Denitrification activity behavior and bacterial community structure in a poly-β-hydroxybutyrate (PHB) embedded soil-column reactor were investigated. The influent containing nitrate (NO₃-N) was applied in a downward flow. The NO₃-N concentration rapidly decreased only in the middle part of the column where PHB was embedded, which indicated that the denitrification activity in the reactor was related to PHB. Influent NO₃-N was effectively removed in the reactor following 60 days of continuous operation. The total number of bacteria in the lower and middle parts of the column decreased after 30 days, and then increased again after 60 days; however, the number in the upper part continuously decreased with time. The number of PHB utilizing denitrifiers increased in both the middle and lower parts with time, contributing to denitrification activity in the reactor. From the result of PCR and denaturing gradient gel electrophoresis (PCR-DGGE), the presence of two denitrifing and a few other bacteria were determined, and which suggested of Ralstonia related species played an important role in the denitrification capability of the reactor.

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Isolation, stock, and in addition, monoxenic and mass culture of specific species of aquatic oligochaetes, Nais sp., Aeolosoma hemprichi, Pristina sp. and Dero sp. were attemped successfully. Small aquatic oligochaetes are the typical metazoa found frequently in the activated sludge and bio-film from domestic sewage treatment plant operated by low BOD loading. Using a sterilized activated sludge as the metazoan food, the monoxenic growth rate of small aquatic oligochaetes, the effects of temperature, pH, phosphate buffer solution concentration, organic concentration, share stress, food concentration and sort of bacteria as food, on the growth rate were measured. The optimum temperature of cultivation for A. hemprichi, Nais sp. and Pristina sp. were 33, 25 and 30°C, respectively. Phosphate buffer concentration which could grow normally were 1/375M for A. hemprichi and Nais sp., 1/150M for Pristina sp. and 1/7,500M for Dero sp. The optimum pH values in cultivating the oligochaetes ranged from 6 to 8. The growth of Nais sp. could not be observed in much stronger share stress.
Specific growth rates (μ) ranged from 0.35 to 0.43 day^-1 for A. hemprichi and 0.10 to 0.13 day^-1 for Nais sp., respectively when the sludge concentration was more than 500 mg · l^-1.

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Surfactants removal capabilities and mechanisms around the rhizosphere of giant duckweed were investigated. From the roots of the weeds cultivated with various environmental water, almost 10¹⁰ CFU/g-roots d.w. of microorganisms were gained and the microbial flora were different from that of water samples. The continuous batch cultivation test showed that the acclimation of the rhizo-communities enhanced (or improved) the rhizodegradation and the symbiotic systems removed the artificial surfactants (LAS, SDS, AE, NPE) immediately. The bacterial degradation activities around the rhizosphere were accelerated by the oxygen release from the plant roots. These results indicate that the plant-microbe systems play an important role in the biodegradation, bioconversion and removal of chemical compounds in the water environment.

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Ammonium removal by swim-bed technology using a novel acryl-fiber biomass carrier (biofringe) was conducted to investigate its effectiveness for treatment of Hanoi groundwater contaminated with high levels of ammonium. Two reactors with different amounts of biofringe—single yarn and double yarn—were used in this study. Ammonium removal efficiencies of 95-100% at volumetric loading rates of up to 0.24 and 0.48 kg-N/m³/d, corresponding to hydraulic retention times (HRTs) as short as 3 and 1.5 hours, were obtained for the single-yarn and double-yarn reactors, respectively. The maximum biomass retention capacities of the biofringe carriers and the influences of operational conditions such as temperature, pH, DO, and alkalinity on treatment efficiencies were investigated. Ammonium was converted to nitrate in this process with effluent nitrite concentrations close to zero, and DO concentrations greater than 3.0 mg/l were required for complete removal of ammonium at loading rates of 0.48 kg-N/m³/d. The pH level in the reactor consistently between 6.9 and 7.5 and alkalinity consumption was from 6.5 to 7.5 mg-CaCO₃ per mg ammonium-nitrogen oxidized to nitrate. Slowly growing nitrifying bacteria were effectively retained in the reactor using the biofringe with effluent suspended-solids concentrations of less than 3-5 mg/l through out most of this study.

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A toxic cyanobacterium Cylindrospermopsis raciborskii was grown in batch culture under initial nitrogen concentrations of 1mg · l^-1 and 16.5 mg · l^-1 as nitrate and production of hepatotoxin cylindrospermopsin (CYN) was investigated. CYN increased with increase of density of C. raciborskii and maximum concentrations of CYN were 0.45 mg CYN · l^-1 and 1.26 mg CYN · l^-1 at initial NO₃-N concentrations of 1mg · l^-1 and 16.5 mg · l^-1 respectively. At initial NO₃-N concentration of 1mg · l^-1, total nitrogen concentration increased to 7 mg · l^-1 showing occurrence of nitrogen fixation. CYN concentration correlated with total nitrogen concentration under initial NO₃-N concentration of 1mg · l^-1. It was suggested that C. raciborskii and CYN might increase by nitrogen fixation in nitrogen limited lakes and reservoirs.

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