The soil profile removes biodegradable organics through filtration, adsorption, and biological reduction and oxidation. Most of the biological activity occurs near the surface, where organics are filtered by the soil, and oxygen is present to support biological oxidation; however, biological activity will continue with depth if a food source and nutrients are present. The BOD loading rate is defined as the average BOD applied over the field area in one application cycle. The oxygen demand created by the BOD is balanced by the atmospheric reaeration of the soil profile during the drying period. Excess organic loading can result in (1) odorous anaerobic conditions, (2) reduced soil environments mobilizing oxidized forms of iron and manganese, or (3) increases in percolate hardness and alkalinity via carbon dioxide dissolution. Prevention from excess loading of organics is a function of maintaining an aerobic soil profile, which is managed by organic loading, hydraulic loading, drying time, oxygen flux, and cycle time, not organic loading alone.
Aerobic conditions and carbon dioxide venting can be maintained by balancing the total oxygen demand with oxygen diffusion into the soil. McMichael and McKee (1966) reviewed methods for determining oxygen diffusion in the soil after an application of wastewater. They discussed three principal mechanisms for reaeration: (1) dissolved air carried in the soil by percolating water, (2) the hydrodynamic flow of air resulting from a "piston-like" movement of a slug of water, and (3) diffusion of air through the soil pores. Dissolved oxygen in wastewater has an insignificant impact on high BOD waste streams. The "piston-like" effect may have a substantial impact on the oxygen available immediately after drainage, but quantifying the exact amount is dependent on the difficult-to-model dynamics of draining soils. McMichael and McKee (1966) solved the non-steady-state equation of oxygen diffusion based on Fick's law. They used the equation as a tool for determining the flux of oxygen (mass of O2 per area) that diffuses in the soil matrix over a given time.
The flux of oxygen across the soil surface does not address the destination of the oxygen, but as long as a gradient exists the oxygen will continue to diffuse into the soil pores. The gradient is based on the oxygen concentration at the soil surface and the initial concentration in the soil. McMichael and McKee (1966) assumed total depletion of oxygen in the soil matrix. Overcash and Pal (1979) assumed a more conservative 140 g/m3 based on a plant-growth-limiting concentration (Hagen et al., 1967).
The total oxygen demand (TOD) is the sum of the BOD and the nitrogenous oxygen demand (NOD) and plant requirement. The NOD is defined as:
NOD = 4.56 x nitrifiable nitrogen (8.17)
Nitrifiable nitrogen is the ammonium concentration, which is often insignificant when compared to high BOD waste streams. Thus, the TOD is defined as:
From the TOD, the time required to diffuse an equivalent amount of oxygen can be determined. The diffusion equation follows:
NO2 = Flux of oxygen crossing the soil surface (g/m2).
CO2 = Vapor phase O2 concentration above the soil surface (310 g/m3).
Cp = Vapor phase O2 concentration required in soil to prevent adverse yields or root growth (140 g/m3).
Dp = Effective diffusion coefficient = 0.6(s)(DO2), where s is the fraction of air-filled soil pore volume at field capacity, and DO2 is the oxygen diffusivity in air (1.62 m2/d). t = Aeration time = cycle time - infiltration time.
Equation 8.6 can be solved with respect to time:
Cycle time is a function of required aeration time plus the time for the soil to reach field capacity. The time to reach field capacity is estimated with the infiltration time calculated by dividing the depth applied by the steady-state infiltration rate:
where ti = Time to infiltrate (hr). d = Depth (cm).
I = Steady-state infiltration rate (cm/s).
Numerous variables are involved in determining the oxygen balance, all of which must be evaluated on a site-specific basis. An important point to note is that supplemental irrigation water without a significant oxygen demand can increase the required cycle time due to increasing drain time. The time required for the upper zone of the soil to drain is a function of climatic conditions and the depth of the wastewater applied. To achieve the desired loading in surface applications, mixing of supplemental water is often required because of larger applications. Most surface applications cannot apply less than 7.6 cm (3 in.) in a uniform manner.
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