Production of Chlorella vulgaris using urban wastewater: Assessment of the nutrient recovery capacity of the biomass and its plant biostimulant effects

The microalga Chlorella vulgaris UAL-1 was produced using urban wastewater that was either left untreated or sterilised. The biomass productivity achieved when using the untreated wastewater was comparable to that of the control, which was freshwater supplemented with synthetic chemicals (0.58 g·L −1 ·day −1 ). The microalga C. vulgaris UAL-1 was able to recover up to 93.8, 73.1, 80.5, and 85.2% of the N-NH 4 + , N-NO 3 − , P-PO 4 3− and COD present in the unprocessed wastewater. The thermal treatment (120 ºC, 10 min) and covering the reactors led to lower biomass productivities and nutrient recoveries. The biomass was then disrupted by sonication (280 W, 24 kHz) and assessed for biostimulant effects. Overall, although the germination index of watercress seeds was improved with the microalgal extracts ( p < 0.05), the observed increase was around 1.0%. All the microalgal extracts significantly promoted the development of roots in soybeans, especially those generated from the biomass produced using freshwater and unprocessed wastewater ( p < 0.05). The percentage increase was close to 200%. Moreover, the results from the cucumber cotyledon expansion test revealed that the microalgal extracts led to a greater weight gain; no major differences were observed between the biomass produced using freshwater and unprocessed wastewater while the autoclaved media and the biomass produced in the dark led to a lower weight gain ( p < 0.05). The microalga C. vulgaris UAL-1 is adaptable, robust, and can be produced using wastewater; the biomass shows potential for evaluation in field trials as a plant biostimulant.