Hydrothermal carbonization aqueous phase (HTCAP) for Arthrospira platensis culture.
Organic nitrogen (N) dominated the total N in HTCAP.
N-limited cultivation mode must be taken for successful N recycling from HTCAP.
Carbohydrate production was enhanced using HTCAP as N source in lab and pilot test.
54% of organic N in HTCAP was utilized by pre-N starved algae under N limitation.