Wastewater treatment in soil as a function of residence time in the root zone

A laboratory study was conducted to determine nitrogen removal rates from a land-applied wastewater as a function of the length of time the wastewater remained in the root zone. A digital simulation model was used as an aid in describing soil-water (and wastewater) movement through the root zone under wet conditions (i.e., root zone 50-75 percent saturated). A procedure was developed to predict the rate and volume of drainage as a function of initial soil moisture content, amount of liquid applied, and time after liquid application. An exact relationship between nitrogen removals and wastewater residence time in the root zone could not be developed. However, removals of up to 95 percent of applied N H4⁺-N were observed in an 18-cm-deep root zone when residence times were as short as 2 hr.