BATCH ANAEROBIC-DIGESTION OF WATER HYACINTH - EFFECTS OF PARTICLE-SIZE, PLANT NITROGEN-CONTENT, AND INOCULUM VOLUME

Batch anaerobic digestion (at 35-degrees-C) of water hyacinth plant material was evaluated for differences in particle size and nitrogen content of plants, and for differences in inoculum volume. Particle sizes of 1.6, 6.4 and 12.7 mm and a nitrogen content of 10 and 34 mg N kg-1 dry plant material were evaluated as well as inoculum volumes of 2.5, 5 and 10 liters. Differences in cumulative biogas production were maximum at 15 days. Cumulative biogas production was highest for a plant particle size of 6.4 mm. Cumulative biogas production at 15 days increased with increasing inoculum volume for plants with a high N content but not for plants with a low N content. Total biogas and methane yields at 60 days were similar for plant material regardless of particle size, N content, or inoculum volume. Total biogas yields ranged from 0.20 to 0.28 liters g-1 volatile solids. Mineralization of organic N-15 to (NH4)-N-15-N accounted for 72% of added N-15 for plant material with high N content and 35% of added N-15 for plant material with low N content.