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Purification of nitrogen-rich digester supernatant by activated sludge from a membrane bioreactor

The potential of activated sludge from a membrane bioreactor (MBR) to remove nitrogen from anaerobic digester supernatant was tested at various volumetric exchange rates (n) (0.1, 0.2 and 0.3 d–1) and dissolved oxygen concentrations (DO) (0.5, 2.5 and 3.0 mg/L). The high abundance of ammonium-oxidizing bacteria (7.2%) in the sludge caused the ammonium removal efficiency to exceed 97%, independently of DO and n. However, DO and n affected the process rate; thus, the time necessary for ammonium removal ranged from 4 to 8 h. At DOs of 2.5 and 3.0 mg/L, nitrogen removal reached 79.8%–89.0% although external carbon was not added. The nitrogen removal efficiency was highest (91.5%) at an n of 0.3 d–1 and a DO of 0.5 mg/L. Thus, this variant, which had the highest availability of substrate and the lowest availability of oxygen, stimulated the cooperation of aerobic nitrifiers, denitrifiers and anammox microorganisms, which were present in the biomass. The high rate of endogenous oxygen uptake indicated the degradation of intracellularly stored organics, which may have supported denitrification. These results indicate that the biological treatment of nitrogen-rich supernatant by highly concentrated MBR sludge in a side-stream is an advantageous alternative to treatment in the main-stream of wastewater treatment plants.