Environmental Engineering Reference
In-Depth Information
The fate of pollutants in the soil matrix can be traced in the three zones of the
subsurface, which are: the infiltration zone, the unsaturated zone and the saturated zone
(or aquifer). The last two form the vadose zone. A schematic representation is shown in
Figure 9.1.
Most of the sewage treatment is performed in the first two zones with the saturated
zone offering dilution and transportation of pollutants away from the source. The
infiltration zone is biologically active and acts as a physical, chemical and biological
filter to remove suspended solids and organics from the wastewater. The filtered solids
are a source of nutrients for the active biomass in the zone. Significant amounts of
chemicals are biologically degraded by microorganisms, or transformed by chemical and
physical processes, and plants take up some of the end products of these processes. The
unsaturated zone acts as a delay barrier for the dissolved pollutants before they reach the
saturated zone. The main processes in the unsaturated zone are solution, dilution and
hydrodynamic dispersion, which result in the attenuation of pollutants. Pathogens die off
in this zone due to anoxic conditions, temperature, starvation and predation. Wastewater
movement in the unsaturated zone is generally vertical with limited dispersion. Pollutants
leave the site in the saturated zone through horizontal flow of groundwater (Freeze &
Cherry 1979).
These natural processes do not attenuate all contaminants, as some remain unchanged
and others are changed to more harmful products. An example for such a transformation
in aquatic ecosystems is the highly toxic methylmercury formed from elemental mercury
by aquatic microorganisms (Fetter 1994).
2.2
Pollutants transformations in soils
2.2.1 Nutrients
2.2.1.1 Nitrogen
The major plant nutrients in sewage sludge are nitrate, phosphate and potassium
compounds. Typical amounts of nitrate and phosphorous in digested sludge are 3% and
2.5% of TS (Pescod 1992). In most land application systems, sludge provides sufficient
nutrients for good plant growth. Nutrient uptake rate by plants is a key parameter for
sludge loading rates in land application systems. A typical rate of nutrients plant uptake
for selected crops is presented in Chapter 10 (Table 10.2).
Nitrogen is present in soils in different forms. The most stable and predominant form
of nitrogen in soil and groundwater is nitrate (NO
3
-
), although it also occurs in other
forms as dissolved ammonium (NH
4
+
), nitrite (NO
2
-
), ammonia (NH
3
), nitrous oxide
(N
2
O) and organic nitrogen. The conversion of organic nitrogen to ammonia is known as
ammonification. Nitrification is the process of converting ammonia to nitrate by
biological means. Both processes occur in the soil above the water table in the presence
of oxygen and organic matter (Freeze & Cherry 1979). Organic nitrogen is readily and
rapidly nitrified biochemically in aerobic soils. Nitrification is a two-stage process,
executed by two different types of microorganisms in the presence of oxygen, as shown
