During what operational conditions might the ammonia concentration in effluent remain relatively low?

The correct response highlights the effect of high pH and long hydraulic retention time (HRT) on the ammonia concentration in effluent. When the pH is high, the ammonia is more likely to exist in its unionized form (NH3), which can enhance its volatilization and reduce its concentration in the effluent. This is particularly true in biological treatment processes, where the pH can influence the activity of specific microorganisms capable of nitrifying ammonia to nitrate.

Furthermore, a long HRT allows for extended contact time between the influent and the microorganisms in the treatment system, providing ample opportunity for the biological processes that convert ammonia into less harmful forms, such as nitrate, to proceed effectively. Therefore, under these conditions, the conversion rates of ammonia to nitrate can be maximized, leading to lower concentrations of ammonia in the effluent.

In contrast, scenarios with low pH typically inhibit the biological activity of nitrifying bacteria and can result in higher ammonia concentrations. Short HRT may not provide sufficient time for effective biological treatment, increasing ammonia levels. Low temperatures can slow down biochemical reactions, also contributing to higher ammonia levels, as the microorganisms may not perform optimally in colder conditions. High solids concentration can indicate an overloaded system, which may not