What can occur if ammonia levels increase in the primary clarifier?

When ammonia levels increase in the primary clarifier, the primary sludge can become septic. This is due to the fact that elevated ammonia can alter the biological processes occurring within the sludge. If conditions in the clarifier become anaerobic or if there is an excessive buildup of ammonia, the bacteria that would typically promote healthy microbial activity may die off or become inhibited. This results in a breakdown of organic matter under anaerobic conditions, leading to the production of gases such as hydrogen sulfide and ultimately causing the sludge to become septic, which is characterized by foul odors and reduced settleability.

In contrast, the other outcomes mentioned do not accurately reflect the consequences of increased ammonia levels. For example, primary sludge does not dissolve in these conditions; instead, it becomes destabilized. The concept of a "swimming pool effect" does not apply in this context, as it is more related to physical mixing rather than biological conditions. Lastly, stable primary sludge would be indicative of healthy microbial activity, which would not be the case with rising ammonia levels leading to septic conditions. Thus, the correct choice relates directly to the biological impact of ammonia on the clarifier and its sludge.