Occurrence of some heavy metals in sewage sludge in some Urban Areas and their uptake by amaranthus (amaranthus hybridus}
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Date
2001
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Publisher
SUA
Abstract
A study was conducted to determine the occurrence of some heavy metals in
sewage sludges from Morogoro and Dar es Salaam urban areas and their uptake by
Amaranthus hybridus. A field study was conducted at the Horticultural Unit,
Sokoine University of Agriculture Morogoro whereby, the soil was characterised
for its physical and chemical properties prior to planting of the test crop. The
analytical data showed that the experimental soil was texture mildly-alkaline with
a pH value of 7.6, very low % O.C. The textural class of the soil was sandy clay
loam. Available and total Cu, Zn, Mn. Cd and Pb ranged from medium to high as
compared to established critical levels. The sewage sludge was collected from the
disposing sites in Morogoro and Dar es Salaam. Prior to incorporation into the soil
the sewage sludge was air dried and characterised for its chemical properties . The
results showed that the available and total Cd, Mn, Cu, Zn and Pb ranged from
medium to high as compared to established critical ranges. A field study was
conducted to study the uptake of some heavy metals and other plants nutrients. In
the field studies a Randomised Complete Block Design (RCBD) was used. The
rates of sewage sludge application on air dry basis were 0, 10, 20, 40, and 60
tons/ha. Amaranthus seeds were measured by using 35 cm3 container and sowed in
1 m2 plots which were separated by 0.5 m strips while the blocks were separated
by 1 m strips. The uptake of N, P, K, Mg, Ca and heavy metals (Cu, Zn, Mn, Cd,
Pb) were determined. Application of sewage sludge resulted in a marked increase in metal content in Amaranthus plant especially for the highest rates of application
during the first and second plantings. The uptake of other essential plant elements
increased with increasing rate of sewage sludge application. No adverse effects due
to application of sewage sludge were observed. At the end of the first growing
season, the soil was analysed to evaluate the residual effects of sewage sludge on
soil chemical properties. The results showed that sewage sludge application
resulted in a marked increase in total and extractable metals as well as essential
elements in the soil, except for total and extractable Mn. The application of sewage
sludge resulted in a significant increase (p < 0.05) in the concentrations of Cd, Zn,
Cu, and Pb. However, the metal content in the soil was below the limits generally
accepted as toxic for most agricultural soils. A second planting was done in the
same plots without further application of sewage sludge in order to determine the
residual concentration of heavy metals in the soil from the previously applied
sewage sludge. The results indicated that, the residual metal concentration in the
soil had significantly increased. Plant dry matter yield of Amaranthus was
determined for the first and second harvests. The application of sewage sludge
from Dar es Salaam increased dry matter yield significantly (P < 0.05) at 60
tons/ha. The maximum yields obtained were 3.66 and 2.4 tons/ha for the first and
second harvests respectively. The Dry matter yield as the result of application of
Morogoro sewage sludge reached a maximum of 2.16 and 1.79 tons/ha at 60
tons/ha for the first and second crop respectively. Generally, the increase in the
Description
Dissertation
Keywords
Sewage sludges, Horticultural Unit, Amaranthus hybridus, Sewage sludge