It is not always easy to keep high efficiency of nitrogen removal in a small-scale oxidation ditch, because its inflow rate generally changes significantly over time, especially for public sewerage facilities without a flow equalization tank. Based on experiments in a real oxidation ditch, a method of combining a DO meter and a timer was found to be useful for controlling intermittent aeration. In this method, a DO set value was used to stop the aerobic condition, and a timer set value was used to keep the anaerobic condition. The DO and timer set values effective for both nitrification and denitrification were about 0.5 mg/l and 1 hour, respectively. A T-N removal efficiency of approximately 90% was achieved under the most effective condition. It was supposed that the effective DO set value was influenced by the installation position of the DO probe and the total capacity of aerators per volume of the aeration tank. However, this study suggested that the DO set value could be determined by finding the rapid rise of the DO concentration, because it rose rapidly as soon as nitrification was completed.

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The filtrate generated from a sludge treatment process at a sewage disposal plant is generally returned to the primary clarifier through a sidestream. The concentration of nitrogen and phosphorus in the sidestream was approximately ten times higher than in the influent raw sewage. Therefore, the sidestream effluent contributed to the overall nitrogen and phosphorus loading of about 40% and 100% in the sewage disposal plant, respectively. However, BOD concentration in the sidestream effluent was notably low, and about 0.1 of BOD·TN^-1 ratio was much lower than the value required for biological denitrification.
An anaerobic fluidized pellet bed bioreactor (AFPB bioreactor) system combined with a contact aerator was applied to remove effectively nitrogenous and phosphorus compounds from the sidestream. The principal advantage of this system is the effective use of solid waste such as the primary sludge as a carbon source for denitrification.
The sludge BOD required for sufficient denitrification was approximately 2.76 mg BOD⋅mg NO₃-N^-1.Even with the varied loading rate of NH₄⁺-N between 0.15 and 0.37 kgN-m^-3⋅day^-1, almost complete nitrification and denitrification were achieved and the nitrogen removal efficiency was maintained over 75% in the sewage treatment system proposed in this study.

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In order to clarify the purification mechanism of eutrophicated lake water, correlation between aquatic environmental factors and micro-animals appeared on the biofilm in a biological treatment reactor was examined. The reactor packed with non-adsorbing carrier, honeycomb tube was continuously run feeding lake water of Lake Kasumigaura as a water source at 2 hydraulic retention time during 5 years from 1990 to 1995. The maximum removal parameter was total Mn with the removal ratio of 78.2% for 2 hours, and the removal of other important parameters; total Fe, turbidity, 2-methylisoborneol (2-MIB), chlorophylla and COD in the eutrophic waters were 75.3%, 71.9%, 67.4%, 62.1% and 28.4%, respectively. As for the aquatic environmental factors influencing to the treatment potential, there was a favorable correlation between the composition of micro-animals on the biofilm and algal removal ratio in the raw water. It was clarified that the higher numbers of filter-feeder and filamentous feeder like Trithigmostoma, Stentor and Stylonychia were correlated with the efficient reduction of filamentous algae, such as the genera of Phormidium and Oscillatoria. High appearence of filamentous feeder, Trithigmostoma and Nassula fed on 2-MIB producer, P. tenue highly corresponded to the removal of 2-MIB.

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The research was carried out to appreciate a slow sand filtration treatment by an investigation of suspended matter distribution in sand beds.The chlorination treatment was given to sand beds. The core-sampler to take suspended matter was inserted in slow sand filter model beds during the filter operation. The following results were obtained.
Normalized head loss value was increased at a chlorination treatment model than at a nonchlorination model (control model) after about 35 days of filter run. From the depth distribution of the suspended solid (SS), a model with chlorination treatment retained large amount of suspended matter than a model treated by non-chlorination. And suspended matter was mainly retained at the extremely thin layer at the sand column of a control model.
Small animals increased in the upper sand layer of a control, but were not existed in a model chlorination treatment.
Measurement of particles in filtered water by a particle counter were decreased at a control model accompany with filter run.

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In batch-type treatment of starch wastewater by Aspergillus niger pellets, effects of nitrogen compounds and pH of the wastewater on the treatment efficiency were examined. The degradation of starch reached 75-85% with the treatment for 15 days, when the wastewater contained 10, 000mg/l TOC (25 g/l starch) and 5-1, 060 mg/l ammonia nitrogen (NH₄-N) . In these experiments, the NH₄-N added to concentrations of 5-53 mg/l was completely consumed with the 3th days. These results indicate that the pellet activity to degrade starch was maintained during the treatment even in the nitrogen insufficient wastewater, suggesting the utilization of cellular nitrogen pool. These results suggest that the nitrogen was recycled to maintain the pellet activity. On the other hand, the removal of TOC increased from 27 to 73% with the increase in the NH₄-N concentration. At lower concentrations of NH₄-N, other TOC components such as glucose and organic acids were accumulated in the treated wastewater. Changes in the starch degradation and the TOC removal were hardly observed when the initial pH was adjusted to ranges from 2.0 to 7.0. At the initial pHs of 3.0-7.0, the pellet state was kept over the treatment time. Since lower pH would be apt to repress bacterial contamination in the fungal treatment reactor, to control wastewater pH at 3.0 was shown to be desirable for both the effective degradation of starch and the maintenance of pellet state.

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Denitrification capability of the tidal flats in Ariake Inland Sea was estimated and factors which affect denitrification were determined. It was found that denitrification capability is largely influencedby the silt and clay content of mud, with a correlation coefficient of 0.518. The denitrification rate was highest in coastal areas where the silt and clay content of the mud was also high. The denitrification rate demonstrated a Michaelis-Menten kinetic dependency on the input of dissolved nitrogen and organics from adjoining land regions. The denitrification rate increased with an increase in water temperature up to 35°C and decreased after there. It was thus concluded that the denitrification capability is highest in summer when the water temperature is high at locations where the mud is rich with silt and clay.

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