Thick and thin: high plant abundance but low surface cover enhances growth and remediation by floating Azolla filiculoides following wastewater acidification

Increased global food demand is paralleled by the generation of greater volumes of nutrient-rich agricultural and food processing wastewaters. These wastewaters, including meat processing wastewater (MPW), contain an abundance of nutrients capable of supporting plant growth. This coincides with agri-food industry requirements to improve valorisation and sustainable circular economy approaches for optimised waste management. Cultivation of aquatic ferns, Azolla spp., has been proposed as a promising means of nutritionally favourable biomass production and nutrient recovery of agri-food wastewaters. Yet optimal cultivation strategies for scaled applications remain largely unknown. The present study assessed the relative growth rate (RGR) of Azolla filiculoides on MPW under different cultivation conditions. These included pH (5, 6, 7.8, 9, 9.4, unadjusted) and dilution (5, 10, 50, 100%) of MPW, alongside physical cultivation parameters of surface cover (10, 20, 40, 60%) and plant abundance (1, 2, 3, 6 sub-plants). Undiluted MPW (100%) at pH 6 facilitated the highest RGR. Optimal density conditions for growth promotion were identified as being remarkably at low (10% surface cover) and equally for plant abundances of 3 and 6 sub-plants, with mean (±SE) removal efficiencies of 37.78 ± 5.01% for total nitrogen and 61.98 ± 2.3% for total phosphorus. These findings demonstrate that MPW can serve as a suitable growth medium for A. filiculoides . Chemical regulation of MPW and plant-physical management emerge as key factors for A. filiculoides biomass production. Given ease of cultivation, MPW grown A. filiculoides could advance circular economy goals through nutrient conservation and high-protein biomass generation. • Meat processing wastewater (MPW) supports enhanced Azolla filiculoides cultivation. • Optimal A. filiculoides cultivation was achieved on 100% MPW at pH 6. • 10% initial A. filiculoides surface cover (3-6 plantlets) enhanced growth. • Optimised growth facilitated, removal of up to 37.8% N and 62.9% P from MPW.