7.1 Wastewater Characteristics & Environmental Effects
Key Takeaways
- Domestic wastewater is relatively consistent; industrial wastewater can be far stronger or more toxic and often needs pretreatment controls.
- BOD5 measures biodegradable organic oxygen demand; COD is faster and usually higher; TSS/VSS describe solids; FOG, nutrients, and pathogens drive other risks.
- Untreated high-BOD discharges deplete stream dissolved oxygen; nutrients drive eutrophication; solids and pathogens harm habitat and public health.
- Typical exam ranges for raw domestic wastewater are about 100–300 mg/L BOD5 and about 100–350 mg/L TSS.
- Operators convert concentration and flow into pounds-per-day loading to anticipate aeration, sludge, and process stress.
7.1 Wastewater Characteristics & Environmental Effects
Quick Answer: Domestic wastewater is relatively consistent in strength, while industrial wastewater varies widely in BOD, COD, solids, FOG, and toxicants. Operators track BOD/COD, TSS/VSS, nutrients, pathogens, and FOG because untreated discharge depletes dissolved oxygen, drives eutrophication, and threatens public health. Memorize typical domestic influent ranges used on Texas TCEQ exams.
Wastewater entering a Texas treatment plant is never “just dirty water.” It is a mixture of water, organic matter, suspended solids, grease, nutrients, microorganisms, and dissolved chemicals. The Texas Commission on Environmental Quality (TCEQ) wastewater operator exams expect you to recognize what is in the flow, how those constituents are measured, and why incomplete treatment harms receiving streams.
Domestic versus industrial wastewater
Domestic (sanitary) wastewater comes from homes, schools, offices, and similar sources. It is relatively predictable: human waste, food scraps, laundry and bath water, and household cleaners. Strength fluctuates with time of day and weather dilution, but the parameter ranges stay familiar enough that plants can be designed around them.
Industrial wastewater comes from manufacturing, food processing, petroleum, metal finishing, and similar operations. Strength can be far higher or chemically different from domestic sewage. A slaughterhouse or dairy may send extremely high biochemical oxygen demand (BOD) and fats, oils, and grease (FOG). A plating shop may send metals that inhibit biological treatment. Many Texas plants accept industrial flow only under a pretreatment program that limits what industries can discharge to the sewer.
On the exam, if a plant suddenly sees a sharp BOD spike, odor change, or biological upset after a wet weather event is ruled out, think industrial slug or FOG dump before rewriting the entire process settings.
| Source | Typical character | Operator concern |
|---|---|---|
| Domestic | Moderate BOD/TSS, pathogens, some FOG | Diurnal peaks, I/I dilution |
| Food processing | High BOD, FOG, variable pH | Oxygen demand, scum, odors |
| Metal finishing | Low BOD, high metals/toxics | Biological inhibition |
| Petroleum/chemical | Solvents, hydrocarbons, variable COD | Toxicity, vapor hazards |
Core strength and solids parameters
BOD (biochemical oxygen demand), usually reported as BOD5, measures how much dissolved oxygen microorganisms consume while oxidizing biodegradable organics over five days at 20°C. It is the classic indicator of organic strength and the main driver of oxygen depletion in streams.
COD (chemical oxygen demand) measures oxygen equivalent of organics oxidized by a strong chemical oxidant. COD is faster to run than BOD and is typically higher because it includes organics that microbes may not fully degrade in five days. A rising COD/BOD ratio can signal more refractory or industrial organics.
TSS (total suspended solids) is the mass of solids retained on a filter. High TSS means cloudy water, solids carryover risk, and sludge production. VSS (volatile suspended solids) is the portion of TSS that burns off and is used as a rough surrogate for organic/biological solids content.
FOG floats, coats pipes, forms scum blankets, and can smother biology if dumped in large amounts. Grease interceptors and skimming are first-line controls.
Nutrients—mainly nitrogen (ammonia, organic nitrogen, nitrate/nitrite) and phosphorus—fuel algal blooms downstream. Even when BOD and TSS limits are met, nutrient loads can still cause eutrophication.
Pathogens include bacteria, viruses, and parasites from human and animal waste. Disinfection later in the plant addresses them; characteristics sections still expect you to know raw wastewater is a public-health hazard, not merely an aesthetic problem.
Environmental effects of untreated or poorly treated wastewater
When high-BOD wastewater enters a stream, microbes consuming the organics pull dissolved oxygen (DO) from the water. Fish and aquatic life need DO; a severe oxygen sag can create a fish kill. That is why secondary treatment targets BOD removal so strongly.
Eutrophication is the nutrient-driven overgrowth of algae and aquatic plants. When blooms die and decay, they also consume oxygen and can shift pH and water clarity. Nitrogen and phosphorus are the usual culprits in exam language.
Solids settle on the stream bed, smother habitat, and release odors as they decompose. Pathogens raise the risk of waterborne illness for recreation and drinking-water intakes downstream. FOG and floatables create visible pollution that triggers complaints and enforcement attention.
Typical domestic influent ranges for exams
Teaching and exam values for medium-strength domestic wastewater commonly fall near:
| Parameter | Typical raw domestic range |
|---|---|
| BOD5 | about 100–300 mg/L (often taught near 200 mg/L) |
| COD | often roughly 1.5–2.5× BOD, commonly ~200–500+ mg/L |
| TSS | about 100–350 mg/L (often taught near 200–250 mg/L) |
| Settleable solids | roughly 5–20 mL/L (Imhoff cone order of magnitude) |
| FOG | highly variable; often tens of mg/L when uncontrolled |
| Ammonia-N | commonly on the order of 15–40 mg/L in many domestic flows |
| Total phosphorus | often a few mg/L in untreated domestic sewage |
| pH | typically near neutral, about 6.5–8.5 |
Exact numbers vary by community and by whether the collection system is diluted by infiltration/inflow. Exam questions usually want the order of magnitude and the ability to spot an abnormal industrial influence, not laboratory-grade precision.
Secondary treatment teaching targets often reference monthly averages near 30 mg/L BOD and 30 mg/L TSS with high percent removal—remember those as effluent goals, not influent values.
How operators use characteristics day to day
Influent sampling tells you what load is arriving. Combined with flow, you calculate pounds of BOD or TSS per day (mg/L × MGD × 8.34). That loading number drives aeration demand, clarifier performance expectations, and sludge production. A plant that ignores a rising influent BOD while keeping the same air and wasting rates is inviting low DO, poor settling, and effluent violations.
Characteristics also explain process selection. High grit and screenings loads argue for robust preliminary treatment. High settleable solids favor primary clarification. High soluble BOD with low settleable solids may send more load straight to biological treatment. High FOG demands skimming and source control. High industrial toxics demand pretreatment enforcement, not just more blowers.
For TCEQ Class D through A candidates, the durable skill is connecting a parameter to an effect: BOD → oxygen demand; nutrients → eutrophication; TSS → turbidity and sludge; pathogens → disinfection need; FOG → scum and clogging. If you can explain that chain, most characteristics questions become straightforward.
Why is high influent BOD a direct threat to a receiving stream if wastewater is discharged untreated?
Which statement best distinguishes domestic wastewater from many industrial discharges for exam purposes?
On a Texas operator exam, which typical raw domestic BOD5 range is most appropriate to memorize?