Groundwater recharge with secondary sewage effluent - a study of removal of nitrogen compounds by soil percolation

Kuruvilla Mathew

Groundwater recharge using sewage effluent is becoming very popular. The removal of nitrogen compounds from sewage effluent by soil percolation can be managed by manipulating flooding and drying periods in a recharge operation. The main processes involved in the removal of nitrogen compounds were established as dispersion, adsorption, nitrification, leaching and denitrification. These processes are reviewed, explained theoretically, investigated experimentally and applied to a recharge situation in Perth, Western Australia. The dispersion coefficient was determined by first solving the equation for non-reactive solute transport according to the boundary conditions of a finite column, then comparing this with the breakthrough curve of a miscible displacement experiment. Adsorption of ammonium during infiltration through soil was found to follow a model combining instantaneous and time dependent adsorption rather than either of these alone. Nitrification was described by Michaelis-Menten kinetics for a wide range of values of ammonium concentration. The equation for non-reactive solute transport in porous media was found to explain the leaching of nitrate in soil. Denitrification in soil was found to follow zero order kinetics in the range of concentrations encountered in recharge situations. When recharge is done by flooding and drying the ammonium in the recharged water. is first adsorbed to the soil and then nitrified in the following drying period. During the next flooding period adsorption of ammonium and simultaneous leaching and denitrification take place. Hence an integrated model of all the processes studied separately has been suggested. As an integrated model is difficult to solve with all the variables that are involved, a set of simplifying assumptions have been made and a simplified model developed. The simplified model has been used to explain the results of field experiences reported in the literature, to select soils at a recharge site and to calculate the movement of nitrogen during recharge. In Perth, Western Australia the soil at the recharge, site is Bassendean sand. It was found to be of no use in the removal of nitrogen, so the major soils in the Swan Coastal Plain and a few mixtures of these soils were investigated. Pyrton Sandy Loam and a mixture of Spearwood sand and Pyrton loam were found to be suitable. Pyrton sandy loam was used for further experiments to verify the models proposed for each of the processes and to determine the constants to be used in the simplified model. From the results of these experiment and applying the simplified model further, general conclusions have been deduced for groundwater recharge management.

Groundwater recharge with secondary sewage effluent - a study of removal of nitrogen compounds by soil percolation

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