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6.2 Sources of Soil Pollutants 157

biological process. The characteristics of the secondary sludge vary with the type
of biological process, and, often, it is mixed with primary sludge before treatment
and disposal. Land application of raw or treated sewage sludge can reduce signifi -
cantly the sludge disposal cost component of sewage treatment as well as providing
a large part of the nitrogen and phosphorus requirements of many crops. But it has
the risk of polluting the environment. Urban sewerage systems transport domestic

sewage, industrial effluents, and storm-water runoff from roads and other paved
areas. Thus, sewage sludge will contain organic waste material and traces of many
pollutants used in our modern society. These substances can be phytotoxic and
some toxic to humans (Dean and Suess 1985 ). Sewage sludge also contains patho-
genic bacteria, viruses, and protozoa along with other parasitic helminths which can
give rise to potential hazards to the health of humans, animals, and plants. A WHO
( 1981 ) report on the risk to health of microbes in sewage sludge applied to land

identified salmonellae and Taenia as giving rise to greatest concern. The numbers of

pathogenic and parasitic organisms in sludge can be significantly reduced before
application to the land by appropriate sludge treatment. Sewage sludge contains
useful concentrations of nitrogen, phosphorus, and organic matter. The availability
of the phosphorus content in the year of application is about 50 % and is indepen-
dent of any prior sludge treatment. The organic matter in sludge can improve the
water retaining capacity and structure of some soils, especially when applied in
the form of dewatered sludge cake. Sewage sludge should be subjected to biological,
chemical, or thermal treatment, long-term storage, or other appropriate processes
designed to reduce its fermentability and health hazards resulting from its use before
being applied in agriculture.
Compost made from lime dewatered sludge tends to have a high pH. When blended
with organic materials such as peat moss or soil, the resulting pH is generally between
7.2 and 8.0. Compost made from polymer dewatered sludge tends to have a pH
between 6.2 and 6.8. When blended with peat moss or pine bark, the resulting pH
is between 4.7 and 6.2, depending on the pH of the peat moss. Compost made from
processed garbage and polymer dewatered sewage sludge (municipal compost) has
a pH between 6.9 and 7.2 and also tends to have a high boron depending on the
bulking material. These composts are advised to apply in landscape plantings, for
shade trees, individually planted trees, flower gardens, etc. Sewage sludge is rich

in organic matter, nitrogen, phosphorus, calcium, magnesium, sulfur, and other
microelements necessary for plants and soil fauna to live. So it is characterized
by the large manurial and soil-forming value. Except the indispensable elements to
live, sludge can contain toxic compounds (heavy metals, pesticides) and pathogenic
organisms (bacteria, eggs of parasites) (Siuta 1999 ).
The chemical composition of sewage sludge is of great importance for developing
recommendations for the rates of sludge applications on agricultural land (Beltran
et al. 1999 ). At the present time, recommendations for sludge applications rates on
land are based on the fertilizer values (N, P, and K) and on the concentrations of
trace metals present in sludge (Delgado et al. 1999 ). The metals of primary concern
are Zn, Cu, Pb, Ni, and Cd which, when applied to soils in excessive amounts, may
reduce plant yields or impair the quality of food or fiber produced (Parr et al. 1989 ).