BOD Removal Efficiency Formula: Calculate Percent Removal
Master the BOD removal efficiency formula with step-by-step examples, common exam traps, and tips to nail percent removal calculations.
What's the BOD Removal Efficiency Formula?
The BOD removal efficiency formula is a straightforward percent-change calculation that tells you how much BOD your plant is removing between two sampling points.
% Removal = ((Influent BOD - Effluent BOD) / Influent BOD) x 100
That's it. Influent minus effluent, divided by influent, times 100. If you've ever calculated a percent change in anything - a price drop, a weight loss, whatever - you already know this formula. The trick on the exam isn't the math itself. It's knowing which numbers go where and not confusing this with a loading rate calculation.
Key Takeaway
BOD removal efficiency measures how much organic matter your treatment process removes, calculated as (Influent BOD - Effluent BOD) / Influent BOD x 100. Secondary treatment plants typically achieve 85-95% BOD removal. If your number falls below 85%, something may be off with your process or you could be out of permit compliance.
How Do You Calculate BOD Removal Step by Step?
To calculate BOD removal, subtract effluent BOD from influent BOD, divide by influent BOD, and multiply by 100. Here's a standard exam problem so you can see exactly how to plug and chug.
Worked Example
Given: A wastewater treatment plant has an influent BOD of 220 mg/L and an effluent BOD of 18 mg/L. What is the BOD removal efficiency?
Step 1: Identify your values Influent BOD = 220 mg/L Effluent BOD = 18 mg/L
Step 2: Subtract effluent from influent 220 mg/L - 18 mg/L = 202 mg/L removed
Step 3: Divide by the influent 202 / 220 = 0.9182
Step 4: Multiply by 100 to get the percentage 0.9182 x 100 = 91.8%
Answer: BOD Removal Efficiency = 91.8%
That 91.8% falls right in the typical range for a well-running secondary treatment plant. If the exam asks whether this plant is meeting secondary treatment standards, you'd say yes - the percent removal exceeds 85%, and the effluent of 18 mg/L is well below the 30 mg/L monthly average concentration standard. Federal secondary treatment standards under 40 CFR Part 133 generally require 85% removal of BOD5 and TSS, though individual permits may express limits differently.
What About Primary vs. Overall BOD Removal?
Here's where exam questions get a little sneakier. They might give you three numbers - raw influent, primary effluent, and final effluent - and ask you to calculate BOD removal for a specific process stage.
Worked Example
Given: Raw influent BOD = 250 mg/L, primary clarifier effluent BOD = 150 mg/L, final effluent BOD = 20 mg/L. What's the BOD removal efficiency for the secondary treatment process only?
Step 1: Identify the correct values for secondary treatment The secondary process starts after the primary clarifier. Influent to secondary = 150 mg/L (primary effluent) Effluent from secondary = 20 mg/L (final effluent)
Step 2: Subtract 150 - 20 = 130 mg/L
Step 3: Divide by the influent to secondary 130 / 150 = 0.8667
Step 4: Multiply by 100 0.8667 x 100 = 86.7%
Answer: Secondary process BOD removal = 86.7%
Bonus - Overall plant removal: (250 - 20) / 250 x 100 = 92.0%
See the difference? The overall plant removal is 92%, but the secondary process alone is only removing 86.7%. The exam loves this kind of multi-step problem because it tests whether you understand the treatment train, not just the formula.
Why Does BOD Removal Get Confused with Loading Rate?
This is the big one. Operators studying for their exam sometimes mix up removal efficiency with BOD loading rate calculations that use the pounds formula. Here's the key difference:
- Removal efficiency is a percentage. It compares what comes in to what goes out. No flow rate needed.
- Loading rate is pounds per day (lbs/day). It tells you how much organic load your plant is handling, and it absolutely requires flow and the 8.34 conversion factor.
If the question asks "what percent," you're doing removal efficiency. If it asks "how many pounds per day," you're doing the loading calculation. Read the question twice before you pick your formula.
Exam Tip
When calculating percent removal from concentrations, you don't need flow rate or the 8.34 factor
- the units cancel out. If you find yourself reaching for flow in MGD on a percent removal problem, stop and reread the question. Percent removal from concentrations only needs two numbers: in and out.
What Are the Common Exam Traps for Percent Removal?
Based on common exam patterns, here are the mistakes that cost operators points on exam day:
Flipping the fraction
The denominator is always the influent (what comes in), not the effluent. Think of it this way - you're asking "what fraction of the original load did we remove?" The original load is always your starting point.
- Wrong: (Influent - Effluent) / Effluent x 100
- Right: (Influent - Effluent) / Influent x 100
Using the wrong sampling points
When a question says "What's the removal efficiency across the secondary clarifier?" it's not asking about the whole plant. Read carefully to figure out which influent and which effluent they want. Primary clarifier influent vs. effluent, secondary process influent vs. effluent, or overall plant influent vs. effluent - these give you very different numbers from the same data set.
Forgetting to multiply by 100
Your calculator shows 0.918, and you bubble in 0.918 instead of 91.8%. On a multiple-choice exam, 0.918 won't be an answer option, so you'll catch it. But on a fill-in-the-blank format, this mistake will cost you the point.
Mixing up BOD removal with TSS removal
The formula structure is identical for BOD, TSS, ammonia, phosphorus - any parameter where you're comparing influent to effluent. The exam might ask for TSS removal in the same problem set. Don't accidentally use your BOD numbers for the TSS question or vice versa. Label your work.
When Do You Use BOD Removal Efficiency in the Plant?
You're calculating removal efficiency more often than you think. Every time you pull your daily sampling results and compare influent to effluent, you're essentially running this formula. Here's when it really matters:
- Monthly operating reports (MORs): Many permits require you to report percent removal for BOD and TSS. If your permit includes a percent removal requirement and you drop below 85%, you could be in violation.
- Process troubleshooting: If your removal efficiency suddenly drops from 92% to 78%, that's a red flag. Something changed - maybe your MLSS dropped and your SVI is climbing, your aeration is off, or you got a slug load of industrial waste.
- Comparing treatment units: If you run parallel treatment trains, comparing removal efficiency across each one helps you spot which unit needs attention.
The EPA's secondary treatment standards (40 CFR Part 133) generally require 85% removal for both BOD and TSS, along with effluent concentration limits (30 mg/L monthly average and 45 mg/L weekly average for BOD5). Your state permit might be tighter or may express limits differently, but 85% removal is the benchmark you'll see on most certification exams.
Quick Reference Table
| Scenario | Typical BOD Removal |
|---|---|
| Primary clarifier only | 25-40% |
| Trickling filter (secondary) | 60-80% |
| Activated sludge (secondary) | 85-95% |
| Overall plant (primary + secondary) | 85-95% |
| Advanced treatment | Can often achieve 95-99% |
These ranges show up on exams as "which process would you expect to achieve the highest BOD removal?" questions. Activated sludge consistently outperforms trickling filters for BOD removal, and knowing those ballpark numbers can turn a tough question into a gimmie.
How Does BOD Removal Connect to Other Formulas?
BOD removal efficiency doesn't live in a vacuum. Once you know your percent removal is slipping, you'll want to check your F:M ratio to see if the bugs are getting the right food-to-organism balance. Low removal efficiency with a high F:M ratio may mean your biomass can't keep up with the load. Low removal with a low F:M ratio might mean your sludge age is too high and the bugs are starving. Keep in mind that removal efficiency can also be affected by other factors like toxicity, low DO, temperature swings, or clarifier performance - so F:M is just one piece of the puzzle.
The percent removal formula also applies directly to TSS, ammonia, and phosphorus - same structure, different parameters. Master it once for BOD, and you've got it for every other parameter on the exam.
Key Takeaway
BOD removal efficiency = (Influent - Effluent) / Influent x 100. No flow rate or 8.34 factor needed. The denominator is always the influent. Secondary treatment plants should achieve 85-95% BOD removal to meet federal standards. If the exam gives you multiple sampling points, read carefully to determine which "in" and "out" they're asking about.