In flows influent, out flows effluent. That's the easy half. The half that catches people is that "effluent" is a relative term — and that a federal rule ties the two numbers together in a way that can fail a plant producing excellent water.
"Effluent" is a relative term, not an absolute one.
One unit's effluent is the next unit's influent. That's why the words confuse people — and why "the effluent BOD is 40" is not a complete sentence.
| Term | Definition |
|---|---|
| Influent | Wastewater flowing into a treatment plant, process or unit operation |
| Effluent | Wastewater flowing out of a treatment plant, process or unit operation |
| Raw influent | What arrives at the headworks, before any treatment |
| Final effluent | Treated water leaving at the permitted outfall — what the NPDES permit regulates |
| Point in the plant | Called | Also called |
|---|---|---|
| Arriving at headworks | Raw influent | Plant influent, raw sewage |
| Leaving primary clarifiers | Primary effluent | Aeration basin influent |
| Leaving secondary clarifiers | Secondary effluent | Filter or disinfection influent |
| Leaving filtration | Filter effluent | Disinfection influent |
| Leaving the plant | Final effluent | Discharge — the permitted stream |
When someone says "the effluent BOD is 40," the first question is always: effluent of what?
Primary effluent at 130 mg/L BOD is normal. Final effluent at 130 mg/L BOD is a violation. On a data sheet, on a phone call, and in a permit, the sample point matters as much as the number. Label everything.
Some scale for what a plant is actually doing: domestic sewage is roughly 99.9 percent water. Everything a treatment plant does is aimed at the remaining tenth of a percent — organic and inorganic solids, suspended and dissolved, plus microorganisms.
| Parameter | Weak | Medium | Strong |
|---|---|---|---|
| BOD₅ (mg/L) | 110 | 220 | 400 |
| TSS (mg/L) | 100 | 220 | 350 |
| FOG (mg/L) | 50 | 100 | 150 |
Tchobanoglous and Burton (1991) — the classic Metcalf & Eddy strength classifications, still the standard reference framework.
| Parameter | Typical domestic range | Note |
|---|---|---|
| TKN | 20–85 mg/L as N | Organic nitrogen plus ammonia. This is the nitrification load. |
| Ammonia (NH₃-N) | A large fraction of TKN | Grows as a share of TKN with collection system detention time |
| pH | Near neutral | Effluent must be held 6.0–9.0 |
| Alkalinity | Varies | Consumed by nitrification. Monitor if nitrifying. |
| BOD₅ : COD ratio | Varies | A useful diagnostic — a low ratio suggests poorly biodegradable or toxic input |
| Particulate fraction | Roughly 50% of BOD | Approximate. Soluble and colloidal make up the rest. |
Influent flow and strength aren't steady. A typical plant sees two daily flow peaks: a pronounced morning peak and a more distributed evening peak. Strength varies with them, and both vary again with rainfall.
Infiltration and inflow dilute the influent. Lower BOD and TSS in the wet season, higher in the dry season.
That's normal. It becomes a compliance problem under the 85 percent removal rule below — which is the single most useful thing in this guide.
| Parameter | 30-day average | 7-day average | Removal |
|---|---|---|---|
| BOD₅ | ≤ 30 mg/L | ≤ 45 mg/L | ≥ 85% (30-day average) |
| CBOD₅ (at permitting authority option) | ≤ 25 mg/L | ≤ 40 mg/L | ≥ 85% |
| SS (TSS) | ≤ 30 mg/L | ≤ 45 mg/L | ≥ 85% |
| pH | 6.0 to 9.0 — maintained within | — | |
40 CFR § 133.102. The pH requirement applies unless the POTW demonstrates that inorganic chemicals are not added as part of treatment and that industrial contributions don't push effluent pH outside the range.
The secondary standards were built on the measured performance of plants using primary treatment followed by suspended growth activated sludge.
Trickling filter and lagoon plants are legitimate technologies that don't perform identically. Rather than requiring them to rebuild, § 133.105 created an equivalent category — where the BOD₅ 30-day average is 45 mg/L rather than 30.
If you run a trickling filter or lagoon plant, check whether your permit was written under 133.102 or 133.105. The numbers differ.
These are technology-based minimums. Your NPDES permit may be more stringent, and frequently is where water-quality-based limits apply.
§ 133.105 also directs permitting authorities to require more stringent limits where past performance shows the works could achieve them. Ammonia, nitrogen, phosphorus, metals and toxics limits come from the permit, not from Part 133. Read your permit.
The 30-day average percent removal shall not be less than 85 percent, for BOD₅ (or CBOD₅) and separately for suspended solids.
That's a relationship between two numbers, not a limit on one. You can hold a beautiful effluent and still violate it.
Colorado's regulation states the same rule from the other direction, which makes the mechanism clearer: the 30-day mean of effluent values shall not exceed 15 percent of the arithmetic mean of the influent values collected at approximately the same time during the same period.
| Scenario | Influent BOD₅ | Effluent BOD₅ | Removal | Compliant? |
|---|---|---|---|---|
| Normal strength | 220 mg/L | 15 mg/L | 93.2% | Yes |
| Normal strength, weak effluent | 220 mg/L | 30 mg/L | 86.4% | Yes — barely |
| Dilute influent, excellent effluent | 90 mg/L | 15 mg/L | 83.3% | NO |
| Very dilute, excellent effluent | 80 mg/L | 13 mg/L | 83.8% | NO |
| Dilute influent, exceptional effluent | 90 mg/L | 10 mg/L | 88.9% | Yes |
Note rows three and four. Both produce an effluent far below the 30 mg/L limit and still fail the removal requirement.
A plant with heavy I&I receives dilute influent. Dilute influent makes 85 percent removal arithmetically harder, even when the effluent is excellent.
The regulation anticipates this. Under § 133.103, a POTW may qualify for an adjusted removal requirement where the less concentrated influent is not the result of excessive I&I — and the determination uses the regulatory definition of excessive infiltration/inflow.
In other words: if your influent is dilute because your collection system leaks, that's your problem to fix. If it's dilute for another reason, there's a route to relief. The regulation adds a further condition — if the dilution comes from clear water industrial discharges, the treatment works must control them under 40 CFR Part 403.
Concentration tells you the strength. Loading tells you the work.
| Example | Value |
|---|---|
| Flow | 4.0 MGD |
| Influent BOD₅ | 220 mg/L |
| Influent BOD loading | 4.0 × 220 × 8.34 = 7,339 lb/day |
| Effluent BOD₅ | 15 mg/L |
| Effluent BOD loading | 4.0 × 15 × 8.34 = 500 lb/day |
| Pounds removed | 6,839 lb/day |
| Percent removal | 93.2% |
A rainfall event can cut influent BOD concentration in half while doubling the flow. Concentration drops; loading is unchanged.
Your process doesn't care about concentration. It cares about pounds of BOD per day against pounds of MLVSS — the F:M ratio. An operator watching only concentration will conclude the load dropped when it didn't.
| Calculation | Uses |
|---|---|
| F:M ratio | Influent (or primary effluent) BOD loading ÷ MLVSS inventory |
| Organic loading rate | Influent BOD loading ÷ basin volume |
| Nitrification oxygen demand | Influent TKN loading × 4.57 lb O₂ per lb NH₃-N |
| Alkalinity demand | Influent TKN loading × 7.14 lb alkalinity per lb NH₃-N |
| Percent removal | Influent and effluent concentrations |
Full formula set: the operator formula sheet, or the calculator.
| COMPOSITE | GRAB | |
|---|---|---|
| What it is | Multiple aliquots combined over a period, typically 24 hours | A single sample at a single moment |
| Represents | Average condition over the period | Condition at that instant |
| Types | Time-proportional or flow-proportional | — |
| Best for | BOD, TSS, TKN, nutrients, metals — where the daily average is the compliance basis | pH, temperature, DO, chlorine residual, cyanide, sulfide, oil and grease |
| Diurnal variation | Averages it out | Captures whatever was happening that minute |
A time-proportional composite weights every hour equally. But a plant doesn't receive equal flow every hour — there's a pronounced morning peak and a distributed evening peak.
A flow-proportional composite weights each aliquot by the flow it represents, so the result reflects the mass that actually arrived. For loading calculations and for the percent removal rule, that's the more representative number. Where your permit allows either, flow-proportional is better practice.
pH cannot be averaged, because the scale is logarithmic — which is why pH limits are applied as instantaneous limits and why § 133.102(c) says effluent values shall be maintained within 6.0 to 9.0.
It also can't be composited. A grab analyzed promptly is the only valid method.
Digester supernatant, dewatering filtrate, centrate, thickener overflow and filter backwash all return to the head of the plant — usually downstream of the influent sample point.
They carry high BOD, suspended solids, ammonia, phosphorus and sometimes sulfides, at low flow. A stream that's a rounding error hydraulically can be a substantial fraction of the nitrogen load. One study reported digester supernatant at 600 mg TKN/L with a COD/N ratio of 2.2 — enormous nitrogen with almost no carbon to denitrify it.
If your calculated F:M doesn't match what the biology is telling you, quantify the sidestreams before you look anywhere else.
| Sidestream | Typically carries |
|---|---|
| Anaerobic digester supernatant | High ammonia, BOD, solids |
| Aerobic digester decant | Solids, nitrate |
| Belt press or centrifuge filtrate/centrate | Very high ammonia and solids at low flow |
| Gravity thickener overflow | Solids, BOD |
| Effluent filter backwash | Solids |
Best practice: return sidestreams slowly and regularly rather than in slugs so the biology can acclimate; sample them, because you can't manage a load you haven't measured; and account for them in your F:M and loading calculations.
| Error | Consequence |
|---|---|
| Saying "effluent" without saying effluent of what | Primary effluent at 130 mg/L is normal; final effluent at 130 is a violation |
| Watching concentration and ignoring loading | A rain event halves concentration and doubles flow. The load didn't change. |
| Assuming a good effluent means compliance | The 85% rule can be violated with an excellent effluent and dilute influent |
| Assuming 85% removal applies only to BOD | It applies to suspended solids as well |
| Not tracking percent removal until month end | It's a leading indicator of an I&I problem |
| Compositing pH | pH is logarithmic and cannot be averaged or composited |
| Using time-proportional where flow-proportional is available | Misweights the diurnal peaks |
| Ignoring sidestreams | They're influent that bypasses the influent sampler |
| Assuming Part 133 is your limit | It's a floor. Your permit governs and may be more stringent. |
| Using 133.102 numbers at a trickling filter or lagoon plant | Check whether your permit was written under 133.105 |
| Sampling influent and effluent at different times for removal | The calculation assumes samples at approximately the same time |
| Item | Value |
|---|---|
| Influent / effluent | Flow into / out of a plant, process or unit — relative terms |
| Domestic sewage composition | ~99.9% water |
| Weak / medium / strong BOD₅ | 110 / 220 / 400 mg/L |
| Weak / medium / strong TSS | 100 / 220 / 350 mg/L |
| Typical domestic TKN | 20–85 mg/L as N |
| Secondary BOD₅ — 30-day / 7-day | ≤ 30 / ≤ 45 mg/L |
| CBOD₅ option — 30-day / 7-day | ≤ 25 / ≤ 40 mg/L |
| Secondary TSS — 30-day / 7-day | ≤ 30 / ≤ 45 mg/L |
| Removal requirement | ≥ 85% for BOD₅ and TSS, 30-day average |
| Equivalent phrasing | Effluent ≤ 15% of the influent mean |
| Effluent pH | 6.0 to 9.0 |
| Equivalent-to-secondary BOD₅ | ≤ 45 mg/L (§ 133.105) |
| Percent removal formula | [(In − Out) ÷ In] × 100 |
| Pounds formula | MGD × mg/L × 8.34 |
| Governing regulation | 40 CFR Part 133; your NPDES permit |
| Sampling methods | 40 CFR Part 136 |
CFR text was quoted from eCFR and Cornell LII as current at the time of research. Verify against the current edition before relying on it.
Part 133 is a floor. Your NPDES permit governs and may be more stringent. Water-quality-based limits, ammonia, nutrients, metals and toxics come from the permit.
Strength classifications are a reference framework, not a specification. Your plant needs its own characterization. The ~50% particulate split is from secondary literature and is approximate.
Only § 133.105(a)(1) BOD₅ at 45 mg/L was verified directly. Confirm the full equivalent-to-secondary set against the regulation.
For compliance work, read 40 CFR Part 133 and your permit directly rather than any summary — including this one.
The Complete Exam Guide covers all 12 major exam topics with 200 conceptual questions and a detailed explanation behind every answer — including loading, percent removal, and the standards that govern them.
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