Growth characteristics of flock forming Microcystis (K-3A, K-3, K-5) isolated from Lake Kasumigaura were studied under the various physical and chemical conditions. Moreover, effects of associated bacteria on the growth of Microcystis were investigated. K-3A was a bacteria free strain, while K-3 and K-5 were bacteria associated strains. The maximum specific growth rate (μ_max) of these strains were observed at 30° to 35°C and the μ_max value was 1.5 day^-1. The light intensity for the maximum cell growth of all strains were between 1, 000 to 3, 000 lux, while the μ_max of K-3 and K-5 was observed at 5, 000 and K-3A was at 3, 000 lux. The light intensity of μ_max was higher than that of the maximum cell growth. The nitrogen, phosphorus and iron concentrations for μ_max of all strains were 0.3 mg/l, 0.005 mg/l and 0.005 mg/l, respectively. It was concluded that physical and chemical environment conditions of Lake Kasumigaura in summer season were suited for the well growth of Microcystis. The μ_max and the maximum cell growth of K-3A were significantly lower than those of K-3. It was considered that the association with bacteria was of great advantage to the growth of Microcystis.

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Since activated sludge flocs have a coagulation effect in addition to the complexity of profile and the structural frailty of flocs, it is very difficult to measure the size distribution of activated sludge flocs.
The authors propose a new method for photographing the sludge flocs using the moment microphotography of the sludge film which flows down an inclined plane. We also have developed the method of semi-automatic measurement of the floc size. This method consists of the following three processes : (1) to judge visually the floc plofile on the picture by means of the slope microphotography, (2) to read in a microcomputer the coordinates used to express a profile with the aid of a digitizer and (3) to carry out data processing by the microcomputer. This apparatus makes it easy to measure the size of zoogleal organism and the length of filamentous organism.

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Raw waters in 8 rivers in Fukuoka Prefecture were examined for coprostanol as an indicator of fecal pollution and the correlations between coprostanol and bacterial indicator groups were studied.
The results obtained were as follows ;
(1) The highest concentration of coprostanol was found in water of the Mikasa River during the drought period, and its value was 5.20 μg/l, which was similar to that of municipal sewage final effluent.
(2) There was a significant relationship between fecal sterols and other pollutants (KMnO₄ consumption, NH₄-N and Cl^-), but no significant relationship between fecal sterols and total coliform.
(3) The residual amounts of coprostanol and bacterial indicator groups in a declining system were in the order total coliform > coprostanol _??_ fecal coliform > fecal streptococci. This result suggests that the finding of one fecal coliform bacteria in 50 ml is equivalent to detecting coprostanol in 800 ml of water.
(4) The rate constants of total coliform, fecal coliform, and fecal streptococci were in the range 0.22-0.29 (day^-1), 0.48-0.98 (day^-1), 0.78-1.22 (day^-1), respectively.

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