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The present study investigates the hydrothermal liquefaction (HTL) of harmful green macroalgal blooms at a temperature of 270 °C with, and without a catalyst with a holding time of 45 min. The effect of different catalysts on the HTL product yield was also studied. Two separation methods were used for recovering the biocrude oil yield from the solid phase. On comparision with other catalyst, Na 2 CO 3 was found to produce higher yiled of bio-oil. The total bio-oil yield was 20.10% with Na 2 CO 3 , 18.74% with TiO 2 , 17.37% with CaO, and 14.6% without a catalyst. The aqueous phase was analyzed for TOC, COD, TN, and TP to determine the nutrient enrichment of water phase for microalgae cultivation. Growth of four microalgae strains viz., Chlorella Minutissima, Chlorella sorokiniana UUIND6, Chlorella singularis UUIND5 and Scenedesmus abundans in the aqueous phase were studied, and compared with a standard growth medium. The results indicate that harmful macroalgal blooms are a suitable feedstock for HTL, and its aqueous phase offers a promising nutrient source for microalgae.

The elevated level of pollutants in water highlights the urgency of effective wastewater treatment. Hence, the concern about the potential of microalgae as an economically and environmentally sustainable for wastewater treatment is addressed. Wastewater treatment using microalgae resulted in significant reductions in micropollutants by 82 % for chemical oxygen demand (COD) and biological oxygen demand (BOD). Iron (Fe) concentration was reduced by 99 %, while calcium (Ca) decreased by 25 %. Algal biomass obtained from wastewater is executed for biochar production and bio-stimulant preparation. Bio-stimulant treatment significantly enhanced seed germination and increased shoot and root lengths in maize (Zea mays) and beans (Phaseolus vulgaris) compared to the control. Conversely, seed germination was completely suppressed in both maize and beans in the presence of biochar of bio-stimulant treatment. The GC-MS and NMR analysis study revealed the presence of key compounds in the microalgal extract that have a great contribution to plant growth. Hence, the study concludes the multifaceted and potential application of microalgae as a remediation and bio-stimulant in a sustainable way. [Display omitted] • Integrated wastewater treatment, recycling and reuse technology (SDG 6a). • Utilization of wastewater treated C. sorokiniana for biochar & bio stimulant generation. • Bio-stimulant influenced germination and growth of Zea Maize and Phaseolus vulgaris. • GC-MS analysis identified key compounds in bio-stimulant for plant growth promotion. [ABSTRACT FROM AUTHOR]