Water resources management in industrial and urban environments requires a deep understanding of the resilience capacity of receiving water bodies.

The phenomenon of self-purification (or natural purification) is, in essence, a set of physical, chemical, and biological processes that interact to reduce the pollutant load of a watercourse, seeking to re-establish the original ecological balance (JORDÃO; PESSÔA, 1995).

However, this capacity is not infinite and depends on kinetic variables that govern the oxygen balance.

The process is primarily based on the oxygen balance within the water body. Self-purification occurs due to the antagonism between two main processes:

• Oxygen Consumption (Deoxygenation): The decay rate of BOD (Biochemical Oxygen Demand) follows first-order kinetics, where the reaction speed is proportional to the remaining concentration of organic matter. This consumption is governed by the deoxygenation coefficient ($K_1$), which is highly dependent on temperature and the biodegradability of the effluent (NUVOLARI, 2003).
• Oxygen Replenishment (Reaeration): Simultaneously, atmospheric oxygen is introduced into the water mass through the air-water interface. This process is quantified by the reaeration coefficient (K2). K2 is influenced by turbulence, flow velocity, and channel depth. Shallow and turbulent rivers exhibit significantly higher reaeration rates than deep, slow-moving rivers.

Streeter-Phelps describes the variation of the oxygen deficit over time (or distance, in lotic systems) as a function of deoxygenation and reaeration rates.

The curve resulting from this interaction is known as the Oxygen Sag Curve (or Dissolved Oxygen Depletion Curve). It demonstrates that, immediately after the point where the effluent mixes with the receiving body, the oxygen deficit tends to grow until it reaches a critical point.

At this location, the consumption rate exactly equals the replenishment rate, representing the minimum DO (Dissolved Oxygen) level the river will reach (VON SPERLING, 2014). From this point forward, if the reaeration capacity becomes predominant, the recovery phase begins.

Along the river's course, self-purification manifests in distinct zones, each with specific biological and chemical characteristics:

• Degradation Zone: Characterized by an abrupt drop in DO content. There is an increase in turbidity and changes in biota (ANDRADE, 2010).
• Active Decomposition Zone: This is the stage of greatest environmental stress. If oxygen is completely consumed, anaerobic processes begin, generating foul odors and making higher aquatic life unsustainable.
• Recovery Zone: Where the reaeration rate exceeds the remaining BOD. DO begins to rise, nitrification occurs (oxidation of ammonia into nitrite and nitrate), and the return of algae and protozoa is observed.
• Clean Water Zone: Equilibrium is restored. DO returns to levels near saturation, and BOD returns to baseline values. Water quality should once again comply with the classifications established by CONAMA Resolution 357/2005.

To evaluate the purification of a water body, other factors that refine technical analysis are also assessed:

• Nitrification (Kn): Oxygen consumption for the oxidation of ammoniacal nitrogen is a critical step in effluents with high nitrogen loads.
• BOD Sedimentation (Ks): Part of the organic matter may be removed from the water column by settling, forming sludge at the bottom that also exerts oxygen demand (Benthic Demand).

The preservation of our water resources requires expertise and commitment. Understanding self-purification is recognizing that human intervention must be based on scientific data and environmental responsibility!

Bibliographic References

• ANDRADE, L. N. Saneamento. São José do Rio Preto: Cultura Acadêmica, 2010.
• JORDÃO, E. P.; PESSÔA, C. A. Tratamento de Esgotos Domésticos. 3. ed. Rio de Janeiro: ABES, 1995.
• NUVOLARI, A. Esgoto Sanitário: Coleta, Transporte, Tratamento e Reúso Agrícola. São Paulo: Blucher, 2003.
• STREETER, H. W.; PHELPS, E. B. A Study of the Pollution and Natural Purification of the Ohio River. (1925).
• VON SPERLING, M. Estudos e modelagem da qualidade da água de rios. Belo Horizonte: Editora UFMG, 2014.