Inputs
Example data table
| Case | Va (m³) | Vc (m³) | Xm (mg/L) | Xc (mg/L) | Qe (m³/d) | Xe (mg/L) | Qw (m³/d) | Xw (mg/L) | Calculated SRT (days) |
|---|---|---|---|---|---|---|---|---|---|
| Example | 5,000 | 1,000 | 3,000 | 5,000 | 20,000 | 15 | 300 | 10,000 | ≈ 6.061 |
Formula used
Sludge age (SRT) is computed as the mass of solids in the system divided by the mass of solids leaving the system each day.
Msystem = (Va·Xm + Vc·Xc) · 0.001
Ṁ = (Q · X) · 0.001
- V in m³, Q in m³/day, X in mg/L.
- The factor 0.001 converts mg/L to kg/m³.
- Disable clarifier inventory if you prefer the simplified approach.
How to use this calculator
- Enter operating volumes for aeration and (optionally) clarifier inventory.
- Enter MLSS, effluent TSS, and WAS concentration from recent lab data.
- Use daily-average flows for effluent and wasting rates.
- Enable extra loss streams if solids leave elsewhere in your process.
- Click Calculate SRT to view results under the header.
- Use the download buttons to store calculations in project records.
Technical guide
1) What sludge age represents
Sludge age, or Solids Retention Time (SRT), is the average time biological solids remain in the activated sludge system. It is a key stability indicator for carbon removal, nitrification, and settling performance. Derived from a solids balance, it supports practical day-to-day control. In design and operations, SRT links process loading to biomass growth, clarifier performance, and wasting needs.
2) Inventory used by this calculator
The calculator estimates solids inventory as mixed liquor mass in the aeration basin plus optional clarifier inventory. Internally, volumes are standardised to m³ and concentrations to mg/L. Mass is computed using the conversion: 1 mg/L = 1 g/m³, therefore kg = (m³ × mg/L) × 0.001.
3) Solids leaving the system each day
Solids loss is the daily mass removed via wasting (Qw × Xw) plus solids escaping in effluent (Qe × Xe). Two optional side streams are included to capture bypasses, scum wasting, filter backwash, or sidestream returns that export solids. If Qw is measured intermittently, use a daily average. Adding side losses and consistent sampling points improves the mass balance and SRT accuracy.
4) Typical design and operating ranges
Many municipal systems operate around 3–8 days for reliable carbon removal, while nitrification commonly requires higher SRT such as 8–15+ days depending on temperature and ammonia targets. At colder temperatures, the required SRT increases. In the example data on this page, the inputs produce about 6.06 days, which aligns with common carbon-removal operation bands. Always confirm targets with project criteria, seasonal temperature, and permit limits.
5) Interpreting results for control
If calculated SRT is below target, increase sludge inventory (higher MLSS/volume) or reduce solids leaving (lower wasting), while watching clarifier loading and oxygen supply. If SRT is above target, increase wasting or reduce inventory to avoid old sludge, poor settleability, and unnecessary aeration demand. Track SRT trends with ammonia results and settling observations to support stable decisions.
FAQs
1) What is a good SRT value?
It depends on objectives. Carbon removal often performs well around 3–8 days, while nitrification typically needs higher SRT, commonly 8–15+ days. Temperature and ammonia limits strongly influence the target.
2) Should I include clarifier solids in SRT?
Include it when significant solids reside in the clarifier or you want a fuller inventory. Excluding it gives a simplified estimate. Use consistent practice across calculations so trends remain comparable.
3) Which solids concentration should I use for Xw?
Use the concentration at the wasting point. If wasting from RAS/underflow, use that solids concentration. If wasting from mixed liquor, use the mixed liquor solids concentration.
4) Why include effluent TSS (Xe) if it is small?
Even low Xe contributes to solids loss at high flows. Including it improves accuracy, especially when wasting is low or when effluent solids rise during bulking, washout, or wet-weather events.
5) What causes unrealistically high SRT results?
Common causes are missing wastage (Qw too low), low Xw due to dilution, or not accounting for side-stream solids losses. Check sampling points, units, and whether extra loss streams should be enabled.
6) Can this calculator be used for step-feed or multiple trains?
Yes, if you use total operating volumes and representative average solids/flows across trains. For separate trains with different controls, calculate SRT per train to avoid masking local issues.
7) How often should SRT be updated?
For operations, daily or several times per week is common. For design checks, update whenever flows, MLSS, wasting rate, or effluent solids assumptions change.