Figure 1: Possible sources of heavy metal in sewage sludge
Use of SS in Agriculture practices and its associated
threats
Impact of SS on soil fertility
Because sewage sludge has a high organic matter, content, its addition
to soil has a considerable influence on soil’s physical qualities and
has a beneficial soil conditioning effect. Because of the stability of
organic components in biosolids, the application of sewage sludge
enhances soil’s physical qualities such as aggregate formation and
stability, bulk density, porosity, and water holding capacity (Angin and
Yaganoglu, 2011, Usman et al., 2012). Wortmann (2005) found that
increasing SSA dosages for growing wheat reduced bulk density while
improving overall porosity. Several additional investigations have found
that when sludge application rates increase, so do the bulk density and
penetration resistance. (Garcia-Orenes et al., 2005; Cogger, 2005). The
increase in aggregate stability and total porosity is thought to be a
contributing factor to the decrease in bulk density. (Lindsay and Logan,
1998). The favorable effect of sludge application on soil physical
qualities is also dependent on soil type, with Improved soil physical
qualities as a consequence of SSA resulting in a higher soil filtration
rate, decreased surface runoff, and therefore reduced water erosion.
(Chambers et al., 2003). Epstein (1975) researched to investigate the
effect of 0.5 percent SS on soil water retention, hydraulic
conductivity, and aggregate stability and found that both raw and
digested sludge enhanced overall soil water retention capacity, with raw
sludge supplemented soil having the largest gain. Sludge addition
resulted in a considerable increase in soil hydraulic conductivity after
27 days of incubation. But the pH has severy decrease in the same case
(Epstein, 1975).
Because of its high organic matter, content, sewage sludge, soil
application aids in the creation of homes compounds and a variety of
other organic acids, which play an important role in the conditioning of
soil qualities. Tsadilas et al. (1995) discovered an increase in soil pH
when municipal sewage sludge was applied to soils. Epstein et al.
(1976), on the other hand, contradict the above result. Changes in soil
pH have been linked to sludge calcium carbonate concentration and acid
generation during the sludge breakdown (Sommers, 1977). In a study on
calcareous soil, Jamil et al. (2006) found that increasing dosages of
sewage sludge (up to 100 t ha–1) lowered soil pH from 8.2 to 8.0. In
principle, applying SS to agricultural soils raises heavy metal
concentrations in both soils and plants (Saha et al., 2015). A composted
sewage sludge (total carbon 28.6 percent, organic carbon 12.8 percent, N
1.5 percent, pH 6.65, EC 7.1 ds/m, P 0.12 percent) was amended into the
top 100 mm of each column at rates of 357, 223 and 22 t/ha dry wt (Gasco
et al., 2005). The soil was irrigated with 5000 ml of water to each
column, and 6-pore volume water was leached from each column. Finally,
soil samples were taken and examined for Cd, Cr, Cu, Pb, Ni, and Zn at
depths of 0–100 mm, 100–300 mm, 300–500 mm, and 500–840 mm.
Analyzing the leachates revealed the metal mass balance. The average
proportion of metals leached was determined to be in the following
order: Cd (0.04%) > Cu (0.02%) = Ni (0.02%) = Zn
(0.02%) > Pb (0.01%) = Cr (0.01 percent ). The mean metal
concentrations and maximum metal concentrations in leachates were found
to be lower than the limit values for irrigation water established by
Branson et al. (1975) for most metals, with the exception of Pb and Ni,
which were higher than the drinking water limits (WHO, 1996). Many
studies have found that applying sewage sludge increases microbial
biomass-C, basal respiration, N-mineralization, and some soil enzyme
activities, all of which promote nutrient recycling for crops (Göcmez
and Okur, 2010; Angin and Yaganoglu, 2011). The increased organic matter
content of the sewage sludge is linked to the favorable effect of the
treatments on microbial biomass and enzyme activity of the soils.
Although sludge application reduced the diversity of the microbial
population, the overall size of the soil microbial biomass and its
nutrient mineralization potential, as well as the potential activities
of soil enzymes were either unchanged or increased, according to
Banerjee et al. (2021). Studies could prove the increment of yeast,
pathogenic bacteria, fungus and viral population after application of
sewage sludge in landfilling (Singh, & Agrawal,2008, Ramulu, 2002). The
similar finding shows metal concentrations that are even below the
European Community’s maximum permitted concentration limits for metals
in sludge-treated soils have harmed microbial activity, populations of
cyanobacteria, Rhizobium leguminosarum bv. trifolii, mycorrhizae, and
total microbial biomass. For example, with soil metal concentrations of
(mg/kg): 127 Zn, 37 Cu, 21 Ni, 3.4 Cd, 52 Cr, and 71 Pb, N2fixation was
hindered by free-living heterotrophic bacteria. N2 fixation by
free-living cyanobacteria was inhibited by 50% at metal concentrations
of (mg/kg): 114 Zn, 33 Cu, 17 Ni, 2.9 Cd, 80 Cr, and 40 Pb. At soil
metal concentrations of (mg/kg): 130-200 Zn, 2748 Cu, 11-I5 Ni, 0.8 to
1.0 Cd and 130-200 Zn, Rhizobium leguminosarum by. trifolii populations
reduced by several orders of magnitude (McGrath et al. 1995). Sludge
application to soil resulted in reduction of diversity of the microbes
(Banerjee et al., 1997). The accumulation of harmful organisms in sewage
sludge is the most serious impediment to its use in agriculture.
Pathogenic microorganisms like viruses and protozoa can be found in
sewage sludge and have the ability to cause diseases in humans, animals
and plants (Usman et al., 2012).
Impact of SS on the plant
The bulk of a study conducted in India and elsewhere revealed that
sewage sludge land application increased crop yield; nevertheless, toxic
metals such as Cadmium, Ni, Pb, and Zn may accumulate in plant tissues
and pollute the food chain. (Singh and Agrawal, 2010a, 2010b). Crops
grown in soil treated with sewage sludge produce yields that are often
equal to or higher than those produced in soil treated with recommended
fertilizer applications (Epstein, 2003) unless the sludge has a high C
to N ratio, excess heavy metals, high soluble salts, or is applied at
extremely high rates (Warman and Termeer, 2005; Angin et al., 2012).
Although the SS over 4.5 kg m–2 boosted rice output,
it also raised the danger of food chain contamination since Ni and Cd
concentrations in rice grains were found to be beyond the Indian
acceptable limits of human consumption above 4.5 kg
m–2 SS and Pb concentrations above 6.0 kg
m–2 SS. (Singh and Agrawal, 2010a). However, in the
case of mungbean, Pb and Ni concentrations in the grains were greater
than the Indian permitted limits at and above 9.0 kg
m2 sludge application rates, while Cd concentrations
were higher than the Indian permissible limits at and above 12.0 kg
m2 sludge application rates. (Singh and Agrawal,
2010b). The increased availability of various important nutrients to the
plants may be one of the causes of the improvement in yield and
productivity of various crops as a result of sludge application. Because
sludge is a key source of nitrogen, phosphate, micronutrients, and
Organic Content, its application improved soil, Organic content and
enhanced the availability of plant critical nutrients in the soil,
notably nitrogen, increasing plant biomass output. This increase was
sometimes observed to be greater than those produced on prescribed NPK
treated soils. However, a significant rise in heavy metal concentrations
in the edible section of the plant was discovered, which should be
considered before proposing sludge application. As a result, before
adding sewage sludge to the soil, the dose should be calibrated based on
heavy metal and other pollutant concentrations for a certain crop. The
use of untreated sewage water in agricultural soils can cause metals to
build up to harmful levels in the topsoil and, as a result, in the crops
grown on it. Saha et al. (2015) examined several crops growing on
long-term sewage-irrigated sites in Kolkata, India, for heavy metal
accumulations and found that Colocasia and Amaranthus acquire the most
metal-based on total metal uptake. The comparison of mean heavy metal
concentrations (Zn, Cu, Pb, Cd, and Ni) in different crops with the
permissible limit of the Prevention of Food Adulteration (PFA) Act 1954
SEPA (2005) revealed that heavy metal concentrations such as Pb, Cd, and
Ni were above the permissible limit in all of the examined crops
generally grown in these areas (Table 2). Previous research has found
elevated amounts of heavy metals in edible food crops cultivated in
sewage-irrigated soils (Kharche et al., 2011). In continuation, a
comparative study has been also documented to showcase the existed
reports on heavy metals found from sewage sludge in different crops in
different countries (Table 2)
Table 2: Metal accumulation in different crops upon long term treatment
of sewage sludge