Coupling hydrothermal carbonization and wastewater treatment by microalgae for biomass resource upcycling loop

Hydrothermal carbonization (HTC) effectively converts microalgae into hydrochar, yet the in-depth research on the related mechanism requires to be further investigated and the disposal of aqueous phase (AP) needs to be considered. In the present work, the HTC of Chlorella sp. resulted in a hydrochar yield of 50.66% with energy yield of 54.71% at 180 ℃. The behaviors and interaction mechanisms of three major components within microalgae were investigated. Proteins drove carbonization, boosting hydrochar yield but reducing its structural order. Carbohydrates facilitated Maillard reactions with protein derivatives, stabilizing N-heterocycles in hydrochar. Conversely, lipids hindered hydrochar formation by generating oil-phase byproducts that suppressed pore development. The HTC-AP diluted 20-fold for cultivation yielded 36.84% higher biomass than BG11 medium, with high N (78.59%) and P (81.78%) removal, during which COD and TOC were reduced by 76.66% and 58.58%, respectively. Notably, the microalgae after cultivating with AP possessed high protein content (42.30%) and long-chain fatty acids content (3.78%). Single-factor dose experiments demonstrated that differences for organic components of AP regulated the microalgae growth, with d-galactose/glycerol serving as readily metabolizable carbon sources that promoted growth, while phenol/pyridine/furan derivatives exhibited growth-inhibitory effects. Additionally, transcriptomic analysis elucidated the regulatory role of AP in microalgal growth.