Land application has been popular for disposing municipal wastewater worldwide. While most of the operations appear to be successful, reports from countries such as China suggested that large-scale irrigation of crops with mostly untreated municipal wastewaters could be harmful to crops and cause injuries to humans because of poorly controlled discharge of toxic constituents. The two approaches, as explained below, differ in their underlying principles and may be employed in the development of guidelines to safeguard the cropland application of wastewaters.

1. Prevent Pollutant Accumulation in Receiving Soils. In land application, if the pollutant input equals to the pollutant output, there will not be a net accumulation of pollutants in the receiving soil. Consequently, the pollutant contents of the soil will remain at the background level and the soil's integrity is preserved. When this requirement is met, the capacity of the soil to sustain any future land uses is guaranteed and the transfer of pollutants up the food chain is kept to a minimum. Numerical limits, therefore, are set to prevent the pollutant concentration of the soil from rising during the course of land application.

2. Maximizing Soil's Capacity to Assimilate Pollutants. Soils possess inherent abilities to assimilate, attenuate, and detoxify pollutants. In land applications, this capacity should be fully utilized. In this manner, the agronomic benefits of applying wastewater may be realized and, when managed properly, accumulation of pollutants in soil can be controlled so that they will not reach levels harmful to human health. Land application guidelines based on this approach set the maximum permissible pollutant loading and provide users the flexibility to develop suitable management practices for using wastewater within the boundary.

Both approaches were considered in this investigation and their principles were employed to develop guidelines for sustainable land application of municipal wastewaters.