Wastewater treatment using black soldier fly larvae: Influence of larval density and solid support

The use of Black Soldier Fly larvae for wastewater treatment has emerged as a promising alternative, coupling high removal efficiency with recovery of high-value products. To overcome the critical issue of larvae mortality in a liquid environment, a patented solution employs a physical porous support to be saturated by liquid, allowing larvae mobility. In view of process optimisation, a knowledge gap persists regarding both the optimal supporting material and larval density. The optimum supporting material and larval density should maximise the porosity (i.e. active volume of the reactor) and the removal efficiencies respectively, without compromising the survival rate. In the present study, three different plastic products (granules, pall rings and geomat) were tested at four different larval densities (4, 8, 16 and 32 larvae/cm(2)). Process performance was compared in terms of larval growth and substrate removal. An increase in larval density only impairs process once threshold density is exceeded. The threshold depended on the supporting material: higher porosity materials allow the use of higher larval densities, and therefore higher organic loads, with reduced reactor volumes. The best performance was achieved at larval density of 16 larvae/cm(2) with pall rings, resulting in a survival rate and carbon removal efficiency of approx. 92 % and 97 % respectively.