The toxicity of 7 polyacrylamide flocculants (AG) was tested on fish. The 48-h LC₅₀ of A and B (nonionic) to carp (Cyprinus carpio), goldfish (Carassius auratus) and Japanese killifish (Oryzias latipes) were high, and those of C, E and G (anionic) were low to some extent, but the range of the values was between 150 and 530 mg·L^-1. Nevertheless, 48-h LC₅₀ of C to nibblers (Girella punctata) in sea water was 840 mg·L^-1.
Safe concentrations and application factors of the flocculants to fish were estimated through the long-term toxicity tests. These values of C, D and E were small to carp and goldfish, that is, safe concentration of C was less than 8 mg·L^-1, those of D and E were 210 mg·L^-1, and application factors of these flocculants were 0.010.03. No effect was observed on nibblers in sea water with 84 mg·L^-1 of C during 14 weeks.
Reduced specific viscosity of the flocculants was measured, dissolving them into the fresh water used in toxicity tests. Moreover, the viscosity of C was measured in sea water. It was lower than in fresh water. There are the tendencies that 48-h LC₅₀ and safe concentration of anionic and nonionic flocculants decrease as the viscosity.

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The formation potential of total trihalomethanes and total organic halides (TOX) on aromatic hydrocarbons such as benzoic acid, salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, catechol, resorcinol, hydroquinone, reacted with chlorine were investigated.
TOX were formed in less than 2 hours after reaction with chlorine, but trihalomethane formation by hydrolysis of trihalomethane intermediates took long terms (3days) to become constant values.
There was a linear relationship between chlorine demand and TOX formation potential to aromatic hydrocarbon reacted with chlorine.
The patterns of TOX formation by chlorination were able to classify at least 3 groups.
The formation rate of trihalomethanes to TOX was 0.3-16.6%, but formation potential of trihalomethanes and TOX was not correlate.

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The population density of the dinoflagellates Peridinium spp., mostly of P. penardiforme, was surveyed from May, 1978 to March, 1981 at two sites in the Ishitegawa Dam Reservoir : near the dam site and at the head of the lake near the inflow site of the Ishite River. Peridinium increased to 10³-10⁴ cells·ml^-1 only at the head of the lake in winter, resulting in a red tide. This abundant winter growth was partly attributed to the low water temperature because of our findings that Peridinium grew more abundantly at 10°C than at 20° or 30°C in the cultures using filtered lake water. It was also found that phosphorus was a growth-limiting nutrient in the lake water from the culture experiments involving N, P, and Fe enrichment. The difference of nutritive water quality between the head and the other parts of the lake was considered to be due to the difference in the available phosphorus supply from the outside. It was suggested that a near-dialysis culture system was naturally constructed at the head of the lake, where little of Peridinium flowed out and PO⁴-P was supplied continually by the inflowing water so that Peridinium grew abundantly, especially when the water was shallow in winter.

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3 protozoa, Cyclidium sp., Tetrahymena pyriformis, and Vorticella microstoma, and an aquatic oligochaete, Aeolosoma hemprichi, were inoculated into 4 separate continuous cultures of Bacteria. All 4 cultures reached their steady states after few weeks of incuvation. The ecological characteristics of those organisms were compared.
Cyclidium sp. and V. microstoma showed an encouragement of the enlargement of bacterial flocs to a greater extent, whereas A. hemprichi showed only a little encouragement. In the case of T. pyriformis, the encouragement was negligible. The number of Cyclidium sp., V. microstoma and A. hemprichi attached to the flocs were found to increase as the floc size increases. The formation and enlargement of bacterial flocs seem to play a significant role in maintaining the steady states.
In addition to the above observations, based on the measurements such as removal rate of polypeptone, transparency, and biomass, etc., it could be suggested that these organisms play important roles in reducing the production of excess biomass and in increasing the transparency of treated water.

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