Heavy metals in sludge from the sewage treatment plant network: a tool to evaluate source and risks of heavy metals to land application

Abstract

ABSTRACT The aim of the study was to evaluate heavy metals concentration based on the total and chemical fractions of sewage sludge, to identify the sources of heavy metals and to assess ecological risks. Thirty-six anaerobic mixed sludge samples were collected from six different types of sewage treatment plant network in Dhaka city, Bangladesh. Total and fraction concentration of heavy metals in sludge samples were analysed by inductively coupled plasma mass spectrometer following acid digestion. The mean ranges of chromium (Cr), nickel (Ni), copper (Cu), arsenic (As), cadmium (Cd), lead (Pb), iron (Fe), manganese (Mn) and zinc (Zn) in the sludges were 237.3–381.8, 108.5–220.2, 116.1–207.2, 14.9–56.1, 0.68–2.86, 72.4–172.0, 12055 –20,015, 64.8–170.6 and 117.5–178.5 mg/kg, respectively. The data of geochemical speciation showed that Fe, Zn, Ni, Cu and Mn were associated with the residual fraction, whereas As, Cd and Pb showed predominant proportion in the exchangeable fraction, indicating the severity of risk by As, Cd and Pb to the surrounding ecosystem. The findings of the study revealed that heavy metals posed moderate to considerable ecological risk. Anthropogenic activities including industrial wastes and agricultural chemicals are substantial sources for heavy metals in the sludge of STP network in the study area.