Input Data
Example Data Table
Example only. Replace with your site values and standards.
| Scenario | Q (m³/d) | S₀ (mg/L) | V (m³) | MLSS (mg/L) | F (kg/d) | M (kg) | F/M (1/d) |
|---|---|---|---|---|---|---|---|
| Conventional | 12,000 | 220 | 6,500 | 3,000 | 2,640 | 19,500 | 0.1354 |
| Extended aeration | 7,500 | 180 | 9,800 | 3,500 | 1,350 | 34,300 | 0.0394 |
Formula Used
The food-to-microorganism ratio expresses the applied biodegradable load per unit of biomass in the aeration basin.
- F/M = F ÷ M
- F = Q × S₀
- M = V × X
Unit conversion used:
- 1 mg/L = 0.001 kg/m³
- F (kg/d) = Q (m³/d) × S₀ (mg/L) × 0.001
- M (kg) = V (m³) × X (mg/L) × 0.001
This tool also reports an optional MLVSS-based F/M using Xᵥ = X × (MLVSS fraction).
How to Use This Calculator
- Select the process type that best matches your plant configuration.
- Enter average flow (Q) and influent BOD (S₀) in the stated units.
- Provide aeration volume (V) directly, or compute from dimensions.
- Enter aeration basin MLSS (X). Optionally enter an MLVSS fraction.
- Press Calculate F/M to view results above the form.
- Use the export buttons to save a CSV or PDF record.
Professional Guide
The food-to-microorganism (F/M) ratio is a practical control indicator for activated sludge systems. It links the incoming biodegradable load (“food”) to the active biomass available to consume it (“microorganisms”) in the aeration basin. When used with solids retention time (SRT), dissolved oxygen, and settling observations, F/M helps operators and designers judge whether a process is overloaded, underloaded, or balanced for stable treatment performance.
Higher F/M generally means more substrate per unit biomass. Plants operating high-rate or during shock loads may see faster growth, higher oxygen demand, and increased risk of poor settling if floc structure is stressed. Lower F/M typically indicates longer sludge ages and more complete oxidation. Extended aeration and oxidation ditch processes often run at low F/M to support nitrification and produce a more stabilized waste sludge, though energy needs can rise due to longer aeration times and mixing requirements.
This calculator uses standard unit handling to reduce mistakes. Flow is entered in m³/d, and BOD and MLSS are entered in mg/L. The tool converts mg/L to kg/m³ and calculates daily food load, basin biomass mass, and F/M on an MLSS basis. An optional MLVSS fraction is included because volatile solids better represent the active biological portion of the mixed liquor. If you track MLVSS routinely, compare your MLVSS-based F/M consistently against your chosen plant targets.
Worked example (same as the table):
- Q = 12,000 m³/d, S₀ = 220 mg/L, V = 6,500 m³, MLSS = 3,000 mg/L
- Food load: F = 12,000 × 220 × 0.001 = 2,640 kg BOD/d
- Biomass mass: M = 6,500 × 3,000 × 0.001 = 19,500 kg
- F/M (MLSS) = 2,640 ÷ 19,500 = 0.1354 1/d
- If MLVSS fraction = 0.75, then MLVSS = 2,250 mg/L and F/M (MLVSS) ≈ 0.1805 1/d
For design documentation, record the basis used (BOD or another parameter), the selected process type, and the volume definition (working volume versus geometric). When results are outside a typical band, review whether the input represents average conditions, whether primary treatment changes the effective load, and whether MLSS values reflect true aeration basin conditions.
FAQs
1) What does the F/M ratio tell me?
It indicates how much biodegradable load is applied per unit of biomass in the aeration basin. It helps screen loading level and supports operational decisions alongside SRT, DO, and settling performance.
2) Which concentration should I enter for X?
Use aeration basin MLSS (mixed liquor), not RAS or WAS concentrations. If you have multiple basins, enter the representative MLSS for the basins included in the volume you selected.
3) Can I use COD instead of BOD?
Yes, but keep the basis consistent. Replace S₀ with your chosen parameter and interpret “F” accordingly. Compare the resulting ratio to targets developed for the same parameter and sampling approach.
4) What volume should I use for V?
Use the working aeration volume that contains the mixed liquor biomass. Exclude dead zones or unused compartments where mixing is not maintained. If using dimensions, apply an operating fraction that matches plant conditions.
5) Why does the tool show an MLVSS-based F/M?
MLVSS better represents the active biological solids fraction. When inert solids are significant, MLVSS can provide a clearer biological loading picture. Use the same fractioning method and sampling frequency for trend comparisons.
6) My F/M seems unusually high or low. What should I check?
Confirm units, average versus peak flow, and whether BOD values reflect primary treatment effects. Verify the aeration volume basis, and ensure the MLSS sample location is within the mixed, aerated zone.
7) How do multiple basins affect the calculation?
Use the total working volume of the basins operating in parallel for the same process stage. If basins operate differently, calculate separate F/M values per basin group and document the flow split assumptions.