Evaluation of the fuel potential of sewage sludge mixtures with beech sawdust and lignite

Sewage sludge (SS) from municipal wastewater treatment has drawn serious attention of scientists around the world in the attempt to find feasible disposal methods. Therefore, in this work, a solution for valorisation of SS as solid fuel is proposed based on an advanced compositional investigation of SS versus conventional solid fuels, namely, lignite (L) and beech sawdust (BS). A characterisation in terms of proximate analysis and ultimate analysis was performed for the studied materials, both to SS, L, and BS as single fuel and to bi-component mixture formulas of SS and L/BS, in variable proportions of 30:70, 50:50, and 70:30. SS:BS based blends had higher hydrogen (from 4.43 wt. % to 5.14 wt. %) and carbon (from 30.76 wt. % to 40.12 wt. %) contents than the SS:L mixtures, very close to the one for BS (5.5 wt. % for hydrogen and 44.00 wt. % for carbon), indicating a more efficient release of energy during combustion. The combustion behaviour of SS and its blends with L and BS was also studied by thermogravimetric analysis (DSC-TGA) by heating the samples from ambient temperature up to 1100 °C at a rate of 20 °C/min under a nitrogen inert atmosphere. Analysis showed differences among SS, L, and BS, and the improvements were observed in the SS combustion profile after the addition of L or BS. The best potential as an alternative fuel was showed by the blends SS+L in a proportion of 30%:70% and SS+BS as a 30%:70% mixture. Additional information regarding the environmental risk induced by SS was obtained through the determination of the content of dioxines, furans, and carcinogenic polycyclic aromatic hydrocarbons of dried sewage sludge and the calculation of the toxicity equivalent factor. The values were within acceptable limits, and therefore, the proposed solution for valorization of waste sewage sludge offers a feasible way to reduce the anthropogenic impact and to enhance the durable development of the society.