Innovative blends to wastewater sludge energy valorisation towards a sustainable circular bioeconomy

This work aimed to determine the elementary, chemical and energy characteristics of pellets produced using sewage sludge combined with elephant grass and residual cooking oil. At first, sludge and grass were mixed in respective proportions of 25%-75% and 50%-50%. Afterwards, two more samples were created by adding 5% residual cooking oil to that mixtures. The parameters analysed were moisture, bulk density, elemental composition, immediate chemical analysis (ash, volatile and fixed carbon content), calorific value, fines content and mechanical durability. The material properties were compared to the European standards for pellets EN 14961-6. The results showed the pellets with a lower proportion of sludge presented higher calorific value and less ash content, but less bulk density and higher fines content. The oil provided an increase in calorific value, however, an increase in the sulphur content. Among the four samples, those with 25% sludge and 75% grass fitted all parameters according to the EN 14961-6 standards.