Variability of metal and metalloid content in sludge as affected by sources and seasons

This study evaluated the effect of seasonal variation on metal and metalloid content in sewage sludge collected from eight different sewage treatment plants (STPs) of Delhi-NCR during summer (June, 2018), rainy (August, 2018) and winter seasons (January, 2019). The micronutrients and pollutant elements content in the sludge varied greatly among the sources and between the three seasons. On an average, micronutrients viz. iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) content in the collected sludge samples in the sources and seasons were 1.48%, 236, 1216 and 314 mg kg⁻¹, respectively. The pollutant elements, i.e. cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni) and arsenic (As) content in sludge samples were 2.85, 48.4, 88.6, 38.6 and 2.49 mg kg⁻¹, respectively. Sludge collected from Delhi Gate Nala, Keshopur and Nilothi tended to have higher pollutant elements compared to the Narella, Pappankallan and Okhla sludge mainly because Delhi Gate Nala, Keshopur and Nilothi sludge is derived mainly from industrial area while the other three sources are a combination of domestic and industrial residues, leading to lower pollutant element concentration. Respect to seasonal variation, higher concentration of micronutrients and pollutant elements were recorded in rainy and summer season, respectively. Content of the metal and metalloid recorded in sludge was within the permissible limit as prescribed by USEPA (75, 85, 4300, 840, 75, 420 and 7500 mg kg⁻¹ for As, Cd, Cu, Pb, Ni, and Zn, respectively). It may be concluded from the present study that sewage sludge generated from STPs of Delhi-NCR can be used as potential source of plant nutrients, particularly in the face of escalating price of chemical fertilizers.