NUMERICAL MODELING OF BIOZONE PERMEABILITY IN WASTEWATER SOIL ABSORPTION SYSTEMS

Wastewater soil absorption systems (WSAS) are often used in the United States to treat domestic wastewater prior to ground water recharge. Soil clogging occurs at the infiltrative surface of WSAS due to the accumulation of suspended solids, organic matter, and chemical precipitates during continued wastewater infiltration. This low permeability zone is referred to as a biozone. A certain degree of clogging enhances wastewater treatment due to increased residence times and nutrient utilization by organisms. Conversely, too much clogging can cause WSAS to backup and ultimately fail. As such, determining the hydraulic conductivity of the biozone and whether it varies in different types of WSAS or for different loading rates are integral to proper WSAS design and installation. Because the system is unsaturated and the low permeability biozone within the soil creates a capillary barrier, simply estimating the hydraulic conductivity of the biozone using Darcy’s law is not feasible. Therefore, inverse modeling was conducted using the numerical model HYDRUS-2D. Input data for the model simulations, which include infiltration rates and soil hydraulic parameters, were determined at a test site at Colorado School of Mines. The test site comprises ten each of three different types of WSAS – gravel fill, chamber, and synthetic – for a total of 30 test cells. Since site initiation, half of the test cells have received a loading rate of 4 cm/day, while the other half have received 8 cm/day. Each WSAS has formed a mature biozone. Constant head tests were conducted to determine infiltration rates in each of the test cells, and at least three estimates of the sub-biozone soil hydraulic properties were obtained using analytical techniques including grain size distribution, hanging column tests, and ring infiltrometer tests. The differences between the optimized biozone hydraulic conductivity values for different loading rates and WSAS types are discussed.