There are two routes to supply oxygen to wastewater infiltrating the soil. The first option is to aerate the wastewater and reduce the oxygen demanding substances. The soil receiving this wastewater does not have to supply much oxygen. However, aeration can be costly, often require energy and demands a high level of management. The more passive and generally less expensive septic tank produces an effluent with high oxygen demand and little or no dissolved oxygen. Septic tank effluent applied to the soil needs appreciable amounts of available soil oxygen to efficiently decompose the added organic matter. The oxygen must pass through soil pores.

Oxygen is supplied to soil from the atmosphere. Once in the soil it is moved through soil pores by diffusion or mass flow and eventually used by respiring soil organisms or the roots of plants. Respiration produces carbon dioxide which needs to move the soil surface. Gas filled soil pores must be available for oxygen to move to infiltration surfaces. It is the same set of pores that is the conduit for the carbon dioxide to exit the soil. Therefore, the soil must have sufficient porosity and a portion of the porosity must be gas filled. If the soil is very wet the pores are filled with water and the gases are neither present in very large quantities nor able to move. Additions of wastewater reduces the gas filled pores and therefore the potential to move oxygen. The wetter the soil the less oxygen will be available.

Since there are limited gas pressure gradients in soil the mass movement of oxygen, an oxygen wind, is unlikely. Diffusion of oxygen is the major mechanism to move oxygen in soil. The oxygen moves from a zone of high concentration to a zone of low concentration. If organisms use oxygen at the infiltration surface as they consume the organic matter from wastewater then the addition of wastewater creates zones of low oxygen concentration. Oxygen will then diffuse from the soil to the infiltration surface. To maximize the delivery of oxygen select a porous soil and use conservative loading rates to maintain pores filled with gases.

Please address any questions to Dr. David Lindbo.

This page created by
Roland O. Coburn, Research Technician I on 1/10/03.