Environmental Engineering Reference
In-Depth Information
phosphorous is either taken up by plants and incorporated into organic phosphorous or
becomes weakly or strongly adsorbed onto aluminum, iron and calcium salt surfaces
depending on pH. Under acidic conditions (pH < 6), phosphorous is precipitated by
aluminum and iron salts. Under alkaline conditions (PH > 8), the presence of calcium and
magnesium cations in soils lead to the formation of phosphorous precipitates in the form
of salts of these ions. Thus the control of phosphorous concentrations in groundwater
depends predominantly on the control of solubility of low soluble phosphate minerals
present in the soil layer. Long-term application results in the top 30 cm of the soil profile
becoming saturated with phosphorous due to adsorption, greater bioactivity and
accumulation of organic matter (Metcalf & Eddy 1991). In a land treatment system, the
major removal processes of phosphorous are precipitation, sorption and plant uptake.
2.2.2 Pathogens
The potential for pathogen transmission exists During irrigation with sewage, if the
system is not properly designed or managed. Domestic sewage may contain four types of
pathogens, namely, eggs of helminthes (worms), protozoa, bacteria and viruses. The
modes of transmission of pathogens include groundwater, surface runoff, direct contact
and consumption of raw crops. However, very few diseases outbreaks have been
associated with sewage application on land, except where inadequately treated sludge
was applied to gardens or other crops that were eaten raw (Pescod 1992). Bacteria and
viruses may be transported with percolating water into groundwater and subsequently
ingested, causing infection. This is in agreement with WRC (1997) observation that this
pathway posed the greatest risk factor to human health.
The fate and transport of microorganisms in groundwater is not well documented, with
the little documentation available indicating that some microorganisms are able to leach
into groundwater, albeit with reduced infection capability (USEPA 1992). Pathogens are
removed by predation, entrapment, ad-sorption and die-off in the soil matrix as the
wastewater flows through the soil matrix. Many researchers have used advection and
dispersion mathematical models to predict the movement of pathogens (Fetter 1994). The
models have a limitation in simulating the behavior of microorganisms in groundwater,
as they assume that microorganisms behave as a dissolved pollutant, while their behavior
actually is closer to a colloidal model (Domenico & Schwartz 1997).
Microorganisms are retained in the soil matrix by filtration, sedimentation and
adsorption, whereas their movement is directly related to the hydraulic infiltration rate
and inversely to the size of soil particles and concentration of cations in the solution
(Pescod 1992). The survival of pathogens depends on different factors such as:
• Type of organism;
• Temperature;
• Soil moisture and type of soil;
• Organic matter, and
• Presence of other organisms.
Pathogens survive longer in moist soils with a neutral pH, low temperature and in the
presence of appropriate organic matter as a source of food. In the first 0.5 m of the soil,
pathogens are retained by soil filtration, inter-grain contacts, sedimentation and
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