A rational method based on oxygen diffusion is proposed as an alternative to numeric BOD loading limits for land application. The rational method determines management parameters, such as cycle time and application depth to insure proper aeration. Even conservative numeric limits may lead to incomplete treatment of organics and increases of TDS in the percolate water if adequate oxygen diffusion does not occur. The rational method uses a single non-steady state Fickian diffusion equation to conservatively estimate available oxygen. Available oxygen is then balanced with the oxygen demand from the wastewater for aerobic oxidation of organics in the land treatment system.