We have surveyed water quality in Samta village in Bangladesh since March 1997, and analyzed the arsenic concentrations of groundwater in the area surrounding Samta. This paper shows the results of the survey, concerning the characteristics of arsenic contamination of groundwater and the nechanism of arsemic elution.
1) The arsenic concentration of groundwater in Samta showed the tendency to increase with the passage of time.
2) The arsenic contamination is observed widely in the area surrounding Samta, although not so high as in Samta.
3) It is characteristic that concentration of HCO₃^- and Fe²⁺ in groundwater are high and their distribution is in accordance with that of arsenic contamination.
4) The pentavalent arsenic concentration in groundwater was higher than that of trivalent in the north of Samta, but in the south trivalent arsenic concentration was higher than that of pentavalent.
5) The distribution of HCO₃^-, Fe²⁺ and EC in groundwater explains that the mechanism of arsenic elution to groundwater is mainly caused via the reduced state of groundwater and partially via oxidation state.
6) There is some correlation between thickness of upper muddy layer and distribution of arsenic concentration of groundwater in Samta.

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Sufficient nutrient supply is one of the key factors for the periphyton growth. The formation of the viscous sublayer near the substratum, however, extremely hampers the nutrient transport through the layer with its low diffusivity. Under two different velocities in a laboratory flume, the transition of Periphyton biomass and species were measured, together with light and nutrient conditions. A higher amount of biomass, mostly Navicula in both currents, occurred in the fast current than in the slow current, although nutrient concentrations in an upper layer and light conditions were almost the same. The estimation of the nutrient diffusion rate indicates that the absorbed amount by the periphyton mat is almost equal to the quantity transported by diffusion. Thus, with its higher diffusion rate, the higher biomass of periphyton occurred in the fast current. This result implies that, under the nutrient limited condition, the periphyton growth may be regulated by the nutrient diffusion rate in the viscous sublayer rather than the absolute concentration in the upper layer.

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In the decolorization treatment of molasses wastewater by electrolysis, the effect of anode material on the decolorization efficiency (η) was studied. Five soats of anode material, which are PdO, PbO₂, IrO₂, Pt and graphite, were certified under the electrolysis condition of wastewater from an alcohol distillery with 5% NaCl. PdO was the most effective anode. η with PdO was 750U·l·kC^-1 three times larger than η with Pt, and η with graphite was 470U·l·kC^-1. In order to study for practical use, the pilot scale electrolysis reactor was made with a graphite anode and a stainless steel cathode. When methane fermentation was used for pre-treatment and the treated water diluted with sea water, η was obtained the highest value. The energy balance of the glutamate wastewater treatment system which was conbined a methane fermentation, electrolysis decolorization and co-generation was estimated. The recovery electric energy from co-generation was calculated as 75% of the required electric energy for electrolysis decolorization.

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The usefulness of colony formation test for the pollution evaluation of the river water was examined. Good dose-response curves were shown by the colony formation test for water samples. From the comparison with the Ames mutagenicity test, it was found that the pollutants which were combined with cytotoxicity and frameshift-type direct mutagenicity mainly presented in the river water. Furthermore, it was possible to partly clarify chemical characteristics of cytotoxic substances by being adsorbed on various solid phase adsorbents. From this fact, the colony formation test, as one of the bioassay techniques, seems to be useful for the water quality evaluation.

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Parallel studies using biological activated carbon (BAC) and sand with bioactivity were conducted to evaluate the response of BAC to changes of substrate inflow. Low molecular weight substances with different biodegradability and adsorbability, i.e, 2,4-dichlorophenol (2,4-DCP), phenol, glucose, were fed into BAC and sand reactors. The influent substrate was changed from glucose to phenol (I), from phenol to glucose (II), and from 2,4-DCP to glucose and back to 2,4-DCP (III). In case of (I), effluent DOC concentration in the sand reactor increased temporarily by the change in influent substrate, because acclimation time was necessary for the new substrate. However, the BAC reactor removed the new substrate without any lag time because the new substrate was removed by adsorption. In case of (II), BAC reactor showed better performance than the sand reactor after the change in influent substrate, however, effluent DOC concentration in the BAC reactor also increased. The difference in the BAC performance between (I) and (II) was due to the different adsorbability of the new substrates, i.e, phenol with high adsorbability easily adsorbed on BAC after the change influent substrate from glucose to phenol and glucose with low adsorbability could hardly adsorb on BAC after the change influent from phenol to glucose. The change in influent substrate from 2,4-DCP to glucose (III) made the effluent DOC concentration increase in both reactors and the biodegradation to glucose in the BAC reactor delayed in comparison with sand reactor. Because 2,4-DCP has high adsorbability and desorbability, desorbed 2,4-DCP inhibited biodegradation of glucose after the change in influent substrate. Re-inflow of 2,4-DCP in day 12 did not increase effluent DOC concentration, because 2,4-DCP has high adsorbability and BAC kept biodegradability to 2,4-DCP by biodegradation of the desorbed 2,4-DCP during inflow of glucose.

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The effect of the circulation of activated sludge in the two intermittent aeration tanks on T-N removal was studied by numerical model simulation and the pilot-plant scale examination. Microfiltration membrane was submerged in the one side of reactors to enrich the MLSS concentration.
The simulated performance to remove nitrogen in domestic wastewater was agreed to the experimental result, and 93.7% of T-N removal was achieved.
The simulated T-N removal was found 17% higher than the continuous aeration mode.
The simulation decided best operation condition as the circulation rate of 3Q and intermitted aeration cycle of 40min/20min and the Pilot-plant had been operated successfully over 6 months.

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