The role of microalgae cultivation in wastewater treatment and reclamation has been studied extensively, as has the potential utility of the resulting algal biomass. Most methods for processing such biomass generate solid residues that must be properly managed to comply with current sustainable resource utilization requirements. Hydrothermal carbonization (HTC) can be used to process both individual wet feedstocks and mixed feedstocks (i.e., co-HTC). Here, we investigate co-HTC using microalgae and digested sewage sludge as feedstocks. The objectives were to (i) study the material’s partitioning into solid and liquid products, and (ii) characterize the products’ physicochemical properties. Co-HTC experiments were conducted at 180–250°C using mixed microalgae/sewage sludge feedstocks with the proportion of sewage sludge ranging from 0 to 100 %. Analyses of the hydrochar composition and the formation and composition of secondary char revealed that the content of carbonized material in the product decreased as the proportion of sewage sludge in the feedstock increased under fixed carbonization conditions. The properties of the hydrochars and the partitioning of material between the liquid phase and the hydrochar correlated linearly with the proportion of microalgae in mixed feedstocks, indicating that adding sewage sludge to microalgae had weak or non-existent synergistic effects on co-HTC outcomes. However, the proportion of sewage sludge in the feedstock did affect the secondary char. For example, adding sewage sludge reduced the abundance of carboxylic acids and ketones as well as the concentrations of higher molecular weight cholesterols. Such changes may alter the viable applications of the hydrochar. 

This study proposes a strategy for phosphorus recovery from algae biomass while simultaneously enhancing the electrochemical capacitance of resulting biochar in supercapacitor applications. Use of sulphuric acid leaching process on both wastewater microalgae and seaweed biochar demonstrated high phosphorus recovery rates, ranging from 93.4% to 95.4%, while at the same time retaining 52.7% to 58.6% of Fe content in the biochars. By modifying the sequential order of leaching (L) and physical activation (A) as LA and AL, the strategy allows for the optimisation of phosphorus recovery and the electrochemical properties of activated biochars. During the LA process, a more porous structure formed and more S-containing functional groups occurred compared to AL which explained the higher specific capacitance (486.3F g−1 at a current density of 1 A g−1) of wastewater microalgae biocarbon with the LA process. Consequently, the life cycle assessment of this strategy revealed a significant global warming reduction potential of 1.34–2.94 tonnes of CO2-eq/yr for each tonne of biochar produced. This indicates that sulphuric acid-assisted biochar production could be a sustainable strategy for waste management and carbon sequestration, while simultaneously generating economic profits from valorised carbon material for energy storage applications.

Microalgae, originating from a tertiary treatment of municipal wastewater, is considered a sustainable feedstock for producing biochar and hydrochar, offering great potential for agricultural use due to nutrient content and carbon storage ability. However, there are risks related to contamination and these need to be carefully assessed to ensure safe use of material from wastewater microalgae. Therefore, this study compared the properties and phototoxicity of biochar and hydrochar produced via pyrolysis and hydrothermal carbonisation (HTC) of microalgae under different temperatures and residence times. While biochar promoted germination and seedling growth by up to 11.0% and 70.0%, respectively, raw hydrochar showed strong phytotoxicity, due to the high content of volatile matter. Two post-treatments, dichloromethane (DCM) washing and further pyrolysis, proved to be effective methods for mitigating phytotoxicity of hydrochar. Additionally, biochar had 35.8–38.6% fixed carbon, resulting in higher carbon sequestration potential compared to hydrochar. © 2023 The Author(s)