Sustainable valorization of digested sewage sludge into added-value products

Before being discharged in a water body, municipal wastewater undergoes filtration, sedimentation, biological oxidation, and nutrient abatement in a wastewater treatment plant (WWTP); typically, the resulting sewage sludge is anaerobically digested for biomethane production. The sludge management practices currently involve landfill disposal and energy-intensive treatments, such as incineration or co-incineration, while agricultural soil applications and composting account for a minor amount. From a sustainability point of view, landfill disposal and incineration of sludge are not efficient in terms of energy consumption or waste material recovery. Thus, scientific research led to the development of innovative technologies aiming to decrease the sewage sludge volume and odour issues and to promote the valorization of the feedstock. In particular, the process described in this thesis, devised by HBI Srl., combines hydrothermal carbonization (HTC) of sewage sludge with hydrochar gasification. The HTC products are hydrochar and HTC liquor; the former can be used for energy production and carbon capture and storage, while the latter can increase the methanogenic sludge potential. Hydrochar then undergoes gasification in an upstream (counter-current) fixed-bed reactor, resulting in syngas and ash production. Once purified from fly ash by a cyclone, the produced syngas is nowadays burned in a torch. Future applications might involve heat and power cogeneration and the production of useful platform chemicals. Since the water excess is recirculated back to the WWTP, the only process by-product is the gasification ash, which can be potentially valorized as a fertilizer additive thanks to its significantly high content of phosphorous and relatively low content of heavy metals. Lastly, CHNS, SEM-EDX, TGA were performed on digested sewage sludge and hydrochar samples, confirming the potential of this promising sludge valorization technology.