| dc.description.abstract | The use of sewage sludge as a manure has for
a long time been considered as the most economical
method for its disposal. However, while serving
as a good source of plant nutrients, application
of sludge on agricultural soils also increases
the heavy metal content of the soils. Excessive
amounts of some of these heavy metals can be
phytotoxic and may, in some cases, enter the food
chain and thus end up being toxic to animals or
human beings feeding on such foods.
In this project, sewage sludge from Kariobangi
Sewage Treatment Works, in Nairobi, has been
analyzed for its heavy metal content. These
analyses have been performed using x-ray fluorescence
technique. Atomic absorption spectroscopy method
has also been used, for some of the elements, for
comparison. Samples analyzed included the sludge
itself, soils on which the sludge has been
used, and plants grown on such soils. For
comparison purposes, soil samples, plants from
areas where sludge is not used, fertilizers, and
manures were also analyzed.
The following metals have been shown to be
present in all the sludge samples: 'i';
Titanium (2220 - 8550 ppm); Chromium (90 - 530 ppm);
Iron (28000 - 61270 ppm); Nickel (26 - 97 ppm);
Copper (296 - 490 ppm); Zinc (1350 - 2400 ppm);
Gallium (11 - 24 ppm); Lead (248 - 580 ppm);
Rubidium (16 - 50 ppm); Strontium (79 - 170 ppm);
yttrium (29 - 67 ppm); Zirconium (210 - 460 ppm) ;
and Niobium (53 - 116 ppm). Soils and manures
were found to contain the same heavy metals at
slightly different levels. In addition to determining
the total heavy metal contents in the soil samples,
the available amount to the plant was also
determined for manganese, copper, iron, and zinc.
The available amounts were determined by atomic
absorption spectroscopy.
The results obtained in this work indicate
that the use of sewage sludge as a·manure results
in accumulation of s~me of these heavy metals
in the soil. The high levels of available
contents of these metals especially manganese and
copper in sludge - enriched soils may be phytotoxic.
Analysis of fertilizer samples indicated that some
metals, such as strontium, may accumulate in soils
if extensive use of fertilizers is practised.
Analysis of plant materials grown on sludge - enriched
soils showed that these plants, for example,
cowpeas, spinach, sugarloaf had high levels of
heavy metals.
While usage of sludge as a manure may still
be the best method of disposal there is a great
need to monitor regularly the levels of heavy
metals in the sludge, soils, and plants. Results
of such monitoring may then be used to decide when
and where sludge may be used as well as the type
of plants that may be grown on sludge - enriched
soils. | en |
