Primary, Secondary and Tertiary Treatment: Differences
The three wastewater treatment levels differ by what they remove: solids, organic matter and fine pollutants. Comparison table and typical uses.
Primary, secondary and tertiary treatment are the three successive stages of a wastewater treatment plant, and they differ by what each one removes: primary takes out solids and grease by physical means, secondary degrades dissolved organic matter biologically, and tertiary polishes the water by removing nutrients, pathogens and residual pollutants. Each level leaves the effluent cleaner and prepares the load for the next stage. Not every plant needs all three: the level required depends on the discharge limit or the final use of the water.
Primary treatment: physical separation
Primary treatment removes the pollutants that separate by gravity or flotation, with no biological action. It includes bar screens that hold back trash, grit chambers that pull out sand and heavy solids, and primary settling tanks where solids settle and grease floats to be skimmed off.
It is a mechanical stage with low operating cost but limited scope: it reduces suspended solids and some organic load without touching the dissolved fraction. Many industrial effluents add physicochemical treatment (coagulation-flocculation) to improve removal before the biological stage.
Secondary treatment: biological degradation
Secondary treatment is the core of purification: microorganisms consume the dissolved organic matter (the BOD) that primary treatment cannot remove. It is carried out by aerobic processes such as activated sludge, or anaerobic ones such as the UASB reactor, depending on the effluent load.
A well-operated secondary stage removes between 85% and 98% of BOD and leaves water fit for discharge to receiving bodies in most jurisdictions. For high-strength industrial effluents, an upstream anaerobic stage cuts aeration costs and generates recoverable biogas.
Tertiary treatment: advanced polishing
Tertiary (or advanced) treatment goes beyond organic matter: it removes nutrients (nitrogen and phosphorus), fine solids, color and pathogens. It is applied when the receiving body is sensitive, when regulation demands strict limits, or when the goal is to reuse the water.
Its technologies include filtration (sand, disc or membrane), nutrient removal, activated-carbon adsorption and disinfection (chlorination, UV or ozone). The result is a high-quality effluent, in some cases fit for industrial reuse or irrigation.
Comparison of the three levels
| Aspect | Primary | Secondary | Tertiary |
|---|---|---|---|
| Mechanism | Physical (settling, flotation) | Biological (aerobic/anaerobic) | Physicochemical and disinfection |
| What it removes | Solids, grit, grease | Dissolved organic matter (BOD) | Nutrients, pathogens, fine solids |
| BOD removal | 25–40% | 85–98% | >98% cumulative |
| TSS removal | 50–70% | 85–95% | >95% cumulative |
| Typical technology | Screens, grit chamber, settler | Activated sludge, UASB | Filtration, UV, membranes |
| Relative cost | Low | Medium | High |
Which level do I need?
The answer depends on the destination of the treated water. To discharge into a watercourse in Paraguay you must meet the discharge limits and hold the corresponding environmental license; it is advisable to verify the current regulation with MADES (Ministry of the Environment and Sustainable Development) under Law 294/93 on Environmental Impact Assessment and Law 3239/07 on Water Resources.
- Primary + secondary: sufficient for most discharges to receiving bodies.
- Primary + secondary + tertiary: required when the receiving body is sensitive or when the goal is to reuse the water.
Where it applies in Tech Tank projects
Tech Tank integrates turn-key wastewater treatment plants (WWTP) where each stage is built in bolted steel tanks and reactors with glass-fused-to-steel (GFS) coating: primary settlers, biological reactors and contact tanks for disinfection. Engineering defines which levels to include based on the effluent, the discharge limit and whether the client seeks reuse.
If you need to define the treatment level for your project, review our wastewater treatment solution and request a technical assessment with your flow and load data.
Frequently Asked Questions
What is the difference between secondary and tertiary treatment?
Secondary treatment removes dissolved organic matter biologically and meets common discharge limits. Tertiary goes further: it removes nutrients, pathogens and fine solids to meet strict limits or enable water reuse.
Does every plant need tertiary treatment?
No. Many discharges are met with primary and secondary treatment alone. Tertiary is justified when the receiving body is sensitive, regulation demands low nutrient or pathogen limits, or when the goal is to reuse the effluent.
Is primary treatment enough to discharge?
Almost never. Primary treatment removes solids and grease but leaves the dissolved organic matter (BOD) intact, so it cannot meet discharge limits on its own. At least a secondary stage is needed.
Where does the UASB reactor fit in these levels?
The UASB reactor is an anaerobic secondary treatment technology. It removes much of the organic load while generating biogas and is usually followed by aerobic or tertiary polishing to reach the final limits.
How much does each stage reduce BOD?
Primary removes on the order of 25–40% of BOD; secondary raises the cumulative removal to 85–98%; and tertiary, when added, brings the effluent to very low levels fit for sensitive receiving bodies or reuse.