Gas transport in the unsaturated zone of soil absorption systems.

摘要

A number of bed-type soil absorption systems (SAS's) have recently been installed to treat and dispose of wastewater generated by small communities. Some of these systems have failed within one to two years after start-up due to rapid declines in their hydraulic acceptance rates. Recent research has indicated that a lack of oxygen within the vadose zone can stimulate "clogging mat" development in SAS's and contribute to this type of failure. Therefore, a two-dimensional model of multicomponent gas transport was developed which could predict the oxygen concentrations within the air-filled pore space of SAS's as a function of their physical properties and substrate oxidation rates.; Operating data used to calibrate and verify the gas transport model was obtained from a bench-scale SAS filled with a medium sand. Using primary effluent as feed, soil water samples were collected at various depths to characterize the carbonaceous and nitrogenous substrate transformations occurring within the vadose zone. From this data, oxygen uptake rates were calculated and used in the model to predict the soil-gas composition throughout the vadose zone. The results were compared with vadose zone soil-gas data gathered from the bench-scale SAS; the comparison showed the model results to reasonably approximate the experimental data. A sensitivity analysis of the model was then performed to determine the impact on oxygen concentrations in the soil-gas phase due to changes in various model parameters. Parameters investigated included trench width, infiltrative surface depth, spacing between adjacent trenches, and air-filled porosity.; It is hoped that the results of this work will prove useful in developing an improved methodology for the design of soil absorption systems which will account for the transfer and uptake of oxygen within the vadose zone as well as the hydraulic acceptance rate of the soil. The computer program used in this work was written for the Apple Macintosh.