Figure 1: Possible sources of heavy metal in sewage sludge

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).

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