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Oxygen Utilization in Soils Flooded with Sewage Water¹

ABSTRACT

Diffusion and mass flow were important mechanisms of oxygen transport in soil columns intermittently flooded with sewage water. The amount of oxygen entering the soil by mass flow during drainage was 30 to 40% of the total amount entering the soil. Most of the estimated available oxygen was used in the oxidation of NH₄⁺ to NO₃^–. Enough oxygen entered the soil during a 5-day dry period to oxidize all of the nitrogen applied when the soil was flooded for 6 days with secondary sewage effluent containing 20 mg/liter of NH₄-N at an infiltration rate of 50 cm/day. Application of NH₄⁺ in excess of that which could be oxidized during the dry period resulted in a gradual increase in the NH₄⁺ concentration of the reclaimed water. An increase in NH₄⁺ concentration can be prevented by balancing the NH₄-N applied against the oxygen entered during the dry period.

Most of the oxygen entered the columns during the first 3–4 days of the dry period. Thus the amount of oxygen entering the soil can be increased more effectively by increasing the frequency rather than the length of the dry periods.

The soil profile remained in a reduced state below the 140-cm depth during a 10-day dry period. This showed that NH₄⁺ stored below this depth would not be nitrified and that an environment favorable for denitrification was maintained in part of the profile at all times. Anaerobic reactions were as effective as aerobic reactions in reducing the chemical oxygen demand (COD) of sewage water from 50 to 18 mg/liter.

Key Words: secondary sewage effluent • diffusion • mass flow • nitrification • denitrification

¹ Contribution from the Agricultural Research Service, USDA.

² Research Soil Chemist, Soil Scientist-Physicist, and Chief Hydraulic Engineer, respectively, U. S. Water Conservation Laboratory, 4331 East Broadway, Phoenix, Arizona 85040.

Received for publication August 15, 1972.