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A MODEL COMPARISON OF CHAMBER AND CONVENTIONAL ON-SITE SYSTEMS

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  701P0104,  . (doi: 10.13031/2013.15783)
Authors:   D.E. Radcliffe, L. T. West, J. Singer
Keywords:   Biomats, individual sewage systems, numerical models, on-site sewage systems, septic systems

In Georgia, chamber on-site sewage systems are approved by the State Department of Human Resources for installation using half the drain line length of conventional systems. This is based on the manufacturers claim that chamber systems have twice the infiltration rate of conventional systems since they do not use gravel in the trench line. Our objective was to use a twodimensional computer model to compare water flow in chamber and conventional systems. We used the finite-element numerical model HYDRUS-2D for our analysis. This model simulates unsaturated and saturated water and solute flow in two dimensions under a variety of boundary conditions. We simulated water movement from the trench bottom into a clay loam layer. We investigated the effect of gravel in a conventional system by imposing alternating 2.5-cm wide segments of no-flux boundary conditions (representing gravel particles) and ponded boundary conditions at the trench-soil interface with a biomat in the top 2 cm of soil. We also tested the effect of a 2-cm thick biomat within the gravel layer in a conventional system by including alternating 2.5-cm wide zones with negligible hydraulic conductivity (representing gravel particles). To simulate a chamber system, we imposed a uniform ponded boundary condition all across the trench-soil interface with a biomat in the top 2-cm of soil. To compare the systems, we examined the infiltration rate once steady state conditions were reached. We found that the chamber system had the highest infiltration rate, but it was not double the rate of the conventional systems. The chamber system infiltration rate was 1.1 times the conventional system rate when the biomat was in the soil. When the biomat was in the gravel layer, the chamber system rate was 1.4 times the conventional system rate. Because flow was twodimensional, water flowed laterally into the areas beneath gravel particles immediately after entering the soil and flowed more rapidly between gravel particles when the biomat was in the gravel layer. As a result, the velocities at the trench-soil interface were greater in the conventional systems and this compensated for the reduced cross-sectional area.

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