Wastewater treatment in soil: effect of residence time

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Virginia Polytechnic Institute and State University

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% 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% of applied NH₄-N were demonstrated in an 18-cm deep root zone with residence times as short as 2 hours. The exact nature of these removals was not determined.