Secondary treatment gets you to 30 mg/L BOD. Tertiary treatment gets you the rest of the way — to strict nutrient limits, reuse standards, or near-drinking-water quality. Here's what it involves and why it matters.
Most people studying wastewater treatment learn primary and secondary treatment first — and stop there. But as permit limits tighten, receiving waters become more sensitive, and water reuse grows as a necessity, tertiary treatment is increasingly part of what operators need to know. It shows up on operator exams, it's installed at a growing number of facilities, and understanding it completes the picture of the full treatment train.
This article covers what tertiary treatment is, why it's needed, the main unit processes involved, typical effluent quality achieved, and what the operator exam tests.
Physical settling. Removes 50–70% TSS, 25–40% BOD. No biology, no chemicals (basic). Produces primary sludge.
Biological treatment. Removes 85%+ BOD and TSS. Required by EPA secondary treatment standards. Produces waste activated sludge.
Advanced polishing. Removes nutrients, fine solids, pathogens, and trace compounds. Required for strict permits or water reuse. Goes beyond secondary standards.
The EPA secondary treatment standard requires effluent BOD and TSS of 30 mg/L or less. But many receiving waters — sensitive bays, estuaries, groundwater recharge zones — have permit limits far below that. Nutrients like nitrogen and phosphorus cause eutrophication and aren't significantly removed by standard secondary treatment. Pathogens require dedicated disinfection. Tertiary treatment addresses what secondary leaves behind.
Tertiary treatment isn't required at every facility — it's driven by specific permit limits, receiving water conditions, or intended end use of the treated water. The main drivers are:
Filtration is the most common tertiary process. Secondary effluent still contains fine suspended solids — residual biological floc, colloidal material, and particulate BOD — that pass through the secondary clarifier. Filtration removes these by passing the effluent through a porous media bed.
Effluent flows downward through one or more layers of granular media — typically anthracite coal over sand, sometimes with garnet as a third layer. Solids are captured in the pore spaces of the media. Filters are periodically backwashed with clean water (and often air scour) to flush captured solids out. Well-operated multimedia filters can reduce effluent TSS to 5–10 mg/L. Simple, robust, widely used for tertiary polishing.
Rotating cloth or disk filters use fine textile media to capture solids as secondary effluent flows through. Smaller footprint than granular media filters, continuous self-cleaning operation, and effective for fine solids removal. Increasingly common at facilities with limited space. TSS removal to 5 mg/L or below achievable.
Microfiltration (MF) and ultrafiltration (UF) membranes provide very fine physical separation — removing essentially all suspended solids, bacteria, and some viruses. Used in membrane bioreactors (MBR) where the membrane replaces the secondary clarifier, or as a standalone tertiary polishing step. Produces very high-quality effluent (TSS near zero) but requires more energy and maintenance than granular filtration.
Nutrient removal is one of the most important and most exam-tested aspects of tertiary treatment. Both nitrogen and phosphorus require specific processes to meet permit limits.
Biological nitrogen removal through nitrification and denitrification (covered in detail in the nitrification and denitrification article) is the primary approach. In a biological nutrient removal (BNR) system, nitrogen is converted to nitrogen gas and stripped from the water. Achieving very low total nitrogen limits (below 3–5 mg/L) often requires:
Phosphorus removal is achieved through biological enhanced phosphorus removal (EBPR/Bio-P), chemical precipitation with metal salts (alum or ferric chloride), or a combination of both:
Standard secondary treatment followed by chlorination meets basic pathogen reduction requirements. Tertiary disinfection goes further — required for water reuse, sensitive discharge, or achieving very low pathogen indicator levels.
Ultraviolet light inactivates pathogens by damaging their DNA — preventing reproduction without adding chemicals. Highly effective against bacteria, viruses, and protozoa (including Cryptosporidium and Giardia, which are resistant to chlorine). No disinfection byproducts. Requires clear, low-turbidity water for effective UV penetration — which is why filtration typically comes before UV in a tertiary train. Increasingly the preferred method for water reuse applications.
Ozone (O₃) is a powerful oxidant that destroys pathogens, oxidizes trace organic compounds, and reduces color and odor. More powerful disinfectant than chlorine for many pathogens. Also breaks down pharmaceuticals and personal care products (PPCPs) — emerging contaminants that UV and chlorine don't effectively remove. Ozone must be generated on-site (unstable — can't be stored) and requires significant energy. Used at advanced water reuse facilities and larger municipal plants.
Combines ozone or hydrogen peroxide with UV to generate hydroxyl radicals — extremely reactive species that destroy virtually any organic compound, including pharmaceuticals, endocrine disruptors, and other trace contaminants. Used at facilities producing indirect or direct potable reuse quality water. The most advanced and energy-intensive disinfection approach.
Granular activated carbon (GAC) or powdered activated carbon (PAC) adsorbs dissolved organic compounds, taste and odor compounds, pharmaceuticals, and other trace contaminants that biological and physical treatment can't remove. GAC contactors are fixed beds that effluent flows through; PAC is added directly to the water stream. Used where trace organic removal is required for reuse or sensitive discharge.
High-pressure membranes that reject essentially everything larger than a water molecule — dissolved salts, nutrients, trace organics, pathogens, and more. Produces near-pure water permeate and a concentrated reject stream that must be disposed of. The core technology in advanced water reuse and indirect potable reuse systems. High energy consumption and cost. Not common in standard municipal wastewater treatment but central to water recycling programs.
| Parameter | After Primary | After Secondary | After Tertiary |
|---|---|---|---|
| BOD5 | 100–250 mg/L | 10–30 mg/L | 2–10 mg/L |
| TSS | 70–150 mg/L | 10–30 mg/L | 1–10 mg/L |
| Total Nitrogen | 25–45 mg/L | 15–35 mg/L | 3–10 mg/L (BNR) |
| Total Phosphorus | 4–8 mg/L | 3–7 mg/L | <0.1–1.0 mg/L |
| Fecal Coliform | 10⁶–10⁸ /100mL | 10³–10⁵ /100mL | <2–200 /100mL |
| Turbidity | 30–100 NTU | 2–10 NTU | <2 NTU |
Water reuse is the fastest-growing application of tertiary treatment. As water scarcity intensifies across much of the US, treated wastewater is increasingly viewed as a resource rather than a waste stream. Common reuse applications include:
| Topic | What to Know |
|---|---|
| Definition | Advanced treatment beyond secondary — polishes effluent to meet strict permit limits or reuse standards |
| Why it's needed | Nutrient limits (N and P), water reuse, sensitive receiving waters, strict TSS/BOD limits below secondary standard |
| Filtration types | Granular media (sand/anthracite), cloth/disk, membrane (MF/UF) — all remove fine suspended solids secondary clarifiers miss |
| Nitrogen removal | Biological — nitrification + denitrification in BNR system; may require external carbon for post-anoxic polishing |
| Phosphorus removal | Biological (EBPR/Bio-P) to 1–2 mg/L; chemical (alum or ferric) to <0.1 mg/L; often combined |
| UV disinfection | Damages pathogen DNA; no disinfection byproducts; effective against Crypto and Giardia; requires low turbidity |
| Ozonation | Powerful oxidant; removes PPCPs and trace organics; must be generated on-site; more energy intensive than chlorine |
| Activated carbon | Adsorbs dissolved organics, taste/odor, pharmaceuticals that biological treatment can't remove |
| Reverse osmosis | Rejects essentially everything; used in advanced reuse; produces concentrate waste stream |
| Water reuse levels | Agricultural/landscape (basic tertiary) → groundwater recharge (IPR) → direct potable reuse (full advanced treatment) |
The WastewaterAce Complete Exam Guide covers tertiary treatment, nutrient removal, disinfection, and all 12 core exam topics — 200 questions with full explanations for every answer.
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