What F/M ratio is listed as best?

• A. 0.5 to 1.5
• B. .5 to .15
• C. 0.05 to 0.2
• D. 0.8 to 1.2

Answer: (B)

The main idea is how much food is available per unit of microorganisms in the treatment system. This Food to Microorganism (F/M) ratio determines how actively the biology can treat the waste without producing excessive sludge or starving the microbes. A balanced F/M keeps the microbes working efficiently while avoiding problems.

The commonly preferred range is about 0.15 to 0.5. Within this window, there is enough substrate to keep the microbial community active and stable, supporting good BOD removal and settling characteristics.

If the ratio gets higher, say toward 0.8 to 1.2 or even higher, there’s too much food relative to biomass, which can cause rapid microbial growth, foaming, poorer settleability, and higher sludge production. If the ratio is too low, such as 0.05 to 0.2, the microbes don’t have enough substrate to stay active, leading to slower treatment and possible process instability.