Since the Ministry of Construction decided the development of the effective aerator for oxidation ditch (OD) process as the theme of construction technique in 1988, the application of OD process for small scale domestic waste water treatment plant have been increasing in Japan.
Authors were engaged in the development of down draft aerator, planned several actual plants and experienced the operation of the plants. The results of the trial and error of introduced “down draft mixing one point aeration type OD process” are discussed in this paper. In order to evaluate comprehensively the results, which were partially already reported, the problem of OD process, the history and the effect of the down draft aerator for “down draft mixing one point aeration type OD process” are here in reported. In addition, the following subjects in field operations are also reported; performance of nitrogen removal, mass balance of alkalinity in nitrification and denitrification, measures for initial low loading operation, countermeasures to flow variation, application to conventional activated sludge process, etc.

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