Land application of treated effluent in the form of surface irrigation was originally conceived as an alternative to tertiary treatment. Land Treatment is designed to utilze the natural treatment mechanisms of the soil and plant growth on the soil. Land Treatment is not specifically designed for reuse purposes and may include a seasonal point discharge. Pretreatment requirements were originally established as 48/48 mg/l for BOD/suspended solids prior to storage with disinfection of applied flows. Although overland flow followed by a point discharge may be proposed as an alternative method of secondary treatment, public health and water quality concerns associated with land treatment will require a minimum of some type of biological system for pretreatment and provision of an established volume of storage. The storage capacity to hold treated effluent is usually based on a nonapplication period of 60 to 120 days. Disinfection of applied effluent may also be required, with a buffer area of land separating the field areas receiving applied effluent from adjacent property and points of public access.
The liquid application depth or loading rate will be specified for hourly, daily and weekly totals based on soil and weather/climate factors. A specified percentage of average soil test hydraulic conductivity measurements can be utilized for calculation of design loading rates. The allowable liquid loading rate cannot either exceed the specified nitrate-nitrogen application limit to support plant growth, or saturate the upper root zone of site vegetation. The site design must also provide the required minimum depth to water table. Irrigation-percolation (slow rate or low intensity) systems should provide five (5) feet of well-drained loamy soils. The seasonal water table should be three (3) feet or more below the irrigation area. Irrigation on filled areas is prohibited. Overland flow application sites are designed with a restrictive infiltration layer of soil, one (1) to two (2) feet below the surface to promote surface runoff through the root zone of surface vegetation. The hydraulic conductivity of such restrictive soil layers should be two-tenths (0.2) or less inches/hour. Overland flow application sites are graded to level flat slopes of eight (8) percent or less and site runoff is collected in gravel lined drainage channels and conveyed to a central point for additional treatment prior to discharge.