Calculators
Loading & Removal
Turn lab results into percent removal stage by stage, pounds per day, organic loading on a filter, RBC or pond, and population equivalent. Your numbers are worked through underneath.
Your numbers, step by step
- Influent → primary effluent(220 − 150) ÷ 220 × 100 = 31.8%
70 mg/L × 3.2 MGD × 8.34 = 1,868 lb/day - Primary effluent → final effluent(150 − 12) ÷ 150 × 100 = 92%
138 mg/L × 3.2 MGD × 8.34 = 3,683 lb/day - Overall, influent → final effluent(220 − 12) ÷ 220 × 100 = 94.5%
208 mg/L × 3.2 MGD × 8.34 = 5,551 lb/day
Why it works
A lab result in mg/L says how strong the water is. A flow says how much of it goes by. Multiply the two, with 8.34 to turn gallons of water into pounds, and you get the pounds per day passing that point. It's the same calculation as a chemical feed rate, which is why the three names page treats them as one idea.
- Percent removal compares what came in with what went out. Flow cancels out, so you only need the two concentrations. Add flow when you want the pounds each stage takes out.
- Stage by stage, each step uses the sample before it as its own "in". That's why the stage percentages don't add up to the overall number: secondary removing 90% of what primary left behind is a smaller share of the raw load.
- Organic loading spreads the daily pounds over the size of the unit doing the work: media volume for a trickling filter, media area for an RBC, surface acres for a pond.
- Population equivalent divides the BOD load by what one person contributes in a day. It tells you how many people a load is "worth", which is handy for sizing up an industrial discharger.
Every formula here is on the formula sheet.
Example: monthly removal report
The monthly average flow is 3.2 MGD. Influent BOD ran 220 mg/L, primary effluent 150 mg/L and final effluent 12 mg/L.
- 220 × 3.2 × 8.34 = 5,871 lb/day coming in
- (220 − 150) ÷ 220 × 100 = 31.8% through the primaries
- (150 − 12) ÷ 150 × 100 = 92% through secondary
- (220 − 12) ÷ 220 × 100 = 94.5% overall
- 208 × 3.2 × 8.34 = 5,551 lb/day of BOD removed
The overall number is the one most permits check, usually against a minimum of 85% for BOD and TSS. The stage numbers tell you where to look when the overall number slips.
Do the numbers look reasonable?
Your own plant's history is the best check. These broad ranges are for catching a typo or a units slip, not for setting targets.
| Number | Typical | If it is far off |
|---|---|---|
| Domestic influent BOD / TSS | About 150–300 mg/L each | Very low often means infiltration and inflow. Very high points to an industrial user or recycle streams. |
| Primary removal | About 25–40% BOD, 50–70% TSS | Check that the primary effluent sample came from after the clarifiers, not the splitter box. |
| Overall secondary removal | 85% or better | Compare with the same day's influent sample; composite timing matters. |
| Trickling filter | About 5–25 lb/1,000 ft³ low rate, 25–100 high rate | Diameter entered as radius cuts the volume to a quarter and quadruples the loading. |
| RBC, soluble BOD | About 2–4 lb/1,000 ft² overall; first stage often kept under ~6 | Total BOD entered instead of soluble BOD will read high. |
| Facultative pond | About 15–50 lb/acre per day | Use the water surface of the cells in service, not the property acreage. |
Before you report it
- Match the samples. Influent and effluent should be composites over the same period. A grab sample against a 24-hour composite can make removal look better or worse than it is.
- Use the flow for the same day. Pounds use the flow over the sampling period, not the design flow.
- Watch below-detection results. Report them the way your permit says. Don't swap in zero on your own.
- Know what the unit actually sees. Organic loading uses what reaches the filter, RBC or pond, including recycle, not raw influent.
This is a study and planning aid. Your permit, plant SOPs and process engineer take priority over any number on this page.