Calculator Inputs
Formula Used
This calculator uses a steady-state solids age (SRT/MCRT) relationship:
- Mass in system (kg) = Vaer·XMLSS + Vclar·Xclar
- Solids leaving (kg/day) = QWAS·XWAS + Qeff·Xeff
- SRT (days) = (Mass in system) / (Solids leaving per day)
Conversions used: 1 mg/L = 0.001 kg/m³. Results assume daily averages and stable process conditions. citeturn0search2turn0search13
How to Use This Calculator
- Choose a mode: target age to find required wasting, or enter current wasting to see achieved age.
- Enter daily flow, basin volumes, and measured solids concentrations.
- Click Calculate to display results above the form.
- Download a CSV for spreadsheets or a PDF for reports.
- Adjust wasting gradually in operations and re-check with new data.
Example Data Table
| Scenario | Flow (m³/d) | Aeration Vol (m³) | MLSS (mg/L) | WAS (mg/L) | Target Age (days) | Result (WAS m³/d) |
|---|---|---|---|---|---|---|
| Typical municipal | 5,000 | 2,500 | 3,000 | 8,000 | 10 | ~92 |
| Higher age objective | 5,000 | 2,500 | 3,000 | 8,000 | 15 | ~55 |
| Thicker WAS | 5,000 | 2,500 | 3,000 | 10,000 | 10 | ~74 |
Process objective and control
Waste activated sludge wasting is a primary lever for controlling biological inventory in suspended growth systems. By matching solids leaving the plant with the desired solids age, operators stabilize effluent quality, oxygen demand, and nutrient removal. This calculator links target solids age to a daily wasting flow, using measured concentrations and basin volumes to represent the active inventory. For design teams, the same framework helps compare scenarios, evaluate wasting pump capacity, and communicate operating targets during commissioning and training for staff.
Key inputs and sampling guidance
Use daily average flow and volumes that reflect the portion of the process holding mixed liquor. Measure MLSS in the aeration basin at consistent locations and times, and record WAS concentration from the wasting line or thickened stream. If clarifier storage is material, include clarifier volume and a representative solids concentration. Enter effluent TSS to account for solids escaping with the final discharge.
Interpreting required wasting flow
The reported WAS flow is the hydraulic rate needed to remove the required solids mass per day. The tool also shows equivalent units, including liters per minute and percent of plant flow, to support pump selection and setpoint entry. If effluent solids losses alone meet the target, the calculator clips wasting to zero and flags the condition for review.
Common adjustment strategies
When achieved solids age is below the target, reduce wasting in small steps and recheck after the system responds. When age is too high, increase wasting gradually or increase WAS concentration by improving thickening. Verify that return rates, aeration capacity, and clarifier performance remain adequate during changes, because wasting affects sludge blanket behavior and oxygen utilization.
Reporting and compliance notes
Exported CSV and PDF outputs support logs, shift handover, and regulatory documentation. Record the measurement date, sampling method, and any process upsets alongside the results. The equations assume steady operation and do not replace detailed process models, but they provide transparent, repeatable calculations that help align wasting practice with performance targets.
FAQs
1) What does WAS rate mean in this tool?
It is the daily wasting flow required to remove a calculated mass of solids from the process, based on the target solids age and measured concentrations.
2) Which flow should I enter as plant flow?
Use the average daily flow through the biological process. If you have a separate effluent flow value, enter it in the effluent field or leave it blank to match plant flow.
3) Should I include clarifier solids storage?
Include it when clarifiers hold significant mixed liquor for long periods, such as during high blanket depths. Exclude it when storage is minimal or when you want a conservative, aeration-only estimate.
4) Why is effluent TSS included?
Solids leaving in the final effluent reduce the solids age even if you do not waste. Accounting for that loss improves realism, especially when effluent TSS is elevated.
5) What if the calculator shows zero required wasting?
It means the estimated effluent solids loss already meets the target leaving rate. Confirm your inputs and consider operational risks before reducing wasting to zero.
6) How often should I update inputs?
Update at least daily during routine operation and more frequently during upsets, seasonal shifts, or major setpoint changes. Use consistent sampling times and record averages rather than single grab outliers.