What mainstream deammonification means
Mainstream deammonification is an autotrophic nitrogen-removal approach for the main liquid line of a municipal or comparable wastewater-treatment plant. It is commonly implemented as partial nitritation–anammox (PN/A): ammonia-oxidizing bacteria (AOB) oxidize only part of the ammonium to nitrite, and anaerobic ammonium-oxidizing bacteria (anammox bacteria) use the remaining ammonium and nitrite to produce nitrogen gas. The process is also called mainstream partial nitritation and anammox, mainstream PN/A, or mainstream anammox. [1] [2]
Engineering principle: mainstream PN/A should be evaluated as a controlled biological system with a complete nitrogen, carbon, solids, oxygen, and greenhouse-gas balance—not as a drop-in replacement for conventional nitrification–denitrification.
Comparative nitrogen-removal pathways
This reference figure contrasts conventional nitrification-denitrification with partial nitritation-anammox and summarizes the associated oxygen, carbon, sludge, and nitrogen-gas pathways. Actual savings depend on wastewater composition, process configuration, controls, energy recovery, and the complete treatment train.
The biological shortcut
Conventional biological nitrogen removal generally nitrifies ammonium to nitrate and then uses organic carbon to denitrify nitrate to nitrogen gas. PN/A avoids fully nitrifying all ammonium to nitrate. Partial nitritation supplies nitrite, while anammox converts ammonium and nitrite under oxygen-limited or anoxic conditions. Anammox produces a relatively small nitrate fraction as part of its metabolism, so less nitrate remains for a downstream heterotrophic denitrification step. This can reduce aeration and external-carbon requirements in suitable treatment trains, while allowing more influent organic carbon to be captured for other uses. [1] [2]
| Functional group | Role in PN/A | Control objective |
|---|---|---|
| AOB | Oxidize part of ammonium to nitrite in the partial-nitritation stage. | Provide the correct ammonium-to-nitrite balance without driving complete nitrification. |
| NOB | Oxidize nitrite to nitrate; they compete with anammox for nitrite. | Suppress or wash out NOB sufficiently to protect nitrite availability and limit unwanted nitrate formation. |
| Anammox bacteria | Convert ammonium and nitrite to nitrogen gas under anoxic conditions. | Retain enough active biomass and protect it from oxygen, toxicants, nitrite shocks, and excessive hydraulic or solids loss. |
| Heterotrophs | Consume readily biodegradable and stored organic carbon and may compete for oxygen or nitrite. | Manage carbon capture and loading so heterotrophic activity does not destabilize the autotrophic pathway. |
Why the mainstream line is difficult
Sidestream deammonification is more established because reject water and similar streams normally have high ammonium concentrations and warmer, more predictable conditions. Mainstream municipal wastewater has lower and variable ammonium, lower temperatures, changing carbon and solids loads, and a continuous influx of organisms from the upstream plant. These conditions make NOB suppression and anammox biomass retention substantially more difficult. A major review reported that mainstream PN/A feasibility had been demonstrated at several scales, while also identifying low loading, elevated effluent nitrogen at low temperatures, NOB control, reactor design, biomass retention, carbon-concentrating pretreatment, and polishing as continuing challenges. [1] [3]
| Condition | Why it matters | Typical engineering response |
|---|---|---|
| Low ammonium concentration | AOB and anammox growth and substrate availability become limiting, while NOB may remain competitive. | Use selective biomass retention, staged operation, careful loading control, and an appropriate effluent-polishing strategy. |
| Low or variable temperature | Anammox growth and reaction rates slow, and the competitive balance among AOB, NOB, anammox, and heterotrophs changes. | Design for seasonal conditions, maintain sufficient active biomass, and validate the winter operating envelope rather than sizing from warm-weather data alone. |
| Readily biodegradable carbon | Heterotrophs can consume oxygen and nitrite, altering the substrate balance expected by PN/A. | Consider primary clarification, high-rate carbon capture, or other pretreatment where compatible with the overall carbon-recovery strategy. |
| NOB intrusion or rebound | NOB consumes nitrite and creates additional nitrate, reducing the shortcut benefit. | Combine DO and aeration control, solids-retention strategy, selective wasting or biofilm management, and routine nitrate/nitrite surveillance. |
| N2O formation | Small direct emissions can materially change the climate footprint of a low-energy nitrogen-removal process. | Measure dissolved and off-gas N2O where material, link emissions to operating conditions, and include mitigation in commissioning and optimization. |
Representative process-flow arrangements
A mainstream PN/A train is not one fixed reactor type. The arrangement can be single-stage, two-stage, granular, suspended-growth, biofilm-based, or hybrid. The most useful process-flow question is where carbon, ammonium, nitrite, nitrate, solids, and oxygen are managed across the train.
| Train element | Purpose | Important design question |
|---|---|---|
| Mechanical and primary treatment | Remove screenings, grit, floatables, and a controllable share of particulate carbon. | How much carbon should be captured for digestion or other recovery, and how much should remain available for downstream polishing? |
| Carbon capture or high-rate pretreatment | Reduce heterotrophic competition and potentially improve energy or carbon recovery. | Will lower soluble and particulate carbon stabilize PN/A without creating an unacceptable downstream polishing burden? |
| Partial nitritation | Convert a controlled fraction of ammonium to nitrite. | Can aeration, pH, alkalinity, temperature, and solids retention maintain the target ammonium-to-nitrite balance? |
| Anammox stage | Convert ammonium and nitrite to nitrogen gas under oxygen-limited conditions. | How will slow-growing anammox biomass be retained and protected during hydraulic, toxicant, and loading transients? |
| Final polishing | Remove residual ammonium, nitrite, nitrate, suspended solids, phosphorus, or trace contaminants as required. | What effluent objective applies, and can polishing absorb seasonal PN/A variability without recreating the full energy and carbon demand of conventional treatment? |
One-stage versus two-stage PN/A
In a one-stage system, partial nitritation and anammox occur in the same biological zone or reactor. This can reduce infrastructure and allows immediate nitrite consumption, but it also couples competing oxygen and solids-retention requirements. A two-stage system separates partial nitritation from anammox, making it easier to give each population a more suitable environment, although it adds equipment, control points, and hydraulic complexity. Neither configuration is universally superior.
| Configuration | Potential advantage | Principal trade-off |
|---|---|---|
| One-stage PN/A | Compact retrofit concept with direct nitrite consumption and potentially lower peak nitrite concentrations. | Balancing AOB oxygen demand, anammox oxygen sensitivity, NOB suppression, and long anammox solids retention in one reactor can be difficult. |
| Two-stage PN/A | Allows partial nitritation and anammox conditions, biomass retention, and monitoring to be managed more independently. | Requires additional reactor volume, transfer control, instrumentation, and protection against upstream imbalance or nitrite shock. |
| Biofilm or granular hybrid | Can retain slow-growing anammox organisms while allowing selective control of suspended or attached populations. | Carrier or granule management, mass transfer, detachment, clogging, and long-term ecological stability become central design issues. |
Operating and control considerations
Successful mainstream PN/A depends on maintaining a narrow biological operating window while the wastewater itself changes. Instrumentation should support control decisions rather than simply record compliance data. The control philosophy should be developed around nitrogen species, alkalinity, temperature, oxygen, solids, and emissions.
| Parameter | Why it is monitored | Interpretation |
|---|---|---|
| NH4-N, NO2-N, NO3-N | Shows whether ammonium conversion, nitrite availability, NOB activity, and final polishing are aligned. | Use trends and load-based balances; a single grab sample can miss cycling and transient behavior. |
| Dissolved oxygen and aeration profile | Supports AOB activity while excessive oxygen can inhibit anammox or favor NOB in the wrong location. | Control aeration by stage and process objective, not by a single universal DO set point. |
| pH, alkalinity, and temperature | Reflects nitrification demand, chemical balance, and seasonal biological activity. | Use actual wastewater conditions to define the operating envelope and chemical-support requirements. |
| SRT, attached biomass, and solids inventory | Anammox organisms grow slowly and must be retained while unwanted populations are controlled. | Track solids loss, granule or carrier condition, and active biomass—not only MLSS concentration. |
| N2O in liquid and off-gas | Provides a direct check on a potentially material greenhouse-gas pathway. | Pair emissions data with DO, nitrite, organic loading, and aeration events to identify mitigation opportunities. |
Potential advantages
Where the feed, effluent objective, and control maturity are suitable, mainstream PN/A can reduce the oxygen needed for nitrogen removal, reduce or avoid external organic-carbon addition, and preserve more organic carbon for biogas or other recovery pathways. It may also reduce sludge production associated with heterotrophic denitrification. These are system-level opportunities, not guaranteed values: carbon-capture equipment, additional polishing, heating or seasonal capacity, instrumentation, and N2O controls must be included in the whole-plant assessment. [2] [3]
Limitations, risks, and commissioning
Mainstream PN/A should be piloted or demonstrated against the actual wastewater whenever uncertainty is material. A successful sidestream system does not automatically establish mainstream performance because ammonium concentration, temperature, carbon load, solids, toxicants, and hydraulic conditions differ. Commissioning should include an inoculation and biomass-retention plan, staged load increases, winter and low-load testing, NOB surveillance, nitrite and ammonium shock response, dissolved-oxygen control verification, and a measured N2O baseline.
Reported pilot work demonstrates that high effluent quality and low-temperature operation are possible, but also shows that nitritation can be rate-limiting and that N2O emissions may be significant. One year-round pilot reported average effluent concentrations of 0.4 mg NH4-N/L, 0.1 mg NO2-N/L, and 0.9 mg NO3-N/L, while observing an average N2O emission factor of 1.2% of influent total nitrogen. These are study-specific results, not design guarantees. [2]
Where the technology can fit
Mainstream deammonification is most relevant to municipal plants and comparable biological treatment systems with a defined nitrogen objective, sufficient process-control capability, and a plan for carbon management and final polishing. It can be considered alongside existing activated-sludge, MBR, AGS, or high-rate carbon-capture configurations. A hybrid approach may retain conventional nitrification–denitrification for a portion of the load, use PN/A for selected zones or seasons, or apply PN/A in a sidestream while the mainstream process is progressively adapted.
For a project review, define the influent nitrogen and carbon loads, temperature range, alkalinity, solids and toxicant profile, effluent limits, available reactor volume, energy and carbon-recovery objectives, monitoring capability, and residual or emissions-management requirements. The technology should be selected against the complete treatment train rather than its nitrogen-removal reaction alone.
Comammox and mainstream nitrogen removal
Comammox means “complete ammonia oxidation.” Unlike the usual division of labor in which ammonia-oxidizing bacteria (AOB) produce nitrite and nitrite-oxidizing bacteria (NOB) convert nitrite to nitrate, a comammox Nitrospira cell can carry out both steps and oxidize ammonia all the way to nitrate. Comammox is therefore not the same as anammox: anammox is an oxygen-sensitive nitrogen-removal pathway that consumes ammonium and nitrite to form nitrogen gas, while comammox is an aerobic nitrification pathway that normally ends in nitrate. [4] [5]
Comammox matters to mainstream PN/A because complete ammonia oxidation can either compete with the shortcut objective or, under selected conditions, help supply nitrite to anammox organisms. If comammox converts too much ammonium to nitrate, less ammonium and nitrite remain available for anammox and the process can lose part of its shortcut benefit. However, reactor ecology is context-dependent. Recent biofilm-MBR work reported comammox-associated partial nitritation supporting anammox under intermittent aeration, rather than universal complete nitrification. [5] [7]
| Comammox question | Why it matters for design | Practical response |
|---|---|---|
| Where can comammox compete? | High substrate affinity and the ability to operate in low-substrate or low-oxygen niches may favor comammox in biofilms, attached-growth systems, and carefully selected low-DO reactors. | Evaluate oxygen profile, ammonium loading, biomass retention, carrier or granule structure, and the actual seasonal operating window. |
| Does nitrate formation help or hurt? | Direct ammonia-to-nitrate conversion can support conventional nitrification but can consume the shortcut opportunity needed by PN/A. | Set the target nitrogen pathway first. Do not treat comammox abundance as a benefit without checking ammonium, nitrite, nitrate, and total-nitrogen balances. |
| Can comammox cooperate with anammox? | Under selected oxygen-limited biofilm conditions, comammox may provide nitrite that anammox consumes. The interaction can also shift toward competition when oxygen, substrate, or retention conditions change. | Use staged or intermittent aeration, retain the desired biomass, and confirm the interaction with activity data rather than taxonomy alone. |
| How should it be monitored? | Routine water-quality data show pathway outcomes but do not identify which nitrifier is responsible. | Combine NH4-N, NO2-N, NO3-N, DO, pH, alkalinity, and N2O data with targeted amoA/nxrB assays, metagenomics, FISH, or activity tests when the distinction affects process decisions. |
Evidence from engineered systems is strong enough to include comammox in process diagnosis, but not strong enough to make it a universal design target. A survey of 16 samples from six full-scale plants detected comammox signatures across conventional, mainstream, and sidestream configurations, while a separate low-DO mainstream reactor study found comammox Nitrospira became the dominant detected ammonia-oxidizer group after long-term selection. Those findings demonstrate ecological and functional potential; they do not transfer a particular abundance, loading rate, or removal performance directly to another plant. [5] [6]
Comammox versus PN/A process pathways
This schematic separates two different biological objectives. Comammox Nitrospira can complete aerobic ammonia oxidation to nitrate, while PN/A deliberately preserves a partial-nitritation step and directs ammonium plus nitrite to anammox for nitrogen-gas production. The dashed connection indicates a possible, site-specific comammox contribution to nitrite supply in selected biofilm conditions—not a guaranteed operating sequence.
Current development directions
More reliable NOB suppression
Combine aeration control, biomass selection, intermittent operation, biofilm or granular retention, and microbial surveillance rather than relying on a single inhibition mechanism.
Low-temperature resilience
Improve retention and activity of anammox biomass, and validate seasonal control strategies under the actual temperature and loading range.
Integrated carbon management
Coordinate primary treatment, high-rate carbon capture, digestion, phosphorus recovery, and PN/A polishing so plant-wide resource benefits are measured together.
Greenhouse-gas-aware control
Pair nitrogen-species control with dissolved and off-gas N2O monitoring, mass balance, and operating strategies that reduce avoidable emissions.
Related engineering resources
For related biological-treatment context, explore membrane bioreactors, MABR, aerobic granular sludge, and anaerobic membrane bioreactors. For project-specific discussion, use our Contact us page.
References
- Cao, Y., van Loosdrecht, M. C. M., & Daigger, G. T., “Mainstream partial nitritation–anammox in municipal wastewater treatment: status, bottlenecks, and further studies,” Applied Microbiology and Biotechnology (2017)
- Hausherr, D. et al., “Successful year-round mainstream partial nitritation anammox: Assessment of effluent quality, performance and N2O emissions,” Water Research X (2022)
- Zheng, M. et al., “One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application,” Water Research X (2023)
- Koch, H. et al., “Insights into the ecophysiology of comammox Nitrospira,” Applied and Environmental Microbiology (2019)
- Annavajhala, M. K. et al., “Comammox Functionality Identified in Diverse Engineered Biological Wastewater Treatment Systems,” Environmental Science & Technology Letters (2018)
- Roots, P. et al., “Comammox Nitrospira are the dominant ammonia oxidizers in a mainstream low dissolved oxygen nitrification reactor,” Water Research (2019)
- Shao, Y.-H., Wu, J.-H. & Chen, H.-W., “Comammox Nitrospira cooperate with anammox bacteria in a partial nitritation–anammox membrane bioreactor treating low-strength ammonium wastewater at high loadings,” Water Research (2024)
- Wang, J. et al., “Wastewater-Derived Comammox Nitrospira for Next-Generation Wastewater Management,” Environmental Science & Technology: Water (2026)