Complete reutilisation of mixed mackerel and brown seaweed wastewater as a high-quality biofertiliser in open-flow lettuce hydroponics

Biodegradation using Bacillus species can convert mixed fishery wastewater into biofertiliser in a clean manner. Herein, fishery wastewater from mackerel and brown seaweed was tested, and the ratio most suitable for biodegradation was 10:1. During the 72-h biodegradation at the optimum mixing ratio, 36.4% hydrolysis occurred due to stable protease, alginate lyase and laminarinase activities, which chemical oxygen demand decreased by 69.1% and total nitrogen by 62.0%. The highest antioxidant activities toward 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals were 84.9% and 93.1% respectively at 24 h, and the total amino acid content at 72 h was 7715.7 μg mL-1. As a biofertiliser, the 72-h culture broth also met the requirements for standard content of nitrogen (N), phosphorus (P), potassium (K) and heavy metals, and the number of viable cells. In open-flow hydroponics using the produced biofertiliser, 1-month-old lettuce plants exhibited significantly enhanced growth rate compared with controls, with high chlorophyll and carotenoid content and high antioxidant activity. Moreover, there was no permeation of pathogens in the circular biofertiliser solution. These results indicate the potential of the biofertiliser for the production of high-quality lettuce, and demonstrate the complete reutilisation of mixed fishery wastewater. [Display omitted] •Quality biofertiliser was produced from mixed wastewater of mackerel and Undaria.•The biofertiliser enhances growth rate and antioxidant activity of lettuce.•Antioxidants present in biofertiliser bio-accumulated in lettuce leaves.•Pathogens did not permeate into the biofertiliser during open-flow hydroponics.•Complete reutilisation of mixed fishery wastewater occurred in lettuce hydroponics.