Biochar production by sewage sludge pyrolysis

Sewage sludge was pyrolyzed in order to assess the effect of pyrolysis temperature, residence time and biomass chemical impregnation on the yield of biochar production. The pyrolysis temperature was a key factor affecting biochar yield, while the highest yield was obtained at a temperature of 300 °C. Biochar surface area increased with increasing pyrolysis temperature and was maximized (90 m2/g) by impregnating biochar with K2CO3. Raw sewage sludge, as well as biochar samples, were subjected to leaching tests in order to investigate the potential release of heavy metals. Pyrolysis suppressed heavy metal release for the non-impregnated biochars, indicating that there is no environmental risk using sludge-derived biochars as soil amendments. Although K2CO3 and H3PO4 impregnation enhanced the solubility of specific heavy metals, the concentrations in the leachates were low. Biochar impregnated with K2CO3 released 85.7% of its potassium content, whereas orthophosphates were bound strongly in the biochar matrix impregnated with H3PO4. The non impregnated biochar was subjected to batch kinetic experiments in order to examine its ability to adsorb As(V) and Cr(III). Biochar removed approximately 70% of Cr(III) at equilibrium time, whereas only 30% of As(V) was adsorbed onto biochar surface, implying that biochar is more efficient in removing cations than anions from aqueous solutions.

► Pyrolysis of sewage sludge for biochar production has received limited attention.
► Impregnation of sludge with potassium and phosphoric acid to catalyze pyrolysis.
► Leaching of heavy metals from biochar samples following TCLP studies.