Hydrogen fermentation, a biological hydrogen production process from organic wastewater, has been drawing attention as a sustainable production process of hydrogen.This paper compiles previous studies on hydrogen producing bacteria and hydrogen fermentation processes and reports the latest trends of hydrogen fermentation. Regarding hydrogen producing bacteria, diverse hydrogen producing bacteria and their different yields of hydrogen, their harvesting methods and microbial community analyses in hydrogen fermentation were summarized. In hydrogen fermentation processes, on the other hand, previous studies using varied reactor configurations and the two phase hydrogen-methane fermentation processes were summarized. The prospect on hydrogen fermentation was discussed at the end.

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We measured the biogas and methane yield from two types of paper sludge and three types of organic waste (food waste, vegetable scraps, and pruning branches) by anaerobic digestion. Batch digestion was carried out under mesophilic and thermophilic conditions at volatile solid loads between 0.18 and 0.60 w/v% per reactor. The volume of methane produced from paper sludge from virgin paper pulp (PS－1) was comparable to that from vegetable scraps. However, the other type of paper sludge, from recycled paper (PS－2) was barely digested, owing to the high aluminum content. A kinetic analysis of the gas yields from PS－1 and food waste was carried out using a first-order kinetics model. The gas production rates were constant at a temperature adequate for the inoculum used. Consequently, co-digestion at the mesophilic temperature might be possible, and the deficiency of nitrogen could be compensated for by the addition of food waste to PS－1.

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Thiobacillus denitrificans is widely used in the autotrophic denitrification process to remove nitrate. The results from batch experiments indicated that the optimum cultivation for the growth and bioactivity of T. denitrificans could be obtained under anaerobic condition at 30℃. Besides the addition of ordinary basal mineral salts, the optimal culture medium should be composed of 40 ml of trace element solution, 15 g/l Na₂S₂O₃·5H₂O and 5.0 g/l KNO₃. Under the optimum conditions, the average ratios of ΔS₂O₃^2–－S/ΔNO₃^–－N and ΔSO₄^2–/ΔNO₃^–－N were 3.8 g/g and 10.9 g/g, respectively, which were comparable with the theoretical values of the autotrophic denitrification using thiosulfate as electron donor, and the SO₄^2– generation, S₂O₃^2–－S consumption and NO₃^–－N denitrification rates were 306.0, 104.9 and 47.3 mg/l·d, respectively. After 12－day cultivation, about 80% of NO₃^–－N could be removed from the culture with 700 mg/l of initial NO₃^–－N contained.

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Two pregrown inocula, pre-acclimated to acetate and propionate respectively and mainly composed of coccus, diplococcus-shaped Methanosarcina-like cells and a few filamentous Methanothrix-like cells, were used separately in batch experiments to investigate the effects of varied initial acetate or propionate concentrations on methanogenesis of mixed acetate and propionate substrate under controlled neutral pH and 35±1℃ conditions. The results indicated that acetate pregrown systems exhibited excellent and stable degradation potentials for acetate, with maximum specific substrate degradation rate (q_max) ranging from 1100 to 1900 mg/g－VSS/d when initial acetate and propionate less than 3300 mg/l and 750 mg/l, respectively. Under the same conditions, no distinctive difference in methane production was found for both pregrown systems, with the highest methane production rates of 107－117 ml/l－reactor/d and methane yields ranging from 0.31 to 0.36 l/g－COD removed. The average acetate and propionate degradation rates obtained were 66.7－307.2 and 3.8－86.0 mg/l－reactor/d and 26.7－265.7 and 5.8－92.0 mg/l－reactor/d for acetate pregrown and propionate pregrown systems, respectively, which were found to have some negative correlations with initial propionate/acetate(P/A) ratio (0－3.0) and initial VFA load (165.7－436.1 mgCOD/l－reactor/d), respectively. It is still a challenge to maintain and control the effectiveness of propionate degradation in pregrown systems.

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To determine the spatial and seasonal variations in bacterial community structure and abundance in the small and steep rivers typically present in Japan, bacterial populations in two rivers, the Yodo River and Kita River, were investigated using a DNA microarray technique. A total of 24 river water samples seasonally collected from four stations in the Yodo River and two stations in the Kita River were analyzed by an oligonucleotide DNA microarray targeting the conserved region of 16S rDNA in 1016 bacterial species. The phyla Proteobacteria, Firmicutes, Actinobacteria, Cyanobacteria, and Bacteroidetes were the dominant bacterial groups in the river water samples investigated, and alpha-Proteobacteria appeared to be the most dominant among the Proteobacteria. Overall diversity, composition and shifts in the bacterial communities depended mainly on the season. Pollution level (as indicated by nutrient concentration) and specific bacterial sources, such as effluent from wastewater treatment plants and backflow of seawater, also appeared to influence bacterial community structure in these small, fast-flowing rivers.

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