How Excessive COD Disrupts Aquatic Equilibrium

Chemical Oxygen Demand (COD) serves as a critical indicator of the organic pollutant load in water. When COD levels exceed a water body’s natural self-purification capacity, a cascade of detrimental effects is triggered, fundamentally disrupting aquatic equilibrium.

The primary disturbance begins with oxygen depletion. Elevated COD signifies an overabundance of organic compounds, which become a food source for aerobic microorganisms. These microbes proliferate rapidly, consuming dissolved oxygen during respiration and decomposition. As oxygen levels plummet, the aquatic environment shifts from oxic to hypoxic, and eventually to anoxic conditions. Fish and many macroinvertebrates suffocate and die, leading to a rapid loss of biodiversity. The water body, once teeming with life, becomes eerily quiet.

In the absence of oxygen, anaerobic bacteria take over the decomposition process. This anaerobic metabolism produces reduced, often toxic by-products such as hydrogen sulfide, ammonia, methane, and volatile organic acids. The water may turn black or grey, emit foul odors reminiscent of rotten eggs, and become hostile to most higher life forms. This represents a fundamental chemical shift in the water column and sediments, locking the system into a stable but polluted state.

The ecological structure is consequently dismantled. The food web collapses as key consumers disappear. With the die-off of oxygen-dependent plants and algae, primary production declines. The ecosystem, once a complex network of producers, consumers, and decomposers, simplifies into a microbial-dominated regime focused on fermentation and putrefaction. The river or lake loses its ecological functions—habitat provision, nutrient cycling, and aesthetic value—transforming into a lifeless conduit or septic basin.

Furthermore, high COD often coincides with excess nutrients. Under anoxic conditions, sediments release stored phosphorus, intensifying eutrophication. This creates a vicious, self-sustaining cycle of pollution that persists even after external pollutant inputs are controlled. Recovery becomes immensely difficult and costly.

In summary, excessive COD acts as a silent disruptor. It initiates a chain reaction: oxygen starvation leads to biological death, which forces a toxic anaerobic regime, culminating in the total collapse of aquatic ecosystem balance. Monitoring and controlling COD is therefore not just about a number; it is about preserving the very life support system of our freshwater resources.