The ornamental design for a biomassgrowthdevice, as shown and described.
FIG. 1 is a perspective view of a biomass growth device showing our new design;
FIG. 2 is a front elevational view thereof, the rear being the same in appearance;
FIG. 3 is a right-side elevational view thereof, the left side being the same in appearance;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the biomass growth device and form no part of the claimed design.