Fig. 3

Critical, strategic and precious element content (in mg per kg of sludge DW) of the studied WWTPs distributed in groups according to their minimum content. A Min content above 1000 mg/kg DW sludge. B Min content in the range 100 to 1000 mg/kg DW sludge. C Min content in the range 20 to 100 mg/kg DW sludge. D Min content in the range 5 to 20 mg/kg DW sludge. E Min content in the range 1 to 5 mg/kg DW sludge