How Tertiary Treatment Fits the Three-Stage Wastewater Process
Tertiary treatment is the third stage in a conventional wastewater train, sitting after primary and secondary treatment to polish effluent before it is discharged or reused. Primary treatment handles the physical work: screening and gravity settling, which one overview of the three-stage process reports removes 40–60% of suspended solids from incoming wastewater (SSI Aeration, 2025-08). Secondary treatment is biological, using aeration or activated sludge to have microorganisms consume dissolved and suspended organics, with the resulting biomass settled out as waste activated sludge. The combined primary–secondary train is often enough where discharge goes to a non-sensitive water body, but it leaves residual nutrients, fine solids, pathogens, and trace organics in the water.
That residual load is what tertiary treatment is built to address. Where primary and secondary deliver "good enough" effluent for many ordinary discharges, tertiary — sometimes called effluent polishing — targets the finer contaminants those two stages cannot fully remove, and is added when reuse or a sensitive receiving water demands it. Not every plant installs it; primary and secondary are still considered sufficient for many municipal discharges, and the capital decision is driven by the end use of the water, not by the technology being available. Knowing where the train ends and the next stage begins is the first step in deciding whether a third block is justified at all.
The Three Building Blocks: Filtration, Nutrient Removal, Disinfection
The tertiary train is conventionally built from three functional blocks, and the order in which water meets them is consistent enough across designs to be a useful mental model for specification work. Filtration comes first, then nutrient removal where it is required, and disinfection is always the final step whenever it is used.
Filtration is the most straightforward tertiary operation. Sand filters are the workhorse: water passes through layers of sand, sometimes with gravel or granular activated carbon above, and particles are trapped in the pore spaces between grains. Membrane filtration is the more advanced option, pushing water through synthetic membranes with very small pores that can catch both particulates and some dissolved species. A practical secondary filter lineup for industrial flows includes three formats documented in a 2025 review: bag filters (felt depth media or mesh surface media, specified to a target micron rating), drum filters (woven cloth around a central drum, with periodic backwash), and disc filters (cloth-covered discs on a central drum, with an inside-out flow pattern) (SSI Aeration, 2025-08). Constructed wetlands are a non-mechanical alternative using plants, root-zone microbes, and substrate to trap pollutants — a lower-energy option at the cost of substantially more land area (Science Insights).
Nutrient removal targets nitrogen and phosphorus because, as the Science Insights overview notes, both feed algal blooms in receiving waters that deplete oxygen and kill aquatic life. Nitrogen removal is sequential biology: nitrification under aerobic conditions converts ammonia to nitrate, then denitrification under anoxic conditions converts nitrate to nitrogen gas — implemented in practice by cycling aeration on and off in dedicated tanks or by routing flow between separate aerobic and anoxic zones. Phosphorus is harder to strip biologically; most plants dose iron or aluminum salts so the phosphate precipitates and is removed by downstream settling or filtration, and engineered wetlands have tested iron oxide, limestone, zeolites, shells, blast furnace slag, and fly ash as phosphorus-binding media.
Disinfection is the final block whenever it is used. The 2025 SSI Aeration guide compares the three main options directly: chlorine is cheap and effective but reacts with residual organics to form byproducts such as trihalomethanes and usually requires dechlorination before discharge; UV damages microorganism DNA and forms no chemical byproducts, but needs low-turbidity influent to work and a regular lamp-and-sleeve maintenance cycle; ozone is highly reactive, generated on-site so there is no bulk chemical to store, leaves no hazardous residuals, but the generation equipment carries a higher capital and maintenance cost. For a deeper dive on UV unit selection, the compact UV disinfection comparison criteria for 2026 reuse applications is a useful starting point. Where sand or multimedia filtration sits at the head of the train, a multi-media filter is the typical equipment anchor.
Tertiary Treatment Parameter Reference
The table below consolidates the operating parameters and design considerations for the common tertiary unit processes discussed in the research, so an engineer evaluating equipment can scan them in one place rather than across multiple sources.
| Unit process | Function | Key design / operating points | Notes from the research |
|---|---|---|---|
| Sand (multimedia) filtration | Remove fine suspended solids | Layered media (sand, sometimes gravel, optionally GAC); particles trapped in inter-grain pore space | Most common tertiary filter type (Science Insights) |
| Bag filter | Reduce particulates to a target micron rating | Felt depth media or mesh surface media; plastic or metal housings; single or multi-bag configurations | Industrial format catalogued by SSI Aeration (2025-08) |
| Drum filter | Continuous solids separation on cloth media | Woven cloth around a central drum; gravity feed; backwash cycle restores media | Industrial format catalogued by SSI Aeration (2025-08) |
| Disc filter | High-area cloth filtration in compact footprint | Cloth-covered discs on a central drum; inside-out flow; backwash on saturation | Industrial format catalogued by SSI Aeration (2025-08) |
| Membrane filtration (incl. MBR / nanofiltration) | Remove particles and some dissolved species | Synthetic membranes with very small pores; tighter pores (NF) extend to dissolved contaminants | Science Insights reports NF achieved >70% removal for 9 contaminants, partial for 22, low for 7; no statistically significant improvement over coarser filtration for hormone-disrupting compounds |
| Biological nitrogen removal | Convert ammonia → nitrate → N₂ | Aerobic zone for nitrification + anoxic zone for denitrification; aeration cycling or separate-tank routing | Standard two-stage biology described by Science Insights |
| Chemical phosphorus precipitation | Remove dissolved phosphorus | Iron or aluminum salt dose; precipitates settled or filtered downstream | Most common P-removal approach (Science Insights) |
| Constructed wetland | Combined filtration + nutrient polishing | Reeds, root-zone microbes, substrate (sand/gravel); low energy, high land area | Alternative for low-energy sites (Science Insights) |
| Chlorine disinfection | Kill/inactivate pathogens | Dose, contact time; dechlorination typically required before discharge | Forms trihalomethanes when organics present (SSI Aeration, 2025-08) |
| UV disinfection | Inactivate pathogens via DNA damage | Lamp banks; influent must be low-turbidity; lamp and quartz-sleeve replacement on a maintenance cycle | No chemical byproducts; ongoing lamp maintenance required (SSI Aeration, 2025-08) |
| Ozone disinfection | Oxidize pathogens and some micropollutants | On-site generation by adding a third O atom to O₂; reactive; no hazardous byproducts | Higher equipment cost than chlorine (SSI Aeration, 2025-08) |
| Advanced oxidation (O₃/H₂O₂/UV) | Break down recalcitrant organics via hydroxyl radicals | Combination of ozone, UV, hydrogen peroxide to generate ·OH; adaptable to plant layout | Used as a final polishing step where micropollutant standards are strict (Science Insights) |
For tighter solids-and-BOD targets where a single packaged unit is preferred, an MBR membrane bioreactor integrates membrane filtration with the biological stage; the MBR design criteria guide walks through the engineering inputs.
Do You Need Tertiary Treatment? A Decision Framework
The right question is rarely "what does tertiary treatment do?" but "which of my drivers actually requires a third block, and which block?" The drivers below map the common reasons engineers add tertiary treatment to the unit processes that satisfy them, in roughly descending order of how often they trigger a full train.
| Driver | Why it triggers tertiary | Minimum tertiary block(s) required | Often escalates to a full train when… |
|---|---|---|---|
| Water reuse (drinking, process, irrigation, oil & gas) | Primary + secondary effluent is not clean enough for the intended end use | Filtration + disinfection | Indirect potable reuse, or reuse where nutrient limits also apply |
| Discharge to a sensitive receiving water (estuary, sluggish river, near coral reefs) | Stringent cleanliness standards apply | Filtration + disinfection; add nutrient removal if N/P limits exist | Multiple stressors (nutrients + pathogens + solids) |
| Industrial color removal (pulp & paper, textiles) | Tertiary is more effective than primary or secondary at stripping color | Targeted filtration (and/or oxidation) | Combined with reuse or nutrient limits |
| Municipal nutrient limits (N and/or P) | Secondary effluent still contains enough N and P to cause ecological damage downstream | Biological N removal; chemical P precipitation | Tight total-N or total-P limits, or year-round algal-bloom pressure |
| Micropollutant / emerging-contaminant limits (PFAS, pharmaceuticals, hormones) | Conventional treatment leaves these largely intact; results vary by method and compound | Advanced oxidation; tight membrane (NF) for some species | Potable reuse, or jurisdiction-specific PFAS or pharmaceutical limits |
A useful rule of thumb: if two or more of these drivers apply to a site, a filtration-plus-nutrient-removal-plus-disinfection train is usually justified, because each driver alone may pull a different block and the blocks tend to compose cleanly. If only one driver applies — color removal, for example, or a simple pathogen limit — a single targeted block is often enough, and the rest of the train can be deferred. The 2025 SSI Aeration overview explicitly notes that after tertiary treatment the water can be clean enough for industrial, manufacturing, oil and gas, and agricultural operations, which is why reuse tends to dominate the decision logic in industrial parks and water-stressed municipalities. For suspended-solids and floatable loads upstream of the tertiary train, a DAF system is often the right primary-stage or pre-treatment anchor, and the manufacturing water-use reduction guide shows how reuse drivers cascade into tertiary block selection.
What Drives the Cost of a Tertiary System
The research is explicit that tertiary systems require additional infrastructure, energy, and chemicals, and that the cost is real, but it does not publish a specific $/m³ or capital figure that can be quoted as an industry norm (Science Insights). For an engineer building a defensible OPEX and CAPEX case, the more useful exercise is to map the cost drivers to specific unit operations so each line item can be priced from supplier inputs.
Energy drivers are tied to the unit processes selected. Aeration cycling for biological nitrogen removal is the dominant electrical load for a biological-nutrient-removal block. Ozone generation is a major load for any plant that selects ozone disinfection, because ozone is produced on-site from electrical energy (SSI Aeration, 2025-08). UV disinfection is comparatively light on energy at the point of use but its consumable-replacement burden is the issue, not its kWh.
Chemical drivers scale with dose and flow. Iron and aluminum salts for phosphorus precipitation are the obvious chemical line, with dose rate set by influent phosphorus and target residual. Chlorine for disinfection — and sulfur dioxide for the dechlorination step that typically follows — adds an ongoing chemical cost; hydrogen peroxide appears as a consumable in advanced-oxidation configurations that combine ozone, UV, and peroxide. Where chemical dose is automated against flow, an automatic chemical dosing skid is the standard equipment anchor.
Consumable and maintenance drivers are where OPEX surprises tend to come from. UV systems need periodic lamp and quartz-sleeve replacement. Membranes need cleaning and eventual replacement on a multi-year cycle. Filter media — sand, multimedia, or activated carbon — need change-out on a defined cycle. The 2025 SSI Aeration guide notes that UV lamps "require a relatively high level of maintenance" and that ozone equipment "is relatively expensive to maintain," both of which translate to recurring OPEX lines rather than one-off capital. Rather than rely on assumed industry averages, the practical step is to request from any shortlisted supplier: specific kWh per cubic metre treated for the proposed train, dose rates in mg/L or kg per cubic metre for each chemical, expected service life for membranes and filter media, and validated residual contaminant concentrations for the target discharge or reuse standard. Those four inputs, multiplied by the local energy and chemical unit costs and the design flow, give a defensible operating-cost number for the specific site.
The 2026 Question: Micropollutants, Advanced Oxidation and Antibiotic Resistance
The forward-looking design question in 2026 is no longer whether tertiary treatment removes the conventional pollutants — the evidence is that it does — but whether it removes the emerging ones, and what the open questions around antibiotic resistance mean for any reuse configuration. A 2025 review of tertiary treatment notes that micropollutants — pharmaceuticals, hormones, industrial chemicals, and PFAS — pass through conventional treatment largely intact, with results varying widely by method and by compound (Science Insights). The same source reports that in a systematic evaluation, nanofiltration achieved removal rates above 70% for 9 different contaminants, partial removal for 22, and low removal for 7, and that hormone-disrupting compounds showed no statistically significant improvement over coarser filtration — a signal that simply tightening membrane pores is not a complete answer to this class of contaminant.
Advanced oxidation is the emerging response. The process generates hydroxyl radicals through combinations of ozone, UV, and hydrogen peroxide; these radicals break chemical bonds in resistant organics and can in some cases fully convert them to carbon dioxide and water (Science Insights). The technology is increasingly being specified as a final polishing step on plants where micropollutant standards are strict, because it complements rather than replaces the filtration and nutrient-removal blocks. The same review also flags the limitation plainly: results vary widely by method and by contaminant, so an advanced-oxidation block is something to evaluate against a specific compound list, not a generic upgrade.
There is also an open research question around antibiotic resistance. A 2025 integrated metagenomic study in Environment International frames tertiary wastewater treatment processes as effectively removing conventional and emerging pollutants, but explicitly notes that their impact on antibiotic resistance genes (ARGs) with potentially higher risk remains an open question for the design community. For any 2026 plant with a downstream reuse or stream-augmentation component, this is a real specification consideration: the tertiary block must be evaluated not only on conventional residuals but on whether the train reduces, propagates, or selects for resistance elements — a question the research has not yet closed. The practical implication is that tertiary treatment is shifting from an optional upgrade to a baseline expectation in regions where reuse volumes are climbing and emerging-contaminant standards are tightening, and advanced oxidation belongs on the evaluation shortlist for any new specification in 2026.
Frequently Asked Questions
What is the difference between secondary and tertiary wastewater treatment?
Secondary treatment is biological — microorganisms consume dissolved organics and the resulting sludge is settled out — and is enough for many ordinary discharges. Tertiary treatment is the third, more rigorous stage that polishes the secondary effluent to remove fine suspended solids, residual nitrogen and phosphorus, pathogens, and trace chemicals, and is required when the water is destined for reuse or for discharge to a sensitive receiving water (SSI Aeration, 2025-08; Science Insights).
What unit processes make up a typical tertiary treatment train?
A typical train is built from three functional blocks in this order: filtration (sand, multimedia, bag, drum, disc, or membrane), nutrient removal (biological nitrification/denitrification for nitrogen, and chemical precipitation with iron or aluminum salts for phosphorus), and disinfection (chlorine, UV, or ozone), with disinfection always the final step when it is used (SSI Aeration, 2025-08; Science Insights).
How much does tertiary wastewater treatment cost to operate?
The research does not publish a specific operating-cost figure that can be quoted as an industry norm. It does identify the cost drivers — additional infrastructure, energy for aeration cycling and ozone generation, chemicals for phosphorus precipitation and chlorination, and recurring consumables such as UV lamps, membrane cleaning/replacement, and filter media change-out (Science Insights). For a defensible OPEX figure for a specific site, request from any shortlisted supplier: kWh per cubic metre treated, chemical dose rates in kg per cubic metre, expected service life for membranes and filter media, and validated residual concentrations for the target discharge or reuse standard, then multiply by the local unit costs for energy and chemicals.
What should I look for when selecting a tertiary treatment supplier?
Look for a supplier that can provide the four quantified inputs listed above for your specific influent and target standard — kWh/m³, chemical dose, consumable life, and validated residuals — rather than generic brochures. For UV-based disinfection trains, the UV sterilizer line is a useful product anchor, and for chemical-disinfection or advanced-oxidation configurations, the chlorine dioxide generator is the relevant equipment reference. Cross-check the supplier's claimed residuals against an independent pilot or against a reference plant on a comparable influent before committing capital.