Which activated sludge process has a detention time of 18 to 20 + hours in the aeration basin?

The activated sludge process with a detention time of 18 to 20+ hours in the aeration basin is the extended aeration process. This process is designed to achieve higher levels of treatment by increasing the time the wastewater remains in the aeration basin. The longer detention time allows for more thorough oxygenation of the mixed liquor, enabling better degradation of organic matter and aiding in the stabilization of the activated sludge.

Extended aeration is particularly effective for smaller treatment facilities, where longer retention times help in the reduction of biochemical oxygen demand (BOD) and ammonia levels, making it ideal for treating wastewater under varying load conditions. This process also often results in a more stable sludge and can handle fluctuations in flow and load more effectively.

While other processes may have varying detention times, they do not typically reach the same length as extended aeration. For example, the contact stabilization process focuses on shorter aeration times followed by a stabilization phase and is designed to promote higher rates of settling in less time. Complete mix processes generally maintain shorter detention times to ensure thorough mixing and immediate treatment. Batch reactors operate in discrete cycles rather than continuous flows, making their operational dynamics different from those of continuous processes like extended aeration.