This study aimed to evaluate the self-purification capacity of the Pernada Stream, located in the municipality of Paraíso do Tocantins – TO. Therefore, it was performed a diagnosis of the existing sanitary sewage system, as well as a surveying of the expansion plans of that. Such surveying was subsidized through information provided by the Sanitation Concessionaire and the City Hall. It was noted that the existing treatment system has satisfied the removal standards established by law, where the effluent, after treated, it is released in the Pernada Stream. However, with the expansion of the system, the purifying capacity of the watercourse may be impaired, thus, it was applied the mathematical model of Streeter-Phelps (1925) to evaluate the self-purification capacity of the water course. For this analysis was considered the period from May to October 2015, for which was performed a systematic monitoring of flow in the receiving body the amount of discharge of effluents by the Sewage Treatment Station of the city, being observed a critical flow of 35.04 L/s in September. In modeling, besides the data of flow, it was used the OD values and BOD5 obtained from monitoring and quality control reports provided by the concessionaire, condition in which the coefficients of deoxygenation (K1), decomposition (Kd) and reaeration (K2) have been established. Thus, from the OD and BOD profiles obtained, it can be concluded that for BOD5 the maximum flow effluent of Pernada Stream is already being operated and increases in the released volume may exceed the limit of its self-purification capacity.
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