Achieving nutrient consistency through acid hydrolysis screening of black soldier fly frass for lettuce hydroponic cultivation

Nutrient-rich frass from black soldier fly bioconversion has shown promise for soil crop production, yet its effect on plant growth remains inconsistent across studies. Its application in hydroponics, which demands precise nutrient control, remains unexplored, likely constrained by batch-to-batch variability arising from heterogeneous waste substrates. This study screened nitric, citric, and phosphoric acid hydrolysis as a nutrient standardisation strategy and evaluated its effectiveness in hydroponic lettuce production. Phosphoric acid hydrolysis at pH 2, 95 °C for 1 h substantially reduced mineral variability across three independent food waste-derived frass batches, yielding statistically comparable concentrations of calcium, zinc, magnesium, and sodium. Nutrient standardisation through acid hydrolysis of frass increased fresh aboveground biomass by 56.5% relative to non-acid hydrolysed frass. Nevertheless, above- and below-ground biomass remained 82.4% and 22.3% lower, respectively, than in the commercial nutrient solution, indicating nutrient insufficiency as the limiting factor rather than nutrient variability. These findings demonstrate a promising method to standardise nutrient composition of biologically variable frass through acid hydrolysis, addressing a key barrier to consistent nutrient delivery in hydroponic systems.