Inhibitory action of mercury on anaerobic digestion, which is composed of many steps degrading organic matters to methane with various bacteria, is studied by batch experiments using glucose as a substrate.
Inhibition by mercury was often observed as a retardation of beginning of acidogenesis and methanogenesis, a decrease of production rate of volatile fatty acids (VFA) and/or methane, and a color change in mixed liquor from coal-black to whitish black. The recovery of color to black was observed when acidogenesis was recovered from inhibition, while methanogenesis remained being inhibited. From detail researches, it is concluded that inhibitory action of mercury is intensely affected by sulfide coexisting in the environment because mercury must capture a sulfide ion from many sulfides in precipitation such as FeS and then precipitate as a very stable HgS to become non-toxic. It is also concluded that each process of digestion has a specific sensitivity to mercury, i.e. methanogenesis is far more sensitive than acidogenesis in total process, and n-butyric acid production is more sensitive than acetic acid production in acidogenesis.

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The purpose of this research was to evaluate the effect of the secondary effluent irrigation by the trench method. Irrigation was continued at a mean rate of 86 1/ (m-trench·d). Soil samples were collected from a test area at each 50-cm-mesh point over the range of 300 cm depth and 500 cm width. Soil permeating water samples were collected with soil-permeating- water sampling devices set at a depth of 120 cm and analyzed together with secondary effluent samples to compare concentrations of chemical components. The average removal of chemical components in soil permeating water based on the secondary effluent was found to be 89.3% in biological oxygen demand (BOD), 92.4% in total phosphorus (T-P), and 22.8% in total nitrogen (T-N). However, accumulation of T-N, T-P, or total carbon (T-C) in soil was rarely observed, and in the lower layer the concentration of these substances was in a negative correlation with the electric conductance of soil extracts (EC_ex), a measure indicating the amount of secondary effluent loading on soil. Concentrations of nitrate nitrogen (NO^-₃-N) and chloride ion (Cl^-) in soil increased in a positive correlation with EC_ex anywhere in the secondary effluent permeating area.

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In the period of the water shortage during summer, the fluctuation characteristics of water quality in river is different from those of the other seasons. This is due to the fact that the solar radiation is so strong in this period that water temperature rises gradually and reaches 25-26°C.
In this study, the changes in the discharge, the velocity of flow, the quantity of solar radiation and the water temperature during this period of the water shortage are investigated and at the same time, the changes in the concentration of Chl-a, COD, SS, PO₄-P, and NO₃-N are investigated. As the results of the two investigations of 1981 and 1984, it is known that the Chl-a concentration increases suddenly as the discharge decreases and that the concentration of NO₃-N and PO₄ -P decreases gradually with the increase of the Chl-a concentration. It is also known that the Chl-a load in this period is influenced greatly by the cumulative product of the values of the water temperature and the quantity of the solar radiation of the previous three days.

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Laboratory scale rotating biological contactors were operated with synthetic wastewater to study the effects of the daily fluctuation of hydraulic and/or organic loadings as observed in wastewater treatment plants for small communities on the removal of organic substrates.
The following results were obtained.
1) Daily average effluent COD values observed in operations with varying inflow rate and strength of wastewater (variable loading) were higher than those of constant loading (no daily fluctuation of inflow rate nor strength of wastewater) with equivalent daily BOD and hydraulic loadings.
2) Little fluctuation in effluent COD was noted by the fluctuation of influent COD concentration under smaller constant flow rate, whereas the fluctuation increased with the increase in hydraulic loading.
3) Attached biomass observed in operations with variable loadings were smaller than those observed in constant loading operations with equivalent daily BOD and hydraulic loadings to high loading period of variable loading. This fact, i.e. the shortage of attached biomass, seemed to result in the higher effluent COD values during high loading period in variable loading operations and, as a consequence, the higher daily average effluent COD values.

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