Co-hydrothermal carbonization of sewage sludge and swine manure: Hydrochar properties and heavy metal chemical speciation

• Sewage sludge is upgraded by co-hydrothermal carbonization with SM. • Evolution of heavy metals in hydrochar is elaborated. • The decarboxylation reaction is enhanced with the addition of FeCl 3 . • The weakly fractions of Mn, Zn, Cr, and Ni dropped during co-HTC. • The presence of FeCl 3 promote the decrease of Cu, Mn, and Zn. Alleviating the risk of heavy metals while improving fuel properties is one of the important challenges of sewage sludge (SS) disposal. In this work, ferric chloride (FeCl 3 ) was chosen as a typical additive (0.005–0.02 mol) to analyse its effect on the co-hydrothermal carbonation (co-HTC) of SS and swine manure (SM). During co-HTC, SS was converted into hydrochar with SM at different mixing ratio (30–70 %) at 230 °C. Chemical speciation of heavy metals (HMs) in hydrochars, along with fuel properties, were evaluated. The results showed that co-HTC was beneficial for the decline of weakly fractions of Cr, Ni, Mn, and Zn. Furthermore, FeCl 3 further decreased the risk of Mn and Zn without significantly expanding the pollution of Ni. More importantly, the risk of Cu was dramatically alleviated by the addition of 0.02 mol FeCl 3 . Compared with the hydrochar produced from SS, the fuel ratio, higher heating values (9.75–13.09 MJ/kg), and combustion behavior of the hydrochar derived from co-HTC were upgraded and improved. The FeCl 3 could notably promoted the dehydration and decarboxylation reaction and prevented the reservation of N content. This study suggested that co-HTC with FeCl 3 was a promising strategy to produce superior solid fuel with lower risk of HMs.