Acta Scientiarum. Biological Sciences Maringá, v. 25, no. 2, p. 253-256, 2003 Effect of initial concentration of dissolved oxygen in aeration coefficient for long-term BOD experiments Marcela Bianchessi da Cunha-Santino 1 and Irineu Bianchini Junior 2 * 1 Pós-Graduação em Ecologia e Recursos Naturais, Universidade Federal de São Carlos, Via Washington Luis, Km 235, 13565-905, Caixa Postal 676, São Carlos, São Paulo, Brasil, e-mail: mbcunha@cosmo.com.br. 2 Universidade Federal de São Carlos, Departamento de Hidrobiologia, Via Washington Luis, Km 235, 13565-905, Caixa Postal 676, São Carlos, São Paulo, Brasil. *Author for correspondence. e-mail: irineu@power.ufscar.br ABSTRACT. Long-term incubations of biochemical oxygen demand (BOD) were performed to determine the aeration coefficient derived from oxygen uptake experiments. The BOD bottles were prepared with distilled water and the incubation occurred in the dark, under controlled temperature (19.4ºC). The bottles were submitted to two different treatments based on the initial dissolved oxygen (DO) concentrations (Treatment 1: 1.43mg L -1 and Treatment 2: 7.67mg L -1 ). The DO concentrations were measured with an OD meter during 52 days. The results were fitted to a first-order kinetics model. The BOD bottles showed an increase in DO concentration for both treatments and the mean aeration coefficient (k a ) was 0.065 a day -1 . The fittings pointed to some dispersion in the k a values, and the initial concentration of DO in the BOD bottles does not interfere in the aeration process, being a random process. Key words: dissolved oxygen, aeration coefficient, long-term BOD experiment. RESUMO. Efeito da concentração inicial de oxigênio no coeficiente de aeração para experimentos de DBO de longo prazo. Incubações de demanda bioquímica de oxigênio (DBO) de longo prazo foram realizadas para determinar o coeficiente de aeração proveniente de experimentos de consumo de oxigênio. Os frascos de DBO foram preparados com água destilada e incubados no escuro e sob temperatura controlada (19,4ºC). Os frascos foram submetidos a dois tratamentos que foram baseados na concentração inicial de oxigênio dissolvido (OD: Tratamento 1 = 1,43mg L -1 e Tratamento 2 = 7,67mg L -1 ). As concentrações de OD foram determinadas com oxímetro durante 52 dias. Os resultados foram ajustados a um modelo cinético de primeira ordem. Os frascos de DBO apresentaram um incremento nas concentrações de OD para ambos os tratamentos e o coeficiente de aeração (k a ) foi de 0,065 dia-1. Os ajustes apontam dispersão nos valores de k a e a concentração inicial de OD nos frascos de DBO não interferiu no processo de aeração sendo um processo aleatório. Palavras-chave: oxigênio dissolvido, coeficiente de aeração, experimentos de DBO de longo prazo. Introduction Biodegradation is an important mechanism in the organic matter removal in natural systems. Biochemical oxygen demand (BOD) is a widely used parameter for the determination of biodegradable organic compounds in aquatic systems, effluents and wastewater (Matos and Sousa, 1996; Hu et al., 1999; Orupõld et al., 1999). During aerobic degradation microorganisms oxidize organic matter in the presence of oxygen; as a result, dissolved oxygen (DO) is consumed by this oxidation. In 1925 Streeter and Phelps formulated a model that evaluated the BOD budget in aquatic systems. Current water quality models are derived from the Streeter-Phelps model and include the effects of sedimentation, advection, dispersion, mixed-order model of BOD decay and aeration from the atmosphere (Rauch et al., 1998; Tyagi et al., 1999; Borsuk and Stow, 2000). The biological oxygen depletion was directly related with oxidation of the organic substrate by microbial communities under aerobic conditions (Henze et al., 1997; Gotvajn and Zagorc-Koncan, 1999). The development of easy-to-use methods for practical applications is important for the mathematical modeling of ecological processes, which have