Effect of selector tank on oxidation ditch system in two full-scale wastewater treatment plants (WWTPs)

The fate of soluble pollutant concentration along the process structures was studied and the actual function of selector tank was discussed in two full-scale oxidation ditch systems of Xi'an city. Samples were taken randomly in winter and autumn after tracking the WWTPs for 3 years because sludge bulking was easy occurred in this season. The results showed that the selector tank was the potent structure to lower the concentration of NH4⁺-N and SCOD, mainly depending on diluting influent with the recycled sludge except for biological adsorption or biodegradation. On the other hand, denitrification could happen uniformly in the two selector tanks, however the process of phosphorus release lied on the concentration of soluble carbon and hydraulic residence time of the selector tank. Moreover, the most important point from this study seemed to indicate a selector tank can't always guarantee good settling characteristic of activated sludge in the oxidation ditch. So, more work need to carry out in the future to look for a suitable strategy to prevent or control sludge bulking in the WWTPs.