Sludge is one of the principal byproducts of wastewater
treatment plants. It may be deposited in landfills, in the sea
(ocean disposal), under the ground, or (to a certain extent)
in the air as a consequence of incineration. In addition,
sludge can be used (recycled) in various ways, including its
use as fertilizer, as a soil conditioner in farmland, in forests,
and in home gardens, as well as in concrete and in
bituminous mixes in buildings and roads (Ødegaard et al.,
2001). Oceanic disposal of sludge is currently forbidden.
Landfilling of sludge has hitherto been an inexpensive
method of disposal, but in the future, the use of landfills
will have the lowest priority in the waste-handling
hierarchy (i.e., waste minimization first, then reuse;
recovery; recycling, composting, and energy production;
and disposal last of all), and will only be chosen when there
are no other ways to dispose of the sludge. The current
trend is towards agricultural use (recycling) and incineration.
Incineration reduces the sludge to ash, which can then
be used for landfilling, but in most cases, supplementary
fuel is needed to burn the sludge, which makes this method
less economical. For these reasons, recycling of sludge for
agricultural purposes seems to be an appealing solution for
the sustainable management of sludge in the coming years.
Recycling of sludge for agricultural purposes has many
beneficial effects. These benefits include supplying nutrients
(nitrogen, phosphorus, and micronutrients) to the crops,
improving soil physical properties, and increasing soil
organic matter content (USEPA, 1983). On the other hand,
there are concerns about the presence of certain toxic
elements in the sludge. The application of sludges in
agriculture may lead to a risk for humans and the
environment as a result of heavy metals and toxic organic
compounds accumulating to levels high enough to cause