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Biological denitrification experiment was conducted using sugar-industry wastes, namely final molasses as a carbon source and bagasse charcoal pellets as supporting media for denitrifying bacteria. We employed an upflow fixed-bed reactor filled with the pellets and biofilm attached onto them. This was fed with potassium-nitrate and dilutemolasses solutions. Total nitrogen removals of more than 85% were achieved at influent carbon-nitrogen (C/N) ratios between 2 and 4, and hydraulic residence times of more than 0.8 h. This demonstrated that final molasses could be used as an alternative carbon source. On the other hand, final molasses also contained some organic/ammonium nitrogen and refractory organic matter including colors, both of which were difficult to remove with the reactor. Accordingly, at higher C/N ratios, these substances caused major increases in effluent total-nitrogen and organic-carbon concentrations. Therefore, an optimum C/N ratio was found to be around 2.

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To establish a sustainable regional biomass use system, clarifying the present condition in the objective area and proposing a proper scenario to improve the present conditions are important. In this study, one suburban village where rice cultivation and livestock industries are popular in Southern Vietnam was selected as the objective area.Literature review, interviews at the site and field surveys were conducted to analyze material flow in agricultural production and excreta treatment of livestock. As a result, it was clarified that surface water quality at some points in the canals, rivers and wells of the village did not meet the national standard, and contributions of discharges of untreated livestock excreta to total nitrogen loads to lower water bodies were large (43% in total). From another aspect, some rice straw was burned without any effective use at the paddy fields. For this situation, two scenarios were proposed and designed as "Plan model 1" and "Plan model 2". The one is to increase biogas digesters in the village for treatment of untreated livestock excreta, decrease washing water rate poured into biogas digesters and use slurry from the biogas digester (hereafter, slurry) at paddy and non-paddy fields as a fertilizer. Another is to use unused rice straw for producing bio-ethanol. By these scenarios, the possibility was shown that approximately 327,000 Nm 3 /year of biogas and more than 240 kL/year ( ≓ 190 t/y) of bio-ethanol can be produced in objective village, 39% of nitrogen load to water bodies and 95% of application rate of chemical fertilizer to agricultural field could be decreased, and greenhouse gas emission rate could be decreased if bio-ethanol is substituted for fossil fuel.