Pyrolysis of different sewage sludge feedstocks for biochar products: Characterization and application

Abstract

Four sewage sludge (SS) feedstocks with distinct properties were converted into biochars by pyrolysis at 300–700 °C, in order to clarify the effects of the composition difference of SS feedstocks. The yields of biochars present a positive correlation with the contents of ash in SS. Notedly, the contents of organic matter (OM) in SS largely determine the quality of biochars. SS feedstocks with high content of OM are more likely to form stable biochars with higher aromaticity/carbonization degree, and the formed biochars possess higher calorific values. The contents of residual OM in biochars derived from SS feedstocks with low content of OM likely fail to meet the needs of soil improvement (10 wt.%). Most of heavy metals (HMs) existing in raw SS are remained in biochars after pyrolysis. The biochar produced from SS feedstocks with high content of HMs usually contains higher contents of HMs. Surprisingly, the leachability of HMs in biochars is all weakened to some extent compared to raw SS. In addition, the biochars show higher thermal stability and pH values, and P/K nutrients are enriched in biochars. The biochars prepared from four SS feedstocks exhibit different adsorption ability of methylene blue, especially at low dosage of biochar.