Land Application of Sewage Sludge: Physicochemical and Microbial Response - Reviews of Environmental Contamination and Toxicology - page 45

Characterization of sewage sludges from Calcutta, India was performed by Maiti
et al. ( 1992 ) to assess the value of the sludge for plant fertilization. The sewage
sludges analyzed were neutral to slightly alkaline and had higher salt content in
winter than during the monsoon season (Maiti et al. 1992 ). Moreover, the cation
exchange capacity (CEC) was reported to be higher during the monsoon season.
Exchangeable Ca +2 was the prevailing cation found in the sludges analyzed, fol-
lowed by Mg +2 , Na + , and K + . The sludges of Calcutta, India were reported to be rich
in organic carbon and available N (Maiti et al. 1992 ).
A comparison of the physicochemical characteristics of sewage sludge collected
from different countries is presented in Table 1 . Results clearly show that sludge pH
varies, and may be either acidic or alkaline (Parkpain et al. 1998 ; Martinez et al. 2002 ;
Singh and Agrawal 2009 ). The organic matter content also varied considerably. By
contrast, the content of total N and P did not vary that much (Table 1 ). Among heavy
metals present, levels of Cu, Zn, and Mn were variable, whereas the Cd content was
more consistent among samples analyzed. The sewage sludge collected from Dindigul
(Tamil Nadu), India was recommended for land application, since this sewage sludge
had nearly neutral pH, high organic matter, good N, P, and Ca content, and was free of
toxic heavy metals such as Cr, Pb, and Hg (Nandakumar et al. 1998 ).
Singh and Agrawal ( 2010a, b, c ) characterized the sewage sludge from Dinapur
Sewage Treatment Plant (DSTP), Varanasi, India. This sewage sludge was neutral in
pH and had high electrical conductance and high concentrations of organic C, total N,
available P, Fe, Na + , K + , Ca 2+ , and Mg 2+ (Singh and Agrawal 2010a, b, c ) (Table 1 ).
Zn was present at the highest concentration in this sewage sludge, followed by levels
of Cu, Mn, Cd, Pb, Ni, and Cr (Singh and Agrawal 2010a, b, c ) (Table 1 ).

3 Effects of Sewage Sludge Application on Soil Properties

The disposal of sewage sludge by applying it to land is increasing in popularity
because of the potential it offers to recycle valuable components (e.g., organic mat-
ter, N, P, and other plant nutrients) (Martinez et al. 2002 ; Singh and Agrawal 2008,
2010b, c ). The aapplication of sewage sludge to agricultural soil not only enables
nutrients to be recycled, but may eliminate the need for commercial fertilization of
cropland (Sommers 1977 ; Singh and Agrawal 2007, 2009 ). Because sludges are
organic fertilizers, application of them to soils increases soil fertility over time
(Archie and Smith 1981 ). Unwise sewage sludge amendment practices, however,
may disturb soil properties, especially when high concentrations of metals and toxic
constituents are present in the sludge.

3.1 Physical Properties

The physical condition of soils has been improved by application of sewage sludges
(Epstein 1975 ; Table 2 ). An increase in soil pH has been reported to occur in soils
to which municipal sewage sludge was applied (Tsadilas et al. 1995 ). Cases of soil