The characteristics of sewage sludge pellet biochar prepared using two different pyrolysis methods

Abstract

The aim of this study was to evaluate and compare the properties of sewage sludge pellet (SSP) biochar prepared using two different pyrolysis methods: (i) in a muffle furnace when feedstock was covered by a double layer of aluminium foil and (ii) in a tube furnace without any cover and under N2 atmosphere. The biochar was produced from dried municipal sewage sludge (MSS) pellets by pyrolysis at temperatures of 300, 400, 500 and 600 °C. The feedstock and different biochar properties (such as pH, ash content, elemental composition and leaching behaviour) were investigated. High amounts of heavy metals, especially of Zn (1300 mg/kg) and Cu (326 mg/kg), were determined in the raw material. An increase in pyrolysis temperature led to an increase in heavy metal (Cr, Cu, Mn, Ni, Pb and Zn) concentrations. Nevertheless, the leaching results showed that pyrolysis of sewage sludge at 500–600 °C produced biochar from which only negligible amounts of heavy metals could leach. The study also revealed that the properties of differently produced biochars differed only slightly between the two preparation methods; therefore, both biochar pyrolysis methods could be considered equivalent. In addition, the adsorption of phosphate (PO4-P) was investigated by using SSP biochar (which was crushed to 1–2-mm size) prepared at 600 °C. The maximum obtained adsorption capacity of such biochar was 1.04 mg/g. Produced SSP biochar can be used for the removal of PO4-P from domestic or industrial wastewater or landfill leachate.