Ecological and human health risk assessment of heavy metals in sewage sludge to ensure safe agricultural use

The use of sewage sludge in agricultural lands is one of the sludge management options that must be ensured to avoid environmental and health consequences. Considering the content of heavy metals in sewage sludge, the aim of this study was to measure the concentration of these elements in sewage sludge and assess the ecological and health risks resulting from its use in agricultural lands in southwestern Iran using Monte Carlo simulation. Eighteen two-liter samples of sewage sludge from Behbahan wastewater treatment plant were sent to the laboratory for analysis of heavy metal concentrations over a six-month period. The results were used to calculate ecological risk based on single-factor pollution index (PI) and Nemerow’s synthetic pollution index (PN), as well as to assess non-carcinogenic risk based on hazard index (HI) and carcinogenic risk based on total cancer risk (TCR). Concentration of the total studied heavy metals in the analyzed samples was 1871.89 mg/kg. The maximum and minimum concentrations detected were associated with iron (1093.26 mg/kg) and cadmium (0.40 mg/kg), respectively. The heavy metal content of the studied sewage sludge produced PI and PN values less than 1, indicating a safe ecological risk class. Also, calculated HI less than 1 indicated a insignificant non-carcinogenic risk level from the use of sewage sludge with the detected heavy metal content for adults and children. The calculated TCR (3.66E-06) indicated an acceptable carcinogenic risk. Under the exposure assumptions and toxicity benchmarks used in this screening assessment, the modeled non-carcinogenic and carcinogenic risks associated with the analyzed sludge concentrations were below the selected risk-management criteria and can be an option considered for sludge management by local policymakers. These results should not be interpreted as evidence that agricultural application is universally risk-free, particularly because long-term soil accumulation and food-chain exposure were not fully characterized. So, long-term monitoring of background concentrations of heavy metals in soil is necessary due to the persistence of these elements in environment.