Microbial community diversity of organically rich cassava sago factory waste waters and their ability to use nitrate and N₂O added as external N-sources for enhancing biomethanation and the purification efficiency.

摘要

Water shortage necessitated South Indian sago factory owners, extracting starch out of cassava tubers, to install biogas plants where a starch utilizing microbial community multiplies and reduces the biological oxygen demand (BOD) of the waste waters by presently about 30%. The purification efficiency of sago factory waste waters, rich in solid particles and having wide C/N ratios, around 250, through unstirred biogas plants needs to be improved. Our approach was to apply instead of animal slurry nitrate (NO₃-) and nitrous oxide (N₂O) as external N-sources anticipating a better N-distribution in the unstirred biogas plants. Estimated cell numbers, bacterial community changes, on the basis of 16S rRNA gene clone libraries and changing CO_2-, CH_4-, N₂O releases due to the presence of nitrate or N₂O suggest that acid tolerant Lactobacillus spp. dominate the biogas plant inflows (pH 3.5). They were very less or not found in the outflows (pH 7.3). Assumingly, the phyla Bacteroidetes (Prevotella spp.), Proteobacteria (Rhizobium spp., Defluvibacter sp.), Firmicutes (Megasphaera spp., Dialister spp., Clostridium spp.) and Synergistetes (Thermanaerovibrio spp.), not-detectable in the biogas plant inflows, replaced them. Anaerobes, about 400 cells ml-1 in the inflows, increased to about 106 cells ml-1 in the outflows. The methane formation, as confirmed by the incubation experiments, suggests that methanogens must have been present among the anaerobes. In the biogas plant in- and outflows also about 300 cells ml-1 denitrifying bacteria and up to 104 cfu fungi were found. Despite the low number of denitrifying bacteria nitrate added to the biogas plant in- and outflows was widely consumed and added N₂O decreased considerably. Thus, wide C/N ratios substrates like sago factory waste waters keep the N₂O emissions low by using N₂O either as electron acceptor or by incorporating it into the growing biomass what needs to be confirmed. The biogas plant inflow samples have emitted tentatively more CO₂ and the outflow samples released more CH₄.