The Guiyang gravel contact oxidation treatment system was constructed to purify combined sewage and rainwater in Taipei. In this study, influent and effluent were sampled monthly for the analyses of dissolved oxygen (DO), suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), ammonia nitrogen and nitrate. Results showed that removal rates of BOD, SS and NH3-N ranged 85-95% (averaged 90%), 70-95% (averaged 85%), and 70-90% (averaged 80%), respectively. Therefore, the gravel contact oxidation system is quite effective in pollution control for combined sewage and rainwater. However, microbial denitrification was not effective due to high concentrations of dissolved oxygen in non-aeration zones of the gravel contact oxidation treatment system. Although the concentration of NH3-N decreased quickly, nitrogen was not removed by microbial degradation. It was only a migration process among different forms of nitrogen. Therefore, it is important to effectively control aeration in the front zone of the gravel contact oxidation treatment system to improve the removal efficiency of total nitrogen.
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