Abstract
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 10days (4days aerobic; dissolved oxygen of 1.8mgL(-1)) and VS content in the feed no lower than 6.7gL(-1). Free ammonia concentration values in the effluent above 1.5mgNL(-1) (around 150mg NH4 (+)-NL(-1) at pH 7) were necessary to promote the BNR over nitrite. Removal efficiencies up to 80% NH4 (+)-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 NH4 (+)-N assimilated in heterotrophic growth was counteracted with that released in ammonification and fermentation, leading to a NH4 (+)-N removal mainly related to biological nitritation/denitritation.
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