Combining the biological nitrogen and sulfur cycles in anaerobic conditions F. Fdz-Polanco, M. Fdz-Polanco, N. Fernández*, M.A. Urueña, P.A. García and S. Villaverde Department of Chemical Engineering, Valladolid University, 47011 Valladolid, Spain * Instituto Superior Politécnico José Antonio Echevarría, La Habana, Cuba Abstract 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 energy (DG 0 ) calculated from thermodynamic data indicate the feasibility of 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 transformation of nitrates using reduced sulfur compounds can be explained considering that nitrate acts as electron acceptor while reduced sulfur compounds are the electron donors. A new N-S interaction in anaerobic conditions (ORP = –425 mV) has been experimentally observed when treating industrial wastewater rich in organic nitrogen and sulfate. The mass balances of the different nitrogenous and sulfur compounds in the liquid and gas phases clearly indicated an uncommon evolution. 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. Simultaneously, only part of the sulfate initially present in the influent appeared as sulfide in the effluent or as hydrogen sulfide in the gas. These experimental observations may suggest a new anaerobic N-S biological interaction involving simultaneous anaerobic ammonium oxidation and sulfate reduction, ammonium being the electron donor and sulfate the electron acceptor. Keywords Anaerobic nitrogen removal; anaerobic sulfur removal; nitrogen/sulfur interaction Introduction The biological cycles of carbonaceous organic matter, sulfurous compounds and nitroge- nous compounds are well established. Anaerobic treatment of industrial wastewater under conventional conditions generates as end products of the anaerobic degradation: methane and carbon dioxide for the organic matter, sulfide for the oxidised sulfur compounds and ammonia for the organic nitrogen compounds (Lettinga, 1995). All the biochemical processes involved can be represented by an oxidation-reduction or electron donor-acceptor scheme. The value of the standard Gibbs free energy (DG 0 ) of a chemical reaction can be calculated following the electron transfer scheme and using the values of DG 0 of the half-reactions. The DG 0 of the half-reactions can be calculated by sim- ple arithmetic as the difference between standard Gibbs free energies of formation (DG 0 f ) of products and reactants. Thermodynamic data are compiled as properties of the substances, not reactions, and are quoted as G 0 f of compounds from their elements at 25°C and in their “standard states” which are generally pure solids, pure liquids or pure gases at one atmosphere partial pres- sure. Because biological reactions occur almost exclusively in aqueous solution, values for DG 0 f are also quoted at a standard concentration of 1 M in aqueous solution. Values for some compounds of biological interest found in anaerobic treatment are listed in Table 1 (Mosey, 1985). As mentioned before, using the data of Table 1 it is possible to estimate DG 0 of the half- reactions. The standard redox potential (E 0 ) is calculated from DG 0 of the half reaction as E 0 = (DG 0 )/(n.F), where n is the number of electrons in the half-reaction and F is the Faraday constant expressed as mV-equivalents (96.42 J/mV). In order to evaluate the redox potential the convention is to write the half-reaction with the electrons on the left, as a Water Science and Technology Vol 44 No 8 pp 77–84 © IWA Publishing 2001 77