Cascade systems to recover resources from sludge by the integration of pretreatments to fermentation-based anaerobic bioleaching process

Future sustainable technological solutions need to balance the development of sludge as resource against the paramount necessity to protect human health and environment. Nutrients, metals, embedded energy, and the avoidance of sludge handling costs are important examples of recoverable assets. The virtuous shift to an efficient resource recovery platform should integrate pretreatments with biological conversion processes to extract values from complex waste streams as waste activated sludge. The aims of this study were: i. to assess the feasibility of sludge pretreatments as ultrasounds to promote the release of metals and organics; ii. to explore the potential of anaerobic microorganisms to leach out embedded resources from sludge through an enhanced hydrolysis/ acidification step. The study proved that the acid addition to the sonicated feedstocks promoted a significant solubilization and acidification of the system during the feeding days. Crucial strategy was the inhibition of methanogens by thermal treatment of the start-up inoculum that assured pH stability, high VFAs production, and elements (As, Co and Se) solubilization. High throughput analysis of 16 S rRNA gene sequencing performed on this system revealed the presence of a high abundance of reads affiliated with hydrolytic and fermentative bacteria, in particular belonging to the phyla Bacteroidetes, Firmicutes, and Actinobacteria. The produced VFAs, fed into a semi-pilot digester with a short HRT of 15 days, were successfully converted into methane thanks to the dominant acetoclastic methanogenic population. Mass and energy balance for a full-scale scenario of this innovative biorefinery platform is reported and compared to a conventional sludge treatment scenario.