Chironomid midges emerging from eutrophic lakes have become intolerable because their high densities are a severe nuisance and cause economic problems. In this paper, we gave an outline of changes in the surrounding environment of chironomids in recent years and we examined the countermeasure for their control in Lake Suwa. Recent factors underlying the increasingly massive flights of midges from Lake Suwa to human living spaces were 1) humans invading the habitats of chironomid, causing human living space and those habitats to overlap (an increase in the intensity and number of lights around the lake drew many chironomid midges to human construction). 2) the progressive eutrophication of the lake provided chironomid larvae with a steady food supply so that the chironomid population density was maintained and stabilized. 3) fishes, which were the main predator of chironomid larvae, have decreased because the structure of the littoral zone was largely changed by human activity which destroyed their reproductive habitats. Basically, the control measures for chironomids in natural lakes were aimed mainly at stabilization of the lake ecosystem for the improvement of lake water quality and restoration of the littoral zone. A plan which effectively promotes the ideal of human coexistence with the organisms as auxiliary measures must be examined.

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The reliability of MPN as a point estimate of the MPN without exclusion to the ordinary two dilution schemes is of interest. Three different sources of the error have been, for years, discussed and treated for obtaining reliable MPN values: improbability of the output quanta, truncation of quantal level to three or so, and positive bias in the MPN value. Thus, first, the two intrinsically different ways of estimating improbability are critically reviewed, and another new way proposed by Kohno and Fukunaga is introduced. Second, the truncation of quantal level to three or so is discussed. Consequently, the use of a program available for computing MPN as well as estimating the improbability is claimed. Thirdly, two different ways of treating the bias in the MPN are introduced and criticized. In every case, the procedure is observed exactly and, if needed, examined through implementing C programs on a Unix machine.

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The confused present state in the method as well as in the value of 95% confidence interval of MPN is of interest. The confidence interval is prevalently expressed by either of the five different methods : Cochran's, Woodward's, Loyer & Hamilton's, Hurley & Roscoe's, and Bayesian methods. The first method assumes lognormal distribution of MPN as the basis for the interval, the second two methods, on the contrary, do not assume any distribution but take the null hypothesis at 5% significance level to select a set of quanta used to determine the interval. The last two methods rely on the likelihood function. The attention has been strictly put on the procedure to calculate the interval, by which difference in concept and feature, problems to be eliminated or reduced, and the applicability for the computer program can be clarified.

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The purpose of this study is to assess the biodegradation rate from the energy balance in the thermophilic oxic process, which is applied to treat the excess sludge, thickened sludge and dewatered sludge cake produced in the wastewater treatment plant. The readily degradable organic matter contained in the sludge was degraded firstly at the thermophilic oxic process according to the temperature curve, on which the temperature rises quickly at the test of sewage sludge added with waste food oil. From the energy budget, about 55-63% of heat produced from bioreaction was used to evaporate water ; around 23-30% of heat produced was used to prevent heat loss. The relationship between organic load and water content was certified, and the addition of energy source is important for the treatment of sewage sludge by the thermophilic oxic process. From the rates of the heat produced from bioreaction to the total calories of organic matter added in these tests, which were varied from 1.02, 1.07 and 0.60, respectively, it is clear that the excess sludge and thickened sludge were degraded more efficiently than dewatered sludge cake. The determination of energy balance could be used to assess the biodegradation rate of organic matter in the thermophilic oxic process.

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A simple kinetic model of the fluidized-bed biofilm reactor is developed based on biological and hydrodynamical knowledge. In order to account for the removal of algae from the lake water, model includes the process in which particulate substrates (algae) attach to and detach from the surface of biofilm-coated medium grain , as well as the processes of biofilm growth and detachment. The result shows that the model can describe the variations of the chlorophyll-a removal in the experiment at Lake Kasumigaura during 1996-1997. Some effects of attachment and detachment processes on the performance of the reactor were examined.

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Degradation kinetics of dichloromethane in water were investigated in a rectangular quartz cell under combination of UV irradiation and hydrogen peroxide (H₂O₂) addition. The presence of H₂O₂ not only enhanced the degradation rate of dichloromethane but prevented the formation of other chlorinated organic compounds as transient intermediates to mineralization. Since H₂O₂ was consumed by UV at a much higher rate than dichloromethane, it was found that the consecutive addition of H₂O₂ was highly effective to maintain high-rate degradation of dichloromethane. When 2,000 mg·l^-1 of H₂O₂ was added every 5 min, the decomposition rate of 1,500 mg·l^-1 dichloromethane exhibited 9.3 times higher than that of one-time addition of H₂O₂. Moreover, we successfully demonstrated that the degradation rate of dichloromethane was almost proportional to the square of incident light intensity.

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UV irradiation method is expected to be one of the remediation technology for the trichloroethylene (TCE) gas extracted from contaminated soil. TCE in gas phase has been known to be decomposed by UV irradiation and acid gas which includes some toxic compounds e.g. dichloroacetyl chloride (DCAC), phosgene, chlorine and hydrogen chloride is produced after irradiation. Therefore they have to be rejected before being released in the air. These components were found to be removed by using absorption tower packed with marbles which were wetted by showering water. DCAC was more easily removed than phosgene and chlorine. Since the pH of the waste water from the tower was almost neutral and calcium cation was dissolved at approximately equivalent amount with chloride and dichloroacetate anions, the acid gas components would be removed by hydrolysis and neutralization reactions.

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The effect of bromide ion in raw water of water supply on oxidative decomposition of humic substances by advanced oxidation process (AOP) was investigated. The following properties became evident after studying the experimental results.
(1) Bromide ion hindered oxidative decomposition of humic substances on ozonation alone and ozonation with UV-irradiation, because of product of hypobromous acid.
(2) On ozonation with H₂O₂ dosage, bromide ion did not hinder the decomposition of humic substances and product of bromate was formed little.
(3) On ozonation with UV-irradiation, all of bromide ion decomposed was oxidized to bromate.
(4) In case of containing bromide ion in raw water, the ozonation with H₂O₂ dosage was better available treatment than the other AOP.

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Effluents containing various dyes, intermediates and auxiliaries, which are typically biorefractory, are known to have high trihalomethane formation potential (THMFP) as well as intense color. In this study, six water-soluble dyes and five intermediates were ozonated to investigate the variations of total organic halide formation potential (TOXFP) and THMFP. The decrease in color and the change in biodegradability were also studied. The variation patterns of TOXFP and THMFP were roughly divided into two groups and the values of TOXFP for dyes (intermediates) decreased to 50.0 (15.0-30.0) μg·mg total organic carbon (TOC)^-1 and that of THMFP to 20.0 (10.0) μg·mg TOC^-1 through ozonation of about 2.50 mgO₃·mg TOC^-1. The rapid decoloration and enhancement in biodegradability were attained by short-term ozonation which was incomplete to decrease TOXFP and THMFP. From the viewpoint of the removal of organic halides, long-term ozonation was found to be necessary.

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