Sludge Age SRT Calculator
Plotly Graph
This chart shows how SRT changes when waste sludge flow changes around the current operating point.
Formula Used
General solids retention time formula
SRT = Total solids inventory in system ÷ Daily solids leaving system
Metric version
SRT (days) = [(Va × MLSS) + (Vc × Xc)] ÷ [(Qw × Xw) + (Qe × Xe)]
Va = aeration volume, MLSS = mixed liquor suspended solids, Vc = clarifier solids volume, Xc = clarifier solids concentration, Qw = waste sludge flow, Xw = waste sludge solids concentration, Qe = effluent flow, Xe = effluent TSS. With metric inputs in m³ and mg/L, the same conversion factor cancels in numerator and denominator.
US customary version
Inventory (lb) = Volume (MG) × Concentration (mg/L) × 8.34
Solids/day (lb/day) = Flow (MGD) × Concentration (mg/L) × 8.34
SRT (days) = Total inventory (lb) ÷ Daily solids leaving (lb/day)
The clarifier inventory is optional in many simplified checks, but it is included here for a fuller solids balance.
How to Use This Calculator
- Choose metric or US customary units.
- Enter aeration basin volume and MLSS concentration.
- Add optional clarifier solids volume and concentration if you want total system inventory.
- Enter waste sludge flow and waste sludge solids concentration.
- Enter final effluent flow and effluent TSS.
- Press Calculate SRT to show the result above the form.
- Review inventory, leaving solids, turnover, and the sensitivity chart.
- Use the CSV and PDF buttons to export the current summary.
Example Data Table
| Example | Unit System | Aeration Volume | MLSS | Clarifier Volume | Clarifier Solids | Waste Flow | Waste Solids | Effluent Flow | Effluent TSS | Estimated SRT |
|---|---|---|---|---|---|---|---|---|---|---|
| Activated Sludge Basin A | Metric | 3200 m³ | 3200 mg/L | 250 m³ | 6000 mg/L | 85 m³/day | 9000 mg/L | 14500 m³/day | 18 mg/L | 11.44 days |
| Compact Utility Plant | Metric | 1800 m³ | 2800 mg/L | 120 m³ | 5000 mg/L | 95 m³/day | 8500 mg/L | 9200 m³/day | 22 mg/L | 7.53 days |
FAQs
1. What is sludge age or SRT?
It is the average time biological solids remain in the treatment system. It links biomass inventory to the solids removed each day through wasting and effluent loss.
2. Why does effluent TSS matter in SRT?
Effluent solids are solids leaving the plant. Ignoring them can overstate SRT, especially when clarifier performance is unstable or final effluent solids rise.
3. Should clarifier solids inventory be included?
For a more complete solids balance, yes. Some quick field checks use only aeration inventory, but full-system SRT is stronger when secondary clarifier inventory is added.
4. What happens when SRT is too low?
Low SRT means solids leave too quickly. Biomass inventory can drop, treatment stability can weaken, and the process may struggle during variable loading conditions.
5. What happens when SRT is too high?
Very high SRT can raise aeration demand, increase older biomass, and reduce operational flexibility. It may also signal under-wasting or excessive solids storage.
6. Can I use this for design and operations?
Yes. It is useful for conceptual construction planning, utility design reviews, and day-to-day operating checks where solids inventory and wasting rates are being evaluated.
7. Why is the graph based on waste sludge flow?
Waste sludge flow is one of the quickest operator controls affecting SRT. The graph shows how raising or lowering wasting changes solids age around the current condition.
8. Are the example values universal targets?
No. They are sample inputs for demonstration only. Actual targets depend on process goals, plant configuration, loading patterns, and operating strategy.