Organic Waste Recycling

Land treatment of wastewater 405

Oxygen at the soil surface must diffuse into the soil layer or waste-soil
matrix, depending on the nature of the wastewater application and soil-water
migration of liquid and waste organics (Figure 8.7). The transfer mechanism is
oxygen diffusion. Because soil pores are usually smaller and having solids
deposits, the diffusion of oxygen into soils may be the rate limiting step in
satisfying the waste oxygen demand and maintaining aerobic soil conditions.
The phenomena of transfer of photosynthetic oxygen from the crop's leaves to
the root zone, similar to that of aquatic weeds (see Chapter 7), is not yet well
understood.
As a result of organic matter decomposition, elements such as nitrogen,
phosphorus, and sulfur are converted from organic to inorganic forms. Many of
these mineralized constituents can be assimilated by plants. Crops are an
essential part of the SR and OF processes (see Table 8.2).

Figure 8.7 Schematic of oxygen transfer to stabilize organic compounds applied to land

The biological nitrification processes in the soil produce nitrate from
ammonia and organic nitrogen under aerobic conditions. However, nitrate
compounds can be reduced into nitrogen gas under anoxic conditions as a result
of denitrification.
It is possible to consider both gaseous nitrogen losses (volatilization and
denitrification) and nitrogen removal by plant uptake as control mechanisms for
the nitrogen in the applied wastewater.
Crop selection and management are important components of the wastewater
irrigation system. Plant uptake of nitrogen is in the range of 100 to 400 kg per
ha per growing season, depending upon specific crop and management
techniques. Data on nutrient (N, P, K) uptake rates for selected crops are given
in Table 8.7. It appears from this table that forage crops have ability to uptake
nutrients better than field crops.

Atmospheric oxygen

diffusion into matrix

Photosynthetic

oxygen transfer to

matrix

ATMOSPHERE

OXYGEN

WASTE SOIL MATRIX