The internal structure of activated sludge flocs was observed under the microscope and its mathematical model was proposed. The model expresses that the volume fraction occupied by microorganisms in a floc decreases monotonously from the center to the surface of the floc. The structure of the floc was characterized by two model parameters named a floc distribution index for volume fraction occupied by microorganisms and a floc core index, respectively. These parameters were determined by measuring the size and the density of the floc. The structural difference between filamentous and pellet-like activated sludge flocs was demonstrated by comparing the numerals of the model parameters.

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Six strains (A, C, F, F, I and J) of Actinomycetes which produced musty odor were isolated from the water and sediments of Lake Kasumigaura. All of six strains were identified as Streptomyces by morphological and taxonomical studies. All strains produced 2-methylisoborneol, and geosmin was produced by only four strains.
Various environmental conditions for musty odor production under the same nutrient concentrations as Lake Kasumigaura water were investigated. The musty odor production was measured by sensory test. The values of threshold odor number were highly correlated to the 2-methylisoborneol and geosmin concentration, respectively. Threshold odor number of 1 for 2-methylisoborneol and geosmin corresponded to 0.007μg/l and 0.01μg/l. The optimum temperature and pH for the musty odor production were 25°C and between 8 and 9. The water of Lake Kasumigaura contained sufficient amount of nutrients to produce intense musty odor with respect to nitrogen, phosphorus, iron and magnesium. Carbohydrates as carbon sources were more effective to the musty odor production than organic acids and amino acids. It was considered that highly musty odor were produced by Streptomyces when adequate carbon sources did exist in Lake Kasumigaura water.

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The writers measured methemoglobin formation with solvent extracts from the sediments of the Tama and the Ayase Rivers, two rivers run through urban areas, and several urban and rural rivers.
Hemoglobin was converted to methemoglobin with the n-hexane extract obtained from the sediments of the upstream of the Tama River and rural rivers, but not with those from the downstream and urban river sediments. The formation of methemoglobin was remarkable with the extracts of ethyl acetate and methanol from polluted river sediments. Doses of solvent extracts for 20% methemoglobin formation in the researched rivers were as follows : below 0.1 mg/ml in hexane extract, 0.1-1.0 mg/ml in ethyl acetate extract and below 0.5 mg/ml in methanol extract.

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The heavy pollutant loading is drained out from the urban area of Ishioka City in the watershed of River Sanno. The loading of particulate matter decreases in going down with sedimentation. The amount of decrease was estimated by our two means. The first is the means with loading budget of the lower reaches from the urban area and the second is by observation of sedimentation rate on artificial substrata in river bed. The river sediments are flushed out by storm event without much decrease by decomposition and therefore become major part of storm runoff loading. The transient storage of sediments in river bed does not mean true self-purification. Consequently, for estimation of exact river loading or evaluation of self-purification abilities in river it is necessary to reevaluate sediments as a part of river pollutant loading.

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Vertical distributions of urea and DON were studied in the northern area of Hiroshima Bay from April to December 1981. It was found that the concentration of urea was remarkedly low in bloom season. On the other hand, relatively high concentration of urea was observed in sea waters above 3m and under 13m from the latter half of July to September. The concentration of urea in the rivers inclined to get larger with an approach to the river mouth. This suggests that domestic and industrial waste waters containing urea inflow into the rivers. The ratios of urea to DON ranged from 2 to 30% (average 15.3%) and fairly high correlation coefficient between them was found.
From the biodegradation experiment of filtered sea water, a part of urea was certified to be produced from the decomposition of DON and its apparent biodegradation rate constant at 15°C was 0.166 day^-1 Urea was observed to be similarly taken into the cells with NH₄⁺and N0₃^- in the culture of Skeletonema costatum. Additive effect of urea on the growth of Skeletonema costatum and Heterosigma sp. were relatively large in the range of 2-10 μg-at N/l.

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