Effects of retention time on the characteristics of bacterial population and organic matter degradation in anaerobic digestion were investigated by using laboratory-scale chemostat-type reactors at 35°C. The heat pretreated waste activated sludge (175°C) was used as a substrate.
At the retention time of 5 days, the following high treatment efficiencies were obtained : COD removal efficiency : 59%, methane gas recovery efficiency : 42.5%, and VSS remaining efficiency : 30%. According to the measurement by the most probable numbers method, the numbers of various groups of methanogenic bacteria in the digesters at the retention times of 1.5 to 10 days were enumerated in the following ranges : total methanogens : 1.9×10⁷ to 2.3×10⁸MPN·ml^-1, H₂ degraders : 3.1×10⁶ to 2.8×10⁷MPN·ml^-1, acetate degraders : 8.2×10⁶ to 1.1×10⁷MPN·ml^-1, methanol degraders : 2.3×10⁶ to 1.3×10⁷MPN·ml^-1, and formate degraders : 3.3×10⁵ to 2.3×10⁷MPN·ml^-1. The number of homoacetogenic bacteria was ranged from 2.6×10⁶ to 3.3×10⁷MPN·ml^-1. It is observed that the bacterial populations were changed with the retention time.
The measurement methods of methanogenic bacteria and the roles of homoacetogenic bacteria in anaerobic digestion were also discussed.

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The authors have already shown the new simultaneous removal method of nitrogen and phosphorous using intermittent aeration treatment with submerged iron contactor. In this method, however, when anti-corrosive membrane, such as iron phosphate, iron oxide and so on, was formed on the surface of iron contactor under high aerobic condition, the removal of phosphorous was gradually decreased. In this study, therefore, the artificial wastewater was treated by intermittent aeration with submerged iron discs rotated at a constant speed corresponding to fluid flow in a practical aeration tank, in order to remove the membrane from the iron surface. The effect of the change of DO concentration on the formation of membrane was also discussed.
From the experimental results, it was cleared that the fluid flow in the tank and the control of DO concentration play an important role to prevent the formation of membrane on the iron surface, and to keep a high rate removal of phosphorous. Mechanism of phosphorous removal was considered that the phosphorous ion in wastewater were conbined with iron ion dissolved from the iron contactor and settled in the sludge.
Submerged iron contactor did not have any detrimental influences on high rate removal of BOD, COD and that of nitrogen.

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By labolatory experiments, the authors have verified that the intermittent aeration treatment with submerged iron contactor was useful for simultaneous removal of nitrogen and phosphorous from the artificial wastewater. In this study, field experiments of this method were carried out in the practical plant in order to develop the new treatment method, adaptable to the smalls-call sewage treatment plant such as those of rural community. The changes of DO and ORP in a treatment cycle were also discussed for the index of high rate removal of nitrogen and phosphorous.
The results obtained were as follows :
1) By submerged iron frames as iron contactor in a practical intermittent aeration tank, high rate removal of phosphorous could be kept without any detrimental influences on the treatment of BOD, COD and that of nitrogen.
2) Since the treatment of nitrogen had close relation to DO and ORP, high rate removal of nitrogen higher than 90% could be obtained by adjusting the aeration volume according to these two factors as operational index.

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[in Japanese]

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