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

Abstract

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 (FeCl3) 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, FeCl3 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 FeCl3. 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 FeCl3 could notably promoted the dehydration and decarboxylation reaction and prevented the reservation of N content. This study suggested that co-HTC with FeCl3 was a promising strategy to produce superior solid fuel with lower risk of HMs.