Self-purification of rivers receiving wastewater discharges: which factors materially contribute to the natural purification capacity?

Introduction / Context:
Rivers possess a finite capacity to assimilate organic loads through dilution, decay, and re-aeration. Understanding the drivers of self-purification helps in setting discharge standards and locating outfalls to protect aquatic life and downstream users.

Given Data / Assumptions:

• Organic wastewater discharge into a flowing river.
• Oxygen-demanding reactions (BOD) consume dissolved oxygen (DO).
• Re-aeration from atmosphere and tributary inflows replenishes DO.

Concept / Approach:

Hydrology governs dilution and travel time; DO level sets the immediate oxygen reserve; temperature controls biochemical kinetics and oxygen solubility; and turbulence elevates gas transfer, enhancing DO recovery. All four together determine the sag curve (DO deficit vs distance downstream) and ultimate recovery point.

Step-by-Step Solution:

Verification / Alternative check:

Apply a DO sag model (e.g., Streeter–Phelps) to simulate deficit and check compliance with water-quality objectives.

Why Other Options Are Wrong:

Each individual factor is indeed a contributor; thus the combined answer “All the above” is correct.

Common Pitfalls:

Ignoring temperature seasonality; overestimating re-aeration in deep, slow reaches; not considering diurnal photosynthesis/respiration effects of algae.

Final Answer:

All the above.