Integrated energy recovery from biogas and dried sewage sludge in a full-scale thermal dryer

Energy recovery from by-products of wastewater treatment plants (WWTPs) can increase operational efficiency and reduce environmental impacts. This full-scale study evaluated the integrated use of biogas from upflow anaerobic sludge blanket (UASB) reactors and the dried sludge produced in a thermal dryer as fuels for the sludge drying process itself. Monitoring covered biogas production and composition, physicochemical and microbiological characterization of the dried sludge, dryer performance, and stack emissions. Biogas production yielded 5.9 × 10 4 MJ d −1 , whereas dried sludge, produced at 1.1 t h −1 with a total solids content of 88.8% (wet basis) and a dry-basis lower heating value (LHV) of 11.2 MJ kg −1 , supplied 2.96 × 10 5 MJ d −1 . Together, these fuels provided 3.55 × 10 5 MJ d −1 against a thermal demand of 3.64 × 10 5 MJ d −1 , meeting approximately 97% of the process requirement. The specific consumption of 3.89 MJ kg −1 of water removed was consistent with values reported for sludge dryers. The dried sludge met the criteria for Class A biosolids under Brazilian regulations, showed relevant macronutrient contents, and, within the measured parameter set, its agricultural use was conditioned on controlling zinc levels. Emissions remained compliant with legal limits, and sludge co-combustion was associated with lower CO and vanadium emission rates and higher particulate matter and NO 2 emission rates than the wood-chip-only test, without loss of regulatory compliance. The results indicate that integrating internal energy streams in WWTPs with UASB reactors is an effective strategy for waste valorization and promotion of the circular economy.