Combining the biological nitrogen and sulfur cycles in anaerobic conditions

The biochemical processes involved in the anaerobic degradation of carbon, nitrogen and sulfur compounds can be represented by an oxidation-reduction or electron donor-acceptor scheme. The theoretic values of Gibbs free energ y (DeltaG(0)) calculated from thermodynamic data indicate the feasibility o f the reactions. The interactions C-S and C-N are well known but there is a lack of information about the interaction N-S. The anaerobic transformatio n of nitrates using reduced sulfur compounds can be explained considering t hat nitrate acts as electron acceptor while reduced sulfur compounds are th e electron donors. A new N-S interaction in anaerobic conditions (ORP = -425 mV) has been expe rimentally observed when treating industrial wastewater rich in organic nit rogen and sulfate. The mass balances of the different nitrogenous and sulfu r compounds in the liquid and gas phases clearly indicated an uncommon evol ution. An important percentage of the nitrogen entering the reactor as TKN was removed from the liquid phase appearing as N-2 in the gas phase, Simult aneously, only part of the sulfate initially present in the influent appear ed as sulfide in the effluent or as hydrogen sulfide in the gas. These expe rimental observations may suggest a new anaerobic N-S biological interactio n involving simultaneous anaerobic ammonium oxidation and sulfate reduction , ammonium being the electron donor and sulfate the electron acceptor.