Heavy metal(loid)s and nutrients in sewage sludge in Portugal – Suitability for use in agricultural soils and assessment of potential risks

The presence of heavy metal(loid)s in sewage sludge is a cause of concern and an obstacle to its agricultural valorisation. This study analysed the elemental composition of sewage sludge from 42 Portuguese wastewater treatment plants (WWTPs) during summer and winter, investigating heavy metal(loid) contamination, nutrient content, and potential risks related to sludge application to agricultural soils. Levels of 8 heavy metal(loid)s were investigated, ranging from not detected (Hg) to 5120 mg kg^-1 dw (Zn), decreasing in the order Zn > Cu > Cr > Ni > Pb > As>Cd > Hg. The legal requirements for agricultural use of sludge were overall met, but elevated levels of Zn and Cu, linked to industrial sources, exceeded the permitted limits in 3 WWTPs. On average, N, P, K, Mg, and Ca comprised 80 % of the sludge nutrient profile. No seasonal variations were found, but sludge composition varied with WWTP size, wastewater origin, and between thickened and digested samples. Environmental hazard indicators showed significant sludge contamination with Zn, Cu, and Cd. However, the geoaccumulation index, potential ecological risk indicators, and risk characterization ratios showed no significant risks to sludge-amended soils, assuming a single application of 5 tons ha^-1. Human health risk assessment for workers handling sewage sludge identified dermal contact as the main route of exposure, with non-carcinogenic risk for Cr and carcinogenic risk for Ni and Cr at the highest reported levels. Sewage sludge produced in Portugal was considered suitable for agricultural use, provided that it is closely monitored and well-managed to meet the needs of crops and receiving soils, while mitigating environmental risks.