Aerobic/anoxic post-treatment of anaerobically digested sewage sludge as an alternative to biological nitrogen removal from reject water

Stabilisation and biological nitrogen removal (BNR) of anaerobically digested sewage sludge were studied in a post-aeration reactor at pilot scale working under alternating anoxic-aerobic conditions. Digested sludge came from a two-stage anaerobic digestion (thermophilic + mesophilic). The best post-aerator performance was achieved when working at an HRT of 10 days (4 days aerobic; dissolved oxygen of 1.8 mg L −1 ) and VS content in the feed no lower than 6.7 g L −1 . Free ammonia concentration values in the effluent above 1.5 mg N L −1 (around 150 mg NH 4 + -N L −1 at pH 7) were necessary to promote the BNR over nitrite. Removal efficiencies up to 80 % NH 4 + -N, 50–55 % total nitrogen and 15–20 % VS were recorded in this study, with no external addition of chemicals. A nitrogen mass balance revealed that the high percent of NH 4 + -N assimilated in heterotrophic growth was counteracted with that released in ammonification and fermentation, leading to a NH 4 + -N removal mainly related to biological nitritation/denitritation.