The paper presents the experimental results collected from seven months of operation of a combined anaerobic/aerobic system treating an industrial effluent with averaged content of organic matter and nitrogen of 10.4 g COD /L and 790 mg NKT /L, respectively. The system was formed by an upflow anaerobic sludge blanket (UASB) and an upflow biological aerated filter (UBAF) connected in series, with a recycling line of the UBAF effluent into the UASB for its denitrification. The best results were obtained when operating the two reactors, UASB and UBAF, with hydraulic retention times (HRT) of 3.3 and 1.3 days, respectively, and a recycling ratio of 6.7. Under these conditions the system removed 98% of the organic matter and ammonia and 91% of the total nitrogen entering the system.

The activity of the different microorganisms was followed through activity assays consisting of measuring the consumption or production rate of any specific substrate or metabolism product. Thus when operating the system under the aforementioned conditions the reported values for the specific activity of methanogenic microorganims and denitrifiers coexisting in the UASB were 1.05 g COD/g VS d and 32.08 mg NO3-N/g VS d, respectively. While the activity of ammonia and nitrite oxidisers within the UBAF were 47.65 and 4.36 mg O2 /g VS h, respectively.

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