝑠𝑠 ) by including an exponential decay as…”

A Review of Surfactant Role in Soil Clogging Processes at Wastewater Exfiltration Locations in Sewers

“…Attention has been recently directed to biomass effects in controlling wastewater infiltration into the soil (Pintelon et al, 2012;Rodgers et al, 2004;Gette-Bouvarot et al, 2013;Baveye et al, 1998;Winstanley and Fowler, 2013). Based on biological studies, the infiltration of nutrient-rich wastewater into the enveloping soil of a sewer pipe is highly likely to increase microbial activity in the clogging zone (Fuchs et al, 2004;Van Cuyk et al, 2001) attaching to the solid surface in order to reduce mobilization.…”

Simple deterministic model of the hydraulic buffer effect in septic tanks

“…The hydraulic buffer effect of septic tanks is often cited but to our knowledge there is still no method for quantifying it. It is often assumed that for the purposes of studying of long‐term phenomena, (like clogging of the treatment unit at month‐long or year‐long scale), the outflow can be considered constant (Winstanley & Fowler ). However, as we gain progressively more knowledge on the actual hydrograph produced by a household, it may be interesting to quantify how effectively the septic tank can buffer large inflow rates.…”