Environmental Engineering Reference
In-Depth Information
Organic matter that is added to soil in the form of sewage sludge composts
improves several soil properties, including bulk density, porosity, and water-holding
capacity (Ramulu
2002
; Table
2
). The chemical properties of sludge-soil mixtures
not only depend on the properties of soil or sludge or sludge application rates but
also on soil pH and on how these components interact (Parkpain et al.
1998
). Epstein
(
1975
) conducted a study to evaluate the effect of 0.5% sewage sludge application
to soil on water retention, hydraulic conductivity and aggregate stability; results
showed that raw and digested sludge improved total soil-water retention capacity,
with the greatest enhancement occurring in raw-sludge-amended soil. Moreover,
the sludge added to soil resulted in a signiicant increase in soil hydraulic conductiv-
ity after 27 days of incubation. The highest percentage of stable aggregates was
reported in a raw sludge treatment that occurred during the irst 118 days of incuba-
tion. After 175 days, the percentage of stable aggregates for sludge treatments
remained the same, averaging 34 vs. 17% for untreated soil (Epstein
1975
).
In a ield experiment designed to study the effect of long-term sewage sludge
application on the chemistry and biology of soils, Hậni et al. (
1996
) reported
increased nutrients (mainly P) and heavy metals from the agricultural use of high
levels of sewage sludge. According to Hậni et al. (
1996
), the soluble fraction of
heavy metals, as well organic pollutants in soil, is a determining factor in deciding
the stage at which heavy metal toxicity to soil microorganisms or microbial pro-
cesses in soil is likely to be evident.
3.2
Chemical Properties
According to Hue and Ranjith (
1994
), the concentrations of metal in sewage sludge
depends on factors that include both the (1) origin of the sewage and the (2) sewage
treatment processes used. The bioavailability of sludge-borne metals in soil is inlu-
enced by several soil properties, including pH, redox potential (Eh), sesquioxide
content, and organic matter content, as well as the rate of sludge application (Hue
and Ranjith
1994
). Adams and Sanders (
1984
) evaluated the effect pH has on the
release rates of Zn, Cu, and Ni from sewage sludges. These authors reported that the
concentration of metal released from sewage sludge to the supernatant liquid
increased as pH decreased below a threshold value; this threshold value was 5.8 for
Zn-loaded sludge, 6.3 for Ni-loaded sludge, and 4.5 for Cu-loaded sludges. The
metal content of the supernatant was small and relatively constant at pHs above the
aforesaid values. In speciation experiments, the proportion of soluble Cu present as
Cu
+2
in CuCl
2
was related to pH, whereas the proportion of soluble Zn present as
Zn
+2
was scarcely correlated with pH (Adams and Sanders
1984
).
Hernandez et al. (
1991
) conducted a study to analyze what inluence sewage
sludge application had to a Calciorthid soil on the soil availability of macronutrients
(N, P, and K) and heavy metals (Fe, Cu, Zn, Mn, Ni, Cr, Cd, and Pb). The total N
and extractable N and P content increased in the sludge-amended soil, whereas the
extractable K remained unaltered. The Cu, Zn, and Pb levels increased, while Fe
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