Kinetics of autotrophic denitrification using elemental sulphur as electron donor C. Manochio 1 , B.S. Moraes 2 and R. P. Rodriguez 1 1 Laboratório de Biotecnologia Anaeróbia, Instituto de Ciência e Tecnologia, UNIFAL-MG, Rodovia José Aurélio Vilela, 11.999, Poços de Caldas-MG, Brasil, 37715-400 (E-mail: renata@biotech.eng.br ) 2 Laboratório de Ciência e Tecnologia de Bioetanol, Rua Giuseppe Máximo Scolfaro, 10.000, Polo II de Alta Tecnologia, Campinas-SP, Brasil, 13083-970. Abstract Sulfur-oxidizing autotrophic denitrification is an advantageous alternative over heterotrophic denitrification, and may have potentials for nitrogen removal of low-strength wastewaters, such as anaerobically pre-treated domestic sewage. This study evaluated the kinetics of this process in batch reactors considering a half-order and a first-order model. The Biot number indicated that external mass transfer resistance was not minimized and the statistical analyses revealed that a half-order model resulted in a better adjustment. The k 1/2 obtained for reactors 1, 2 and 3 were 2.15, 2.45 and 1.81 mgN 1/2 l -1/2 h -1/2 . The results confirmed some studies in the literature and indicated that foam cubes matrices and sulphur particles had negatively influenced the diffusion of the substrate. The observed nitrogen removal ratio could be related to some results presented earlier in the literature. Keywords Autotrophic denitrification, nitrate, elemental sulphur, kinetics, half-order INTRODUCTION The overall nitrogen removal that occurs in conventional sewage treatment stations does not reach the requirements for receiving water bodies, especially those environments impacted for oxidized forms of nitrogen. The usual method applied for the removal of nitrogen from water bodies includes nitrification in an aerobic environment and denitrification in anoxic environment. Usually the denitrification step utilizes a heterotrophic pathway with organic compounds such as ethanol or complex organic substrates. However, nitrite will be produced and accumulated in water when the added organic is limiting. Contrarily, the residual organic compound will pollute the treated water when the added organic is excessive (Liu et al., 2009). Denitrification with sulphur compounds as electron donors is an alternative to heterotrophic denitrification for wastewaters with high nitrate concentration and low organic matter content. Therefore, compared with conventional heterotrophic denitrification, autotrophic denitrification has two clear advantages: (1) no need for an external organic carbon source, e.g. methanol or ethanol, which may decreases the cost of the process; and (2) less biomass production, which minimizes the handling of sludge (Campos et al., 2007). Kinetics of biofilm reactors has been discussed considerably in the literature (Wan et al, 2009; Liu and Koenig, 2001; Jasen and Harremoes, 1985). According to Liu and Koenig (2001), the predominant phenomena associated with autotrophic denitrification in sulphur packed-bed biofilm reactors are: (1) sulphur ‘‘dissolves’’ into the biofilm clinging to the surface of the sulphur particles, possibly under the effect of an enzyme, and diffuses through the biofilm; (2) nitrate diffuses from the bulk liquid into the biofilm; (3) autotrophic denitrification takes place inside the biofilm; and, (4) the reaction products (SO 4 2- , N 2 , etc.) diffuse from the biofilm to the bulk liquid. Many studies (Liu and Koenig, 2001; Wan et al, 2009; Moraes and Foresti, 2012) had been discussed kinetic models for the autotrophic denitrification with sulphur compounds. Most of all have been