Wastewater treatment
Wastewater treatment cleans used water from homes and businesses in four stages: preliminary (screens and grit removal), primary (settling), secondary (microorganisms eat the dissolved waste) and tertiary or advanced polishing, then disinfection before discharge. Federal secondary treatment rules cap effluent BOD and suspended solids at 30 mg/L as a 30-day average.
Wastewater treatment stages at a glance
| Stage | What it removes | Typical equipment |
|---|---|---|
| Preliminary | Rags, grit, sand and large debris that would damage pumps | Bar screens, grit chambers |
| Primary | Settleable solids and floating grease | Primary clarifiers |
| Secondary | Dissolved organic matter (BOD), consumed by microorganisms | Activated sludge aeration basins, trickling filters, lagoons |
| Tertiary / advanced | Nutrients (nitrogen, phosphorus) and remaining solids, where the permit requires it | Filters, nutrient removal processes |
| Disinfection | Pathogens before discharge | Chlorination and dechlorination, UV |
| Federal secondary standard | BOD5 and TSS: 30 mg/L 30-day average, 45 mg/L 7-day average, 85% removal, pH 6.0 to 9.0 | 40 CFR 133.102 |
What it means for operators
Wastewater treatment is the sequence of physical, biological, and chemical processes that removes pollutants from used water so it can be returned safely to the environment. Water arriving at the plant from homes, businesses, and industry passes through stages that screen out debris, settle solids, harness microorganisms to consume dissolved waste, and disinfect the flow before discharge. The heart of most plants is biological: a managed population of microorganisms doing the actual cleaning.
Wastewater treatment is also one of the standardized certification exam series, parallel in structure to the water side, with class levels that rise alongside plant complexity. Exam content covers treatment processes, biology and chemistry, equipment, sampling and laboratory work, safety, regulations, and operator math. Certified wastewater treatment operators carry legal responsibility for running processes that protect rivers, lakes, and coastal waters.
Operators who thrive in this series often describe the plant as a living system they tend rather than a machine they run. Reading the process through observation, adjusting to changing flows, and keeping the biology healthy is a genuinely scientific kind of work performed in work boots. The environmental mission is direct: the waterway downstream stays healthy because the plant did its job.
The water leaving the plant is the effluent, and its quality is set by the plant's discharge permit. Treatment performance is usually expressed as removal efficiency, the percent of a pollutant taken out between influent and effluent, which is one of the most common calculations on the wastewater exam.
The pipes and lift stations that deliver the flow are a separate job and a separate certification: wastewater collection. If the plant side appeals to you, the wastewater operator career guide covers the job, pay and certification path.
Common questions
How is wastewater treatment and disposal handled?
After the treatment stages, a plant disinfects the effluent and discharges it to a river, lake or ocean under its discharge permit, or reuses it, for example for irrigation. Solids removed along the way (biosolids) are separately stabilized and then land applied, landfilled or incinerated under their own rules.
What are the stages of wastewater treatment?
Preliminary treatment removes debris and grit, primary treatment settles solids, secondary treatment uses microorganisms to remove dissolved organic matter, and tertiary treatment polishes the water where a permit requires it. Disinfection follows before discharge.
What does treated wastewater mean?
Treated wastewater, or effluent, is used water that has gone through a treatment plant and meets the limits in the plant's discharge permit, so it can be returned to a river, lake, ocean or reused for purposes such as irrigation.
How clean does treated wastewater have to be?
Under the federal secondary treatment regulation, municipal plants must generally reach 30 mg/L BOD5 and 30 mg/L total suspended solids as a 30-day average with at least 85 percent removal. Individual permits are often stricter.
Is wastewater treatment a good career?
It is steady, certified work that cannot be outsourced, with a published class ladder. The BLS reports a 2025 median of $60,020 a year for water and wastewater operators; see the salary breakdown.
Related terms
| Wastewater collection | The network of sewers and lift stations that carries wastewater to the treatment plant, and its certification series. |
| Effluent | Treated water flowing out of a treatment plant, the finished product of the wastewater side. |
| Disinfection | The treatment step that inactivates disease-causing microorganisms in water or wastewater. |
Sources
- U.S. EPA: Municipal wastewater
- 40 CFR Part 133: Secondary treatment regulation (eCFR)
- U.S. EPA: NPDES permit basics
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