A granular activated carbon (GAC) anaerobic fluidised-bed reactor treating vinasse from an ethanol distillery of sugar beet molasses was operated for 90 days, the first 40 days of start-up followed by 50 days of operation at constant organic loading rate of 1.7 g COD/Ld . The reactor showed good performance in terms of organic matter removal but an anomalous behaviour in terms of unusual high concentrations of molecular nitrogen in the biogas. The analysis of the different nitrogenous and sulphur compounds and the mass balances of these compounds in the liquid and gas phases clearly indicated an uncommon evolution of nitrogen and sulphur in the reactor. About 50% of the nitrogen entering the reactor as total Kjeldahl nitrogen (TKN) was removed from the liquid phase appearing as N 2 in the gas phase. Simultaneously, only 20% of the S-SO 4 2- initially present in the influent appears as S-S 2- in the effluent or S-H 2 S in the biogas, indicating that 80% of the sulphur is removed. This behaviour has not been reported previously in the literature. These observations may suggest a new anaerobic removal process of ammonia and sulphate according to an uncommon mechanism involving simultaneous anaerobic ammonium oxidation and sulphate reduction. © 2001 Elsevier Science Ltd.
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