Modelagem da poluição hídrica por matéria orgânica: aplicação do modelo de Streeter-Phelps para depleção de oxigênio dissolvido

Abstract

This study aimed to apply the Streeter-Phelps mathematical model to simulate the dissolved oxygen (DO) depletion curve in a segment of the Tibagi River, located in the state of Paraná, Brazil, impacted by organic loads from the discharge of treated domestic sewage. The rationale lies in the growing concern over water pollution and the need for predictive tools to support water quality management in receiving bodies. Publicly available data on water quality and flow, obtained from reports by the Instituto Água e Terra (IAT) for the period from 2016 to 2023, were used. The methodology included data preprocessing, estimation of kinetic parameters for deoxygenation, reaeration, and degradation, and the simulation of the DO curve under five distinct scenarios, considering different treatment efficiencies and seasonal conditions. The results showed that both the efficiency of the sewage treatment system and the river flow have a direct influence on the DO concentration after the discharge point. Scenarios with higher organic load removal efficiency or greater water availability exhibited better DO recovery, indicating a higher self-purification capacity under such conditions. In contrast, scenarios with no treatment or during dry periods revealed critical depletions, with DO levels falling below the legal limit of 5.0 mg/L established by Brazilian Resolution CONAMA No. 357/2005. Although the Streeter-Phelps model presents limitations related to simplifying assumptions, such as instantaneous mixing and steady-state flow, its application proved effective for preliminary analyses and environmental planning. It is concluded that mathematical modeling is a valuable tool for assessing river self-purification and supporting decision-making, highlighting the need to strengthen monitoring networks and adopt more complex models in future studies.