Integrated valorization of digestate and concentrate from a centralized biogas plant via hydrothermal co-carbonization

Centralized biogas production plants treating biowaste and/or sewage sludge generate high organic and nutrients containing waste streams (e.g., dewatered digestate and reject water) for further valorization. However, evaporation of reject water results in ammonium-rich liquid for recovery via stripping, leaving a concentrate without proper disposal or reuse. This study aims to recover energy and nutrients from the concentrate, the dewatered digestate, and their mixture through hydrothermal carbonization (HTC). Specifically, HTC improved the fuel properties of the concentrate by generating hydrochar with energy densification of 1.26 % and low ash and oxygen contents. Filtrate from HTC of the concentrate demonstrated the highest methane production (250–261 mL CH₄/g-COD). Nonetheless, after individual HTC, nutrients were mostly recovered in hydrochar for the digestate, while nutrients mostly ended up in the aqueous fraction for the concentrate. In hydrothermal co-carbonization (co-HTC) of their mixture, the nutrients were distributed in the two fractions with phosphorus mostly in hydrochar and most of nitrogen and nearly all of potassium in the aqueous fraction. Despite the attractive fuel properties of the hydrochar from the concentrate, co-HTC of the mixture rather than individual HTC of the concentrate could be favorably considered as a promising option for large scale applications due to its selectiveness towards nutrients recovery as well as higher energy yield.