Which version of the activated sludge process will have a F/M ratio of 0.03 to 0.15 and a hydraulic detention time of 20 to 30 hours?

• A. Complete mix
• B. Oxidation ditch
• C. Conventional
• D. Extended aeration

Answer: (D)

In activated sludge design, the food-to-microorganism ratio (F/M) and the hydraulic detention time (HRT) are controlled to shape how aggressively biomass grows and how long wastewater stays in the reactor. The extended aeration setup is designed to run with a very low F/M—meaning only a small amount of organic food per unit of biomass—and a long detention time, typically around 20 to 30 hours. This combination encourages slow, thorough stabilization of the wastewater and minimizes sludge production, producing higher-quality effluent. The specified F/M range of 0.03 to 0.15 and the 20–30 hour HRT fit this configuration perfectly. Other configurations, like conventional or complete-mix systems, generally operate with higher F/M and shorter detention times, so they don’t match these values as closely. Oxidation ditch can have longer retention than basic systems, but it’s not typically associated with such a low F/M and such long HRT as a standard design.