8.1 Wastewater Characteristics & Plant Loadings
Key Takeaways
- Typical medium-strength domestic wastewater runs about 200 mg/L BOD, 200 mg/L TSS, 40 mg/L total nitrogen, and 7 mg/L total phosphorus.
- The standard domestic design basis is roughly 100 gallons per capita per day and about 0.17 to 0.20 pounds of BOD per person per day.
- Pounds of loading equal flow in MGD times concentration in mg/L times 8.34.
- BOD measures the biodegradable oxygen demand over five days while COD measures total chemically oxidizable material in about two hours.
- Diurnal flow peaks in late morning and early evening, and infiltration and inflow show up as sustained wet-weather flow increases with diluted strength.
8.1 Wastewater Characteristics & Plant Loadings
The standardized wastewater exam weights Treatment Process Evaluation and Adjustment at 38 to 42 questions out of 100 — the largest area on every class of the exam, and every one of those adjustments starts from the same question: what is coming in? Concentration alone never answers it. Pounds do.
1. Typical Domestic Wastewater Strength
| Parameter | Weak | Medium (typical) | Strong |
|---|---|---|---|
| BOD₅ | 110 mg/L | 200 mg/L | 400 mg/L |
| COD | 250 mg/L | 500 mg/L | 1,000 mg/L |
| TSS | 100 mg/L | 200 mg/L | 350 mg/L |
| Total nitrogen (TN) | 20 mg/L | 40 mg/L | 85 mg/L |
| Ammonia-N | 12 mg/L | 25 mg/L | 50 mg/L |
| Total phosphorus (TP) | 4 mg/L | 7 mg/L | 15 mg/L |
| Alkalinity | 50 mg/L | 100 mg/L | 200 mg/L |
Per-capita design basis
- Flow: roughly 100 gallons per capita per day (gpcd) including a normal allowance for infiltration; some references use 80–120 gpcd.
- BOD: roughly 0.17 to 0.20 lb BOD per person per day.
- TSS: roughly 0.20 lb per person per day.
These let you sanity-check a plant. A 10,000-person community should deliver about 1.0 MGD carrying roughly 1,700–2,000 lb/day of BOD. If your plant sees 4,000 lb/day, you have an industrial contributor, hauled septage, or a metering error.
2. The Pounds Formula — the most important equation in wastewater
The 8.34 is pounds per gallon of water. Concentration tells you quality; pounds tell you load, and biological processes respond to load.
Worked example. A plant receives 2.4 MGD at 215 mg/L BOD.
Why it matters. Suppose influent BOD drops to 140 mg/L but flow rises to 3.8 MGD during a storm:
Concentration fell by 35%, but the load actually went up. An operator who wastes sludge based on the falling concentration will under-feed the biology and lose the process. Always convert to pounds first.
3. The Oxygen Demand Tests
| Test | What it measures | Time | Notes |
|---|---|---|---|
| BOD₅ | Oxygen consumed by microorganisms degrading organic matter | 5 days at 20 °C | The regulatory standard; slow, so useless for same-day control |
| CBOD₅ | BOD with nitrification suppressed by an inhibitor | 5 days | Isolates carbonaceous demand; most modern NPDES permits use CBOD |
| COD | Total chemically oxidizable material, via strong acid dichromate digestion | ~2 hours | Fast; always higher than BOD because it oxidizes non-biodegradable material too |
| TOC | Total organic carbon | Minutes | Fastest; used as a surrogate once correlated |
Using the COD:BOD ratio
Once a plant establishes its own COD-to-BOD ratio on domestic influent (commonly around 2:1 to 2.5:1), COD becomes a same-day surrogate for process control. A sudden jump in the ratio signals arrival of material that is chemically oxidizable but not biodegradable — an industrial discharge that the biology cannot treat and that may be toxic to it.
4. Solids Terminology
| Term | Definition |
|---|---|
| Total Solids (TS) | Everything remaining after evaporation at 103–105 °C |
| Total Suspended Solids (TSS) | Retained on a glass fiber filter |
| Total Dissolved Solids (TDS) | Passes the filter |
| Volatile Suspended Solids (VSS) | The portion of TSS lost on ignition at 550 °C — the organic fraction, typically 70–80% of TSS in domestic wastewater |
| Fixed Suspended Solids (FSS) | The mineral residue remaining after ignition — the inert fraction |
| Settleable Solids | Measured in an Imhoff cone after 1 hour, reported in mL/L |
VSS is used as a proxy for biological mass in the activated sludge process, which is why MLVSS rather than MLSS is often the control parameter.
5. Diurnal Variation
Domestic flow follows a daily rhythm:
- Minimum flow in the early morning hours, roughly 3–6 a.m.
- Morning peak in the late morning as the community wakes and showers.
- Evening peak in the early evening.
- Peak hourly flow commonly runs 1.5 to 2.5 times average daily flow in a typical municipal system, higher in small systems.
Strength peaks track flow peaks, so the peak pound loading arrives with the peak flow — the process is stressed on both axes at once. Equalization basins exist to flatten this.
6. Infiltration and Inflow
| Infiltration | Inflow | |
|---|---|---|
| Path | Groundwater entering through cracks, defective joints, and deteriorated pipe | Stormwater entering directly through roof drains, cross-connections, manhole covers, sump pumps |
| Timing | Slow, sustained; rises with the water table and persists for days or weeks | Fast, sharp; peaks within hours of rainfall and recedes quickly |
| Signature at the plant | Chronically elevated flow with diluted influent strength | Sudden hydraulic spike, washout risk, diluted strength |
How I&I shows up in your numbers: flow up, concentrations down, but pounds roughly unchanged. That is the diagnostic. If both flow and pounds rise together, you have a new load, not water in the collection system.
Consequences: hydraulic overloading, solids washout from clarifiers, reduced detention time, lost treatment capacity paid for by ratepayers, and in severe cases sanitary sewer overflows. See Section 7.2 for collection system source control.
7. Other Influent Characteristics Worth Watching
- Temperature — biological rates fall sharply in winter; nitrification is the first process to suffer.
- pH and alkalinity — nitrification consumes roughly 7.14 mg/L of alkalinity as CaCO₃ per mg/L of ammonia oxidized, so low-alkalinity influent can crash pH and stall the process.
- Septage and hauled waste — extremely strong, often 20–50 times domestic strength; must be metered in slowly.
- FOG (fats, oils, grease) — causes collection blockages and, at the plant, filamentous foaming.
- Toxic and inhibitory loads — heavy metals, solvents, and strong acids or bases from industry. Prevention is the industrial pretreatment program in Section 9.4.
A plant treats 3.1 MGD with an influent BOD of 185 mg/L. What is the BOD loading in pounds per day?
During a wet-weather event, influent flow doubles while BOD and TSS concentrations fall by roughly half, leaving pounds per day nearly unchanged. What does this indicate?
Why is COD useful for daily process control even though NPDES permits are written on BOD or CBOD?
Volatile suspended solids are determined by igniting the sample residue at which temperature?