The removal efficiency of nitrogen, phosphorus and organic substances by oxidation ditch process was evaluated by using bench scale simulators in which both aerobic and anaerobic/anoxic reactions were introduced in a cycle of 20 min.
The results derived from these experiments were as follows ;
1) The introduction of anaerobic condition into the downstream of wastewater inflow caused less sludge bulking than that into the upstream of inflow. The introduction of anaerobic reaction, however, was not as effective to prevent sludge bulking as observed in other activated sludge processes.
2) Nitrogen removal was successful under proper operational conditions, i.e., the introduction of anaerobic conditions of 10 min per cycle and intermediate organic loadings. Prolonged aeration and/ or low organic loadings prevented denitrification and shortage of aeration and/or excess organic loadings resulted in poor nitrification. Also, wastewater inflow into the anaerobic section enhanced nitrogen removal.
3) The more phosphorus was removed under the higher loadings with high sludge yield and with anaerobic reaction.

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Results of laboratory experiments on the change of solid-liquid separation characteristics and the behaviour of suspended solids, dissolved organic carbon and dissolved solids during the freezing process of excess activated sludge are reported.
Several cylinders containing municipal excess activated sludge were put in a freezer chamber maintained at a constant temperature -16.5°C. Each cylinder was taken one after another out of the chamber in sequence of freezing time. A cylinder taken out at the halfway freeze contained partially frozen part. The liquid unfrozen part was separated from the solid frozen part by decantation. One of the separated frozen part (S1) was immediately thawed and analyzed. Another frozen part (S2) was returned and kept at freezer temperature for extended freezing and analyzed after thawing.
The improvement of solid-liquid separation characteristics was little with half-frozen sludge (S1), but remarkable with extended frozen part (S2). So, it was supposed that in half-frozen sludge (S1), the cells of excess activated sludge did not freeze completely. And this hypothesis was supported by the increase of dissolved organic carbon and dissolved solids concentration in thawed sludge samples.
If the sludge froze completely, the solid-liquid separation characteristics of the sludge were improved effectively, regardless of position in the freezing column.

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A project has been initiated by introducing secondary treated wastewater after sand filtration into a vacant small urban channel, Nobidome Channel, to create amiable space around the small stream. Comprehensive water quality surveys were carried out from the start of discharge for 2 and half years to make an assessment of the feasibility of usage of treated water for this purpose.
Suspended solid concentration was increased along the flow by the erosion of the river bed composed of soil. Occasionally, smells and foaming characteristics to sewage were noted. Remarkable change in the water and the environment was not noted for 2.5 years. Although self purification was not noted for BOD, nitrification of ammonium ion in the wastewater was observed since rather high temperature was kept in the stream water even in the winter. Stormwater caused rapid increase in suspended solid concentrations, whereas little increase in organic and nutrient concentrations were noted. Also the stream water seemed hygenically safe from the number of fecal colic.

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[in Japanese]

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