Environmental Engineering Reference
In-Depth Information
by equations 9.1 and 9.2. Nitrification rates are reported to be in the range of 0.010-0.016
gN/m
2
per day (Polprasert 1996).
(9.1)
(9.2)
Denitrification is the reduction of nitrate to nitrogen gas, and occurs in anaerobic
conditions. Conditions that are favorable to denitrification are:
• High organic content;
• Limited free oxygen;
• Fine textured soils;
• Moisture;
• Neutral to alkaline pH;
• Vegetative cover; and
• Warm temperature.
For the overland flow treatment system, denitrification and volatilization losses can range
from 20% to 30% of the applied nitrogen (Polprasert 1996). Nitrite is significantly more
toxic to humans than nitrate (DWAF 1996a).
Due to its anionic form, the soil exchange complex (which is mainly a cation
exchanger) very weakly sorbs nitrate and its movement in soil is affected to a limited
extent by exchange reactions (Foth 1984). Therefore nitrates are easily transported in
groundwater and leach freely, with their relative concentration and distribution in the soil
matrix being largely determined by the leaching fraction (DWAF 1996c). However, the
organic content and pH of soils also influence the nitrate concentration in the soil solution
(Barry et al. 1995). The major control on its transfer from soil to aquifer is the
permeability of the vadose zone (McLay et al. 2001). Nitrate is very mobile in
groundwater, because of its solubility. In a strongly oxidizing environment, nitrate is
stable in dissolved form, and moves in groundwater with no retardation or
transformation. Thus, it can migrate large distances in highly permeable soils or fractured
rock.
Nitrogen is an essential plant nutrient in the soil. However, at elevated concentrations,
in its various forms, it becomes a pollutant. Elevated concentrations of nitrate in water
consumed for potable purposes may result in methaemoglobinaemia (infantile cyanosis)
and carcinogenesis (gastric cancer). Ruminants are sensitive to nitrogen and trace
application of nitrogen to pastures can cause its accumulation to levels that are hazardous
to animals (DWAF 1996b).
2.2.1.2 Phosphorous
Phosphorous is one of the essential plant nutrients and is frequently a limiting factor in
vegetative productivity (Foth 1984). Phosphorous in natural treatment systems is
extracted by both biotic and abiotic processes. Biotic processes include plant uptake and
mineralization by biological means, while the abiotic processes include sedimentation,
sorption, precipitation and exchange processes between soil and water interface. Applied
