Why US Municipal Plants Are Upgrading Primary, Secondary, and Tertiary Treatment in 2026
Primary, secondary, and tertiary treatment upgrades for US municipal wastewater facilities in 2026 are driven by tighter ammonia, nutrient, and PFAS-related permit limits that legacy 1970s activated-sludge and trickling-filter plants were not designed to meet. Modern retrofits add biological nutrient removal, IFAS, MBBR, MBR, or ozone-biological activated carbon polishing to lift BOD₅ removal from the EPA baseline of 85–95% toward 99% and effluent ammonia below 1 mg/L, while cutting footprint up to 60%.
The plant cohort affected is large and old. The 1968 EPA inventory counted 9,353 secondary treatment plants serving roughly 75.8 million people, with zero advanced treatment facilities nationwide (per EPA, 1977). Approximately $18 billion in 1977 construction dollars was needed to meet the Federal Water Pollution Control Act Amendments of 1972, and the agency estimated about one-fourth of that spend went to upgrading existing secondary plants while the rest went to converting primary works to secondary. Those 1968 basins are now 50+ years old; many have reached the end of their structural and hydraulic service life, and their 1972 "best practicable technology" baseline no longer satisfies 2026 effluent expectations.
Three regulatory pressures are now converging on the same 2026–2028 capital window. First, tighter NPDES ammonia and total nitrogen limits in consent decrees across the Chesapeake, Long Island Sound, and San Francisco Bay watersheds push seasonal effluent ammonia below 1 mg/L and annual total nitrogen below 5–8 mg/L. Second, EPA's 2024 ELG updates and an expanding body of state PFAS NPDES expectations are forcing utilities to plan for granular activated carbon or oxidation-coupled biological polishing ahead of any formal numeric limit. Third, the 2024 ScienceDirect review of biological process upgrades (S4) concludes that traditional A²/O, oxidation ditch, and SBR plants "consume large amounts of energy and produce a substantial volume of waste-activated sludge," which is precisely the condition utilities are being told to leave behind.
The signal is not just US-domestic. Watercare's NZ$500M Southwest programme and the MABR installation at Waiuku in September 2026, summarized in our Watercare MABR upgrade case study, show that municipal utilities on tight consent-decree timelines are betting on biofilm-based retrofits over full tank reconstruction. For US engineers, the question is no longer whether to upgrade the secondary train, but which of the five available biological reactor configurations to install under an existing basin footprint.
What Each Stage Actually Does — and Where Upgrades Add the Most Value
Primary treatment is a risk-management stage, not a polishing stage. The 1977 EPA performance tables (S5, Table 1) put plain sedimentation at 25–40% BOD₅ and 40–70% suspended solids removal, while chemical precipitation lifts that to 50–85% BOD₅ and 70–90% SS. For most 2026 US retrofits, the upgrade value at the primary stage is mechanical, not chemical: better screening, reliable grit removal, and clarifier hydraulic profiling that protects downstream equipment. Tightening primary capture without confirming downstream thickening, polymer, and dewatering capacity creates more operational headaches than it solves.
Secondary treatment is where 50 years of technology divergence shows up. Trickling filters deliver 50–95% BOD₅ and 50–92% SS, conventional activated sludge delivers 85–95% BOD₅, 70–95% SS, 50–80% COD, and 90–98% coliform reduction (EPA, 1977). The plant-engineering problem is that those numbers do not address nitrogen, phosphorus, or trace organics — the parameters 2026 permit renewals are written around. A conventional plug-and-play secondary plant that meets 1972 BPT can still fail a 2026 ammonia limit by a factor of ten, which is why almost every biological nutrient removal retrofit in the US begins at the secondary stage rather than at the polish step.
Tertiary treatment covers the polish operations added after secondary: chemical precipitation, sand/dual/multi-media filtration, microscreening, carbon adsorption, and the newer ozone-plus-biologically active filtration sequences for micropollutants (S5; S2). The 1977 EPA tables show chemical precipitation at 50–85% BOD₅ and 70–90% SS as a stand-alone polish, but 2026 tertiary retrofits are increasingly driven by PFAS, trace organics, and reuse-grade turbidity rather than by nutrient polishing alone. Disc filters and multi-media filters now sit ahead of UV for reuse projects, and ozone-GAC trains are being designed explicitly for trace organic control, not for residual BOD.
The municipal-versus-industrial framing matters here: municipal wastewater relies on biological degradation, so the upgrade decision is overwhelmingly about reactor configuration rather than chemistry. Where a refinery or textile plant might add chemical precipitation or ion exchange, a municipal utility upgrading in 2026 is choosing between IFAS, MBBR, MBR, AGS, or MABR, then deciding what polish step sits behind it. Our disc filter retrofit guide covers the tertiary filter side of that decision in detail.
Secondary Treatment Upgrades: MBR vs MBBR vs IFAS vs AGS vs MABR
The 2024 ScienceDirect review of biological process upgrades (S4) identifies IFAS, aerobic granular sludge (AGS), and MBBR as the most promising retrofits to enhance and transform conventional activated-sludge processes, with full-scale mainstream partial denitrification/anammox as the emerging frontier. The table below puts the five leading options head-to-head on the parameters a US design engineer has to defend in a basis-of-design memo. All values are typical full-scale municipal ranges, not vendor nameplate maximums.
| Parameter | MBR | MBBR | IFAS | AGS | MABR |
|---|---|---|---|---|---|
| BOD₅ removal | 95–99% | 85–95% | 90–97% | 90–97% | 85–95% |
| Effluent ammonia (as N) | < 1 mg/L | 1–3 mg/L | < 1–2 mg/L | < 1–2 mg/L | < 1 mg/L |
| Total nitrogen capability | 5–10 mg/L with denite zone | 5–10 mg/L (multi-stage) | 5–10 mg/L (multi-stage) | 3–8 mg/L (granule SND) | 3–6 mg/L (counter-diffusion) |
| Biological TP removal | Limited; chem-P typical | Limited; chem-P typical | Limited; chem-P typical | Yes (PAO granules) | Limited; chem-P typical |
| Footprint vs. conventional | ~40% (60% smaller) | ~60% | ~50% | ~40–50% | ~50% |
| Effluent TSS / turbidity | < 1 mg/L / < 0.5 NTU | 10–30 mg/L | 10–30 mg/L | 10–20 mg/L | 10–30 mg/L |
| Retrofit complexity | High (membranes, RAS, CIP) | Low–moderate (carriers) | Moderate (carriers + mixed liquor) | High (selector hydraulics, WAS) | Low–moderate (membrane modules) |
| Energy demand (kWh/m³) | 0.4–0.8 | 0.2–0.4 | 0.3–0.5 | 0.3–0.5 | 0.15–0.3 |
MBR is the highest-effluent option. An integrated MBR membrane bioreactor system delivers sub-micron effluent (< 1 μm nominal pore) at roughly 60% of the footprint of a conventional activated-sludge train, with unit capacities from 10 to 2,000 m³/day suitable for satellite plants and reuse packages. The flat-sheet variant covered by the MBR flat sheet membrane module uses 0.1 μm pores, runs 10–20× lower energy than external cross-flow designs, and produces 32–135 m³/day per cassette (HydropureWater product data, 2026). MBR is the right answer when the permit is reuse-driven or when downstream RO needs very low SDI.
MBBR and IFAS are the workhorse retrofits for plants that need to push ammonia and TN without abandoning their existing tankage. The detailed trade-off is covered in our MBBR vs IFAS comparison; the short version is that MBBR adds carriers to a reactor with no return activated sludge, while IFAS keeps the suspended-growth MLSS and adds carriers in the same basin, which gives IFAS a higher volumetric nitrification rate at the cost of more complex mixed-liquor management. AGS is the emerging option when the utility can accept a selector-and-flux-control retrofit for biological TP and partial SND, and MABR is the option for utilities that want oxygen-efficient nitrification in a small footprint — the same configuration the Watercare MABR upgrade case study documents for September 2026 commissioning. Long-term flux recovery and cleaning intervals for MBR retrofits are covered in our MBR membrane cleaning troubleshooting guide.
Tertiary Treatment Upgrades: Filtration, Disinfection, and Micropollutant Removal
Tertiary retrofits in 2026 typically layer three functions: solids capture to protect downstream equipment, disinfection to meet NPDES or reuse pathogen limits, and micropollutant removal for PFAS or trace organics. The most common polish filter is still a multi-media or disc filter; both are common retrofits because they fit inside an existing filter building and run at 5–15 gpm/ft² with backwash water under 5% of throughput (S2).
Micropollutant control is rarely a single-unit fix. The practitioner guidance is to run an oxidation step — typically ozone at 5–15 mg/L dose — to transform PFAS precursors and trace organics, then follow with biologically active filtration or granular activated carbon to remove the byproducts. Expect integration headaches: headloss accumulation, backwash water returning nutrients to the head of the plant, and a measurable increase in solids handling are the typical commissioning surprises. A multi-media filter for RO membrane protection is the standard choice when reuse is the end use, because the filter drops SDI to under 3 and protects the RO train from fouling.
Disinfection closes the train. UV is the default for chlorine-resistant Cryptosporidium and Giardia with no DBPs; a UV sterilizer for tertiary disinfection typically doses 30–40 mJ/cm² at 65% UVT to meet a 4-log virus target. Chlorine dioxide is the right call for systems that need a residual in the collection system; a chlorine dioxide generator typically doses 1–5 mg/L as ClO₂ and produces fewer THMs than free chlorine at equivalent CT. The Isles of Scilly petition covered in our Isles of Scilly UV upgrade brief is a real-world example of a community weighing secondary expansion against UV tertiary addition under a tightening permit.
Retrofit Logic: Permits, Footprint, Hydraulics, and a 10 MGD Worked Example
The selection logic that actually survives a value-engineering review starts with the permit, not the technology. The practitioner sequence from S2 is: commission a 90-day monitoring campaign (composite influent/effluent plus grab pathogen checks) aimed at the specific parameters the renewed permit will enforce; run a solids mass balance that includes anticipated tertiary solids and chemistry impacts; shortlist two technologies using a lifecycle cost filter that weights energy, chemical use, and operator skill; and require a manufacturer-led pilot and operator training clause in procurement. Skipping any of those four steps is the most common reason 2026 retrofits end up over-budget or under-performing.
Worked example for a 10 MGD primary works expansion: at a conservative surface overflow rate (SOR) of around 1,000 gpd/ft², the required clarifier surface area is approximately 10,000 ft² — typically two 70-ft-diameter circular clarifiers or one rectangular basin with adequate weir length. The depth and hopper geometry must permit at least 24 hours of primary sludge storage, and the scum and sludge piping has to be sized for the increased solids load if chemical phosphorus precipitation is added downstream. On the headworks side, a rotary mechanical bar screen with 6 mm openings protects new biological and tertiary equipment from rag and grit damage — a small line item that prevents outsized maintenance costs later.
Solids handling must be in scope from day one because tighter primary capture and tertiary chemical addition both push more mass to thickening and dewatering. A plate and frame filter press for sludge dewatering producing 22–28% DS cake, or a decanter centrifuge producing 20–25% DS cake, is the typical end-of-pipe choice. The decision aids in our DAF and clarifier selection guides — including the 2026 DAF/clarifier guide for York — apply when primary or tertiary solids separation is the bottleneck rather than the biological reactor.
| Worked item | 10 MGD baseline | 2026 retrofit assumption |
|---|---|---|
| Primary clarifier surface area | ~10,000 ft² | Two 70-ft circular or equivalent rectangular |
| Primary sludge storage | 24 h minimum | Hopper + sludge-well design |
| Secondary biological upgrade | Conventional AS, 85–95% BOD₅ | IFAS / MBBR / MBR to < 1 mg/L NH₃-N |
| Tertiary polish | Chlorination only | Multi-media + UV (or ozone-GAC for PFAS) |
| Parameter | 1977 baseline (EPA, 1977) | 2026 retrofit target |
| BOD₅ removal | 85–95% | 95–99% |
| Effluent ammonia (as N) | Not regulated | < 1 mg/L seasonal |
| Effluent TN | Not regulated | 5–8 mg/L annual |
| Effluent TP | Not regulated | 0.1–0.5 mg/L (chem-P or AGS) |
| Footprint vs. 1977 conventional | 1.0× | 0.4–0.6× |
Frequently Asked Questions
What is the difference between primary, secondary, and tertiary wastewater treatment?
Primary is mechanical sedimentation that removes 25–40% of BOD₅ and 40–70% of suspended solids; secondary is biological oxidation that lifts BOD₅ removal to 85–95% and adds 90–98% coliform reduction; tertiary is polish — filtration, advanced nutrient removal, disinfection, and increasingly PFAS or trace organic control (per EPA, 1977; CWT, 2024). Each stage is a separate upgrade decision point for a 2026 US municipal retrofit.
Which biological upgrade — MBR, MBBR, IFAS, AGS, or MABR — is best for a 2026 US municipal retrofit?
It depends on the permit driver. MBR is the right choice for reuse or for sub-1 mg/L ammonia in a tight footprint (HydropureWater product data, 2026). MBBR and IFAS are the workhorse retrofits for ammonia and TN at lower capex; AGS adds biological TP; MABR is the lowest-energy option for oxygen-limited sites (per ScienceDirect, 2024; Watercare case, 2026-09).
What BOD₅ and ammonia performance can a modern secondary retrofit deliver versus a 1970s baseline?
Modern retrofits lift BOD₅ removal from the 1977 EPA baseline of 85–95% to 95–99%, and bring effluent ammonia (as N) below 1 mg/L year-round versus essentially unregulated in 1977 (per EPA, 1977; ScienceDirect, 2024). Total nitrogen can be driven to 3–10 mg/L and total phosphorus to 0.1–0.5 mg/L with the right reactor configuration.
How is PFAS treated in a 2026 municipal wastewater upgrade?
PFAS control typically layers ozone or AOP for precursor transformation with granular activated carbon or ion-exchange resin for the long-chain species, sized to the plant's effluent flow and target ppt level (per practitioner guidance, 2024). No single-unit fix reliably meets current state PFAS expectations; expect a multi-stage polish ahead of any reuse or discharge permit.
What does a 10 MGD primary clarifier retrofit actually require in footprint and hydraulics?
At a conservative SOR around 1,000 gpd/ft², a 10 MGD primary clarifier needs roughly 10,000 ft² of surface area, a hopper volume that provides at least 24 hours of sludge storage, and a headworks screen with 6 mm or finer openings to protect downstream biological and tertiary equipment (per practitioner sizing guidance, 2024; EPA, 1977).
