If an activated sludge plant experiences a high organic load, what most likely occurs in the aeration tank?

In an activated sludge process, if there is a high organic load, microorganisms in the aeration tank consume the organic material as their food source. This increased activity leads to a higher demand for oxygen, as these microorganisms require dissolved oxygen (DO) to metabolize the organic matter efficiently.

As the organic load increases, the competition for available dissolved oxygen rises. If the aeration system cannot supply enough oxygen to meet the demands of the microorganisms, it will result in a significant decrease in dissolved oxygen levels within the tank. Low DO conditions can lead to stress on the microbial population and can hinder effective treatment, potentially resulting in anaerobic conditions.

In contrast, a decrease in pH could occur due to increased production of organic acids during high loading conditions, but it is not the most direct effect observed in the aeration tank. Similarly, significant increases in DO and no changes in DO are not typical outcomes under high organic load conditions due to the described oxygen consumption dynamics. Thus, in the scenario posed by the question, the significant decrease in dissolved oxygen is the most likely outcome.