Analysis of metals and EOX in sludge from municipal wastewater treatment plants: A case study

The monitoring of extractable organic halogen (EOX) and heavy metal contents in sludge coming from 10 different municipal wastewater treatment plants (MWWTP) located in Friuli Venezia Giulia (Italy) is reported. In this work, sludge samples drawn from sludge treatment units have been digested and analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) for metal evaluation. Samples were also extracted and analyzed by microcoulometric titrations, following modified DIN 38414 T17 standard, for EOX analysis. Analytical results showed a slight enrichment of the contents of certain metals (Cd < 2 mg/kg, Cr < 51.5 mg/kg, Cu < 105.8 mg/kg, Hg < 1.4 mg/kg, Ni < 35.9 mg/kg, Pb < 58.7 mg/kg, Zn < 410.1 mg/kg, Ba < 317.1 mg/kg, Co < 1 mg/kg, Mo < 5 mg/kg, Mn < 106.7 mg/kg), so almost all of the sludge would be suitable for agricultural use following Italian and European regulations. The evaluation of EOX was carried out by using hexane and ethyl acetate as extraction solvents, and a measurable organic halogen content (ranging from 0.31 to 39.5 mg Cl/kg DM) was clearly detected in the sludge. The lowest concentrations of EOX were found in sludge coming from the smallest MWWTPs, which is to be considered more suitable for agricultural use. Additionally, analytical assays on composts, peat and soils were performed to compare EOX concentrations between these matrices and sludge.