402 Organic waste reuse and recycling: technology and management
8.3.2 Chemical removal mechanisms
Chemical reactions in the soil affect the mobility of dissolved ions or
compounds with the result that some constituents are retained within the soil
profile for extended periods of time while the movement of others may only be
temporarily restricted. Liquid residence times for normal wastewater irrigation
rates are on the order of weeks, while some constituents may be retained in the
soil for a much longer period.
Adsorption and chemical precipitation are the most important chemical
reactions governing the movement of constituents in the irrigated wastewater
with cation exchange being the most important adsorption phenomenon. The
cation exchange capacity (CEC) of soils can range from 2 to 60 meq/100 g of
soil, with most soils having a CEC value between 10 and 30. The differences
occur because soils vary widely in their humus and clay content, the
components that have the highest CEC.
Typical soils have considerable capacity to adsorb many of the cations in
wastewater, including many of the metals, which may adversely affect the
health of human and animals eating the crops grown on the irrigated fields.
Cation exchange of ammonium nitrogen is a possible control mechanism for
nitrogen. However, the ammonium ion is biologically oxidized to nitrate in
aerobic soils. Nitrate is an anion and will move with the soil water.
Phosphate is the only anion appreciably retained in soil. The primary
mechanism is the formation of insoluble or slowly soluble precipitates.
In arid regions, wastewater irrigation rates may not be enough to avoid the
accumulation of sodium ions in the soil. Such accumulations can lead to a
degradation of soil structure and reduction in water percolation rates.
The relationship between the principal cations in wastewater (calcium,
magnesium, sodium, and potassium) is of importance. When the ratio of sodium
to the other cations, especially calcium and magnesium, becomes too high, the
sodium tends to replace the calcium and magnesium ions on clay particles. The
predominance of sodium ions on clay particles has the effect of dispersing the
soil particles and decreasing the soil permeability. To determine the sodium
hazard, the Sodium Adsorption Ratio (SAR) has been developed. It is defined as
follows:
[]()Ca Mg / 20.^5Na
SAR
+= (8.1)
Where Na, Ca, and Mg are concentrations of the respective ions in milli
equivalents per liter of water (meq/L).