Geoscience Reference
In-Depth Information
planar surfaces, which leads to a high adsorption capacity. In the case of smectite,
hydraulic conductivity decreases through partial blocking of pores by short-dis-
tance migration of dispersed particles.
18.3.3 Impact of Sludge and Compost
Land disposal of untreated effluents and of sludge from wastewater purification
plants, as well as urban waste compost, constitutes an additional factor leading to
anthropogenically induced changes in soil and subsurface physical properties.
Application of sludge obtained from wastewater purification treatments was
considered, in the last half of the past century, to be a sound method for increasing
soil fertility and improving soil physical properties.
Some of the organic carbon found in effluents, sludge, and urban waste compost
is resistant to degradation. The organic matter content of the soil increases under
repeated application; in this case, irreversible alteration of the physical properties
is possible. The organic components of sewage effluents, sludge, and compost lead
to changes in bulk soil density, aggregation, and porosity, affecting also the water
transmission properties of the native soil-subsurface regime.
Many studies have shown an increase in the solid phase organic content after
sludge application, which is usually explained by a parallel decrease in the bulk
density and an increase in the total porosity. For soils with a wide range of
textures, hydraulic conductivity in sludge-amended plots is usually higher than
that in control plots, although a large quantity per unit area of sludge or compost
additions is necessary to induce statistically significant changes in hydraulic
conductivity (Metzger and Yaron
1987
, and references therein).
Bulk density and porosity of the soil-subsurface solid phase is reduced after
sludge application on land surface. Several studies have reported that the bulk
density of a sludge-amended sandy soil is negatively correlated to the organic
carbon content of the applied sludge, and to the application rate, and positively
correlated to the soil porosity (Lindsay and Logan
1998
; Bahremand et al.
2003
).
Based on analysis of 12 studies dealing with short- and long-term application of
sludge and compost on bulk density, Khaleel et al. (
1981
) proposed an empirical
relationship between the soil organic carbon content and the reduction in bulk
density.
The effect of sludge application rate on the bulk density and porosity of a silt
loam from Columbus, Ohio, USA, is shown in Fig.
18.35
. The soil porosity
increased approximately linearly (r
2
= 0.728; P \ 0.01), while the bulk density
decreased approximately linearly (r
2
= 0.815; P \ 0.01) as a function of sludge
application rate. Lindsay and Logan (
1998
) considered the effect of sludge on soil
physical properties to depend largely on the rate of sludge decomposition and on
the contribution to soil organic carbon. In the reported cases, these effects could
still be seen four years after sludge application. This finding is in accord with the
results of Paglia et al. (
1981
), who found that six months after sludge application
