Glossary term

Food-to-Microorganism Ratio

Activated-sludge loading metric comparing biodegradable organic load with biomass inventory, used to interpret aeration basin stability, SRT, oxygen demand and settleability.

Branch: Environmental Engineering
Glossary type: metric
Content: Glossary term
Updated: Jun 25, 2026
Revision: v1.0.0 · reviewed

Definition

metric

Food-to-microorganism ratio is the organic loading rate applied to an activated-sludge biomass inventory, usually expressed as kilograms of BOD or COD per kilogram of MLVSS per day.

Food-to-microorganism ratio, commonly written as F/M ratio, compares biodegradable organic load with the biomass available to consume it in an activated-sludge aeration basin. It connects influent BOD or COD, flow rate, aeration basin volume, MLSS or MLVSS, sludge wasting, solids retention time, oxygen demand, floc condition, sludge volume index and effluent stability. It is a screening and operating metric, not a substitute for a full mass balance, respirometry, SRT calculation or effluent compliance evidence.

Food-to-microorganism ratio is the organic loading rate applied to an activated-sludge biomass inventory. It is usually abbreviated F/M and used to judge whether the aeration basin has enough active biomass for the incoming biodegradable load.

The metric is useful because the same influent load can behave differently in a small basin with low biomass than in a larger basin with a high mixed-liquor inventory. F/M helps operators connect flow, BOD or COD, MLSS, wasting, oxygen demand and sludge settleability.

Engineering Meaning

A common form is:

\displaystyle \frac{F}{M}=\frac{Q S_0}{V X_v}

where $Q$ is influent flow, $S_0$ is biodegradable substrate concentration, $V$ is aeration basin volume and $X_v$ is volatile suspended biomass concentration. The result is normally reported as:

\text{kg BOD}/\text{kg MLVSS}/\text{d}

or on a COD basis if the plant uses COD fractions.

Food Load

The food term must state the analytical basis:

F=Q S_0 10^{-3}

when $Q$ is in $\text{m}^3/\text{d}$ and $S_0$ is in $\text{mg/L}$ . If:

Q=18000\ \text{m}^3/\text{d},\quad S_0=220\ \text{mg BOD/L}

then:

F=18000(220)10^{-3}=3960\ \text{kg BOD/d}

Primary effluent BOD, soluble BOD, total COD and readily biodegradable COD are not interchangeable without a stated reason.

Biomass Inventory

The microorganism term is the active biomass inventory:

M=V X_v 10^{-3}

For:

V=7200\ \text{m}^3,\quad X_v=2500\ \text{mg/L}

then:

M=7200(2500)10^{-3}=18000\ \text{kg MLVSS}

Using MLSS instead of MLVSS can be acceptable for quick screening, but the inert fraction should be understood.

Worked Screen

The resulting loading is:

\displaystyle \frac{F}{M}=\frac{3960}{18000}=0.22\ \text{kg BOD}/\text{kg MLVSS}/\text{d}

This value is not automatically good or bad. It must be interpreted with process type, temperature, nitrification objective, SRT, oxygen-transfer capacity, clarifier limits and effluent permit requirements.

For troubleshooting, the averaging period matters. A daily average F/M can hide morning peak loads, storm dilution, sidestream returns or short-term biomass loss after excessive wasting.

Relation to SRT and Oxygen

F/M and solids retention time often move in opposite directions, but they are not the same calculation. Increasing wasting can reduce biomass inventory, increase F/M and reduce sludge age. Lower F/M can indicate more biomass relative to load, but it can also accompany old sludge, oxygen demand, endogenous respiration, foaming or poor settleability.

Oxygen demand must still be checked independently:

O_{req}=O_C+O_N

because a moderate F/M ratio does not prove that the aeration basin can transfer enough oxygen where the load is applied.

Validation Evidence

Useful evidence includes flow-weighted influent and primary-effluent BOD or COD, MLSS and MLVSS samples, aeration basin active volume, wasting records, SRT calculation, oxygen-transfer capacity, DO profile, sludge volume index, ammonia trend and effluent BOD or COD. A reliable F/M trend uses consistent sampling locations and time averaging.

Common Mistakes

Common mistakes are mixing BOD and COD bases, using total tank volume instead of active aeration volume, using MLSS without noting volatile fraction, ignoring return sludge and wasting changes, treating a textbook F/M range as a permit criterion and diagnosing settleability from F/M alone.

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Disciplines