Analysis of Chemical and Phytotoxic Properties of Frass Derived from Black Soldier Fly-Based Bioconversion of Biosolids

Black soldier fly (BSF)-based bioconversion can reduce significant volumes of biosolids and other organic waste while generating high-value BSF larvae (BSFL) and frass. While the mass of frass is greater than the BSFL biomass, its end use is less explored, especially when the bioprocessed waste, such as biosolids, contains high concentrations of contaminants. We assessed the potential to use frass from bioconverted biosolids as fertiliser by analysing chemical parameters and conducting phytotoxicity germination tests. We included frass from bioconverted food waste and wheat bran as comparisons. The chemical composition and phytotoxicity of the frass was related to the type of feedstock. Frass originating from biosolids and from wheat bran had the highest phytotoxicity, which was correlated with increased NH4+-N and EC. Initially, these feedstocks had significantly higher total N compared to food waste. Frass derived from food waste showed the lowest phytotoxicity, which was related to low NH4+-N and EC. This study demonstrates that frass from BSF-based bioconversion could be used as fertiliser; however, the original feedstock will dictate how this by-product has to be used. In this study, frass from food waste was most suitable as fertiliser. Frass originating from bioconverted biosolids needs to be applied at similar rates as unconverted biosolids to avoid phytotoxicity.