Recycling the nutrients in residues from methane digesters of aquatic macrophytes for new biomass production

The floating freshwater macrophyte Eichhornia crassipes (water hyacinth) was fermented anaerobically to produce 0.4 1 of biogas/g volatile solids at 60% methane with a bioconversion efficiency of 47%. Both the liquid and solid digester residues were a rich source of nutrients that were recycled to produce additional biomass. An approximate balance of the nitrogen recycled through the culture—digester—culture system indicated that nitrogen was conserved within the digester. All of the nitrogen originally added to the digester in the form of shredded water hyacinths could be found in the liquid (48%) and solid (52%) residues; 64.5% of the nitrogen in these residues could be reassimilated by cultures of water hyacinths. This study indicated the potential of bioconversion of aquatic macrophytes to methane as a possible means of both producing and conserving energy.