In order to improve the nitrogen removal efficiency and to achieve the sludge reduction in traditional wastewater treatment plants, a combined hydrolysis-anoxic-oxic (H-A-O) pilot-scaled reactor was used in this study to investigate the possibility and validity of using excess activated sludge (EAS) fermentation liquids to enhance the nitrogen removal. The results clearly showed that sludge acidification rate in fermentation reactor can reach to 43.2%. The percentages of acetic acid, propionic acid and butyric acid in the fermentation liquids were 68.4, 25.3 and 6.3%, respectively, while those in domestic wastewater were 73.0, 12.2 and 13.8%, respectively. Bioavailability of soluble chemical oxygen demand (SCOD) from fermentation liquids and domestic wastewater were investigated in batch reactors with nitrate as the electron accepter as well. The corresponding specific denitrification rates were 0.15 g NO3-N/g VSS d−1 and 0.09 g NO3-N/g VSS d−1. When the substances were enough, the denitrification reaction appeared to follow the zero-order kinetics. The results also showed that, when the H-A-O pilot-scaled reactor was operated continuously and sludge fermentation liquids were applied as additional carbon source in the A-O reactor, the removal efficiencies of SCOD, NH4+-N and total nitrogen (TN) were higher than 90, 95 and 79%, respectively. EAS reduction rate in this system was able to reach 40.4%, and the sludge VSS/SS ratio decreased from 0.82 to 0.59 after hydrolysis step.

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