Which factor would be a consequence of decreased RAS while keeping WAS constant?

The correct choice is that RAS concentration increases as a consequence of decreased return activated sludge (RAS) while keeping waste activated sludge (WAS) constant.

When the amount of RAS returning to the treatment process is decreased, the concentration of RAS that remains in the system actually increases because there is less volume of liquid returning to mix with the solids. This increased concentration occurs due to the same mass of solids being retained in a smaller volume, which raises the concentration level.

In a typical activated sludge process, maintaining a balance between RAS and WAS is crucial for effective treatment. A decrease in RAS implies that there is a reduced flow of mixed liquor back into the aeration basin, leading to a higher concentration of solids per unit volume in the remaining sludge stream. This situation can result in operational challenges, as the maintenance of appropriate sludge concentration is key to effective biological treatment performance.