Extended aeration systems are very common in rural parts around the world and in small communities. These are a variant of activated sludge process with high (20 to 30 days) SRT to achieve stabilization in situ, which helps in reducing sludge disposal requirements significantly. Oxidation Ditch is a special type of extended aeration system where a ring or oval-shaped channel is equipped with mechanical aeration and mixing devices, followed by a secondary settler. The screened influent enters the oxidation ditch for biological treatment to remove BOD. These may also be configured for nitrification, denitrification and phosphorus removal, if needed. These are easy to maintain and operate but require a significant amount of space.
Important to understand is also that upgrading activated sludge plants with IFAS to increase capacity and improve performance can be done without increasing the capacity of existing clarifiers, a huge advantage of IFAS. In this case, the TSS generated from the biological process has better settling characteristics, which means the existing clarifiers can often handle higher flows than before without increasing their volume or surface area.