Environmental Engineering Reference
In-Depth Information
content decreased. Extractability of Fe, Cu, Mn, Zn, and Pb increased, when sludge
was applied, as compared to the unamended control.
Relatively high rates of sludge application increased the soil cation exchange
capacity, which helped to retain essential plant nutrients within the rooting zone.
Such nutrients are retained as a result of additional cation binding sites being
created (Soon
1981
). Such responses, however, depend upon the sewage soil ratio.
The higher the proportion of organic matter in sludges, the more bulk density was
decreased and aggregate stability increased (Ojeda et al.
2003
; Table
2
). Sludge-
related improvements in soil physical properties also increased water-holding
capacity by promoting higher water retention in sludge-amended soils (Ojeda et al.
2003
; Table
2
).
Analysis of sewage-sludge-fed agricultural soil layers (0-15 and 15-30 cm)
around Calcutta, collected from different upland and lowland sites, showed slightly
alkaline pHs (Maiti et al.
1992
). Subsurface soil had a higher pH (7.5) than did the
surface ones (7.3). The soil of upland sites had slightly higher CEC [18.4-22.8
(cmol (p
+
kg
−1
))] than did lowland ones [15.1-19.1 (cmol (p
+
kg
−1
))]. Surface soil
contained higher amounts of organic carbon (1.31%) than did subsurface ones
(1.16%). Ca
+2
was the dominant cation [11.5-19.3 cmol (p
+
kg
−1
)] in the sewage-fed
soil, followed by Mg
+2
[2.1-2.7 cmol (p
+
kg
−1
)], Na
+
[0.4-0.9 cmol (p
+
kg
−1
)], and K
+
[0.1-0.3 cmol (p
+
kg
−1
)]. Available N and P were at moderate to high levels.
Sewage sludge amendment always poses an environment risk, resulting from
nutrient imbalances and toxic element accumulation, and leaching. Transfer of
metal from sewage sludge to soil and later to groundwater via leaching poses poten-
tial health and environmental risks from plant uptake (McBride et al.
1997
; Bhogal
et al.
2003
; Mahdavi and Jafari
2010
). Korboulewsky et al. (
2002
) studied the effects
of sewage sludge composts applied at the rates of 10, 30, and 90 tons ha
−1
fresh wt,
on a vineyard in southeastern France. These authors quantiied the rate of in situ N
mineralization, soil organic matter levels, and evaluated selected environmental
risks, including N and P leaching rates, and levels of heavy-metal accumulation in
soil. It was found that soil organic matter levels increased at all the treatment doses,
but neither total nor available heavy metal concentrations increased. Because the
sewage sludge studied contained very low levels of heavy metals, and existed mainly
in nonextractable and nonexchangeable forms (Breslin
1999
), composting it reduced
the heavy metal availability by adsorption or complexing processes with humic
substances. The levels of mineral nitrogen present increased in the plots in which
the topsoil was amended during the irst and the second summers. The risk of N
leaching was very low in contrast to P at the recommended sludge amendment rate.
The increase of P content in amended soil was signiicant in both top and subsoil
layers in all treated plots. The maximum increase in P content occurred at the high-
est rate of sludge applied. However, at lower sludge amendment rates no signiicant
differences were observed. It has been shown, in column leaching (Ashworth
and Alloway
2004
) studies, and in batch (Burton et al.
2003
) experiments, that
heavy metal ions may leach more easily in the presence of sewage sludge than in its
absence. Moreover, as dissolved organic matter (DOM) concentration increases,
the movement and translocation of heavy metals in soil increases; by contrast, an
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