Anoxic–Aerobic Biological Treatment

Anoxic and aerobic biological zones work together to reduce biodegradable organic matter and convert nitrogen through nitrification, nitrate recycle and denitrification.

Process function: Biological organic matter removal; Nitrification and denitrification

PROCESS / AO-01CONCEPTUAL TRAIN

01Influent
02Anoxic zone
03Aerobic zone
04Secondary clarification
05Effluent

Internal nitrate recycle

Nitrate-bearing mixed liquor returns from the aerobic zone to the anoxic zone; rate and oxygen carryover are design-specific.

Conceptual continuous-flow arrangement. Sludge return, wasting, polishing and disinfection depend on the selected process configuration.

Quick definition

What Is Anoxic–Aerobic Biological Treatment?

Anoxic–aerobic biological treatment combines a mixed zone with limited free dissolved oxygen and an aerated zone within one biological treatment train. In the aerobic zone, biomass oxidizes biodegradable organic matter and can convert ammonia to nitrite and nitrate. Nitrate-bearing mixed liquor can then be returned to the anoxic zone, where denitrifying organisms use nitrate or nitrite while oxidizing an available carbon source.

The biological reactors convert dissolved and colloidal pollutants; they do not by themselves complete physical solids separation. A downstream clarification, membrane or timed settling step must separate treated water from suspended biomass according to the selected system configuration.

On this page, A/O means anoxic–aerobic. The abbreviation is not universal: some wastewater references use A/O for anaerobic/oxic phosphorus-removal processes. Anoxic and anaerobic conditions are not interchangeable, so every design document should define the zones and recycle paths explicitly.

The U.S. EPA Nutrient Control Design Manual provides independent engineering context for nitrification, denitrification, carbon availability, recycle arrangements and suspended-, attached- and hybrid-growth nitrogen-removal configurations.

InputConditioned wastewater

ProcessAnoxic + aerobic biology

OutputsTreated mixed liquor + Biological solids

Process purpose

Why Is Anoxic–Aerobic Treatment Used?

A/O treatment is used when a project needs biological organic-matter conversion and an engineered nitrogen-removal pathway rather than aerobic carbon removal alone.

01
Reduce biodegradable organic load
Heterotrophic biomass converts biodegradable carbon under the conditions established in the biological reactors.
02
Oxidize ammonia
Sufficient oxygen, alkalinity, temperature and biomass retention allow nitrifying organisms to convert ammonia in the aerobic zone.
03
Reduce oxidized nitrogen
An anoxic zone gives denitrifying organisms access to nitrate or nitrite and an electron donor, commonly biodegradable carbon.
04
Coordinate biomass retention
Sludge return, wasting, fixed media or hybrid retention can maintain the active biomass needed for the selected duty.
05
Support decentralized treatment
The process can be configured in municipal, rural, institutional and packaged treatment systems when the wastewater and operating context are suitable.
06
Create a controllable biological stage
Aeration, mixing, recycle and solids management provide operating variables that can be monitored against the design basis.

BIOLOGICAL RELATIONSHIPCONCEPTUAL

Carbon: Converted and used for denitrification
Nitrogen: Oxidized, recycled and reduced
Biomass: Retained, returned or wasted

Conceptual relationship only; bar lengths do not represent oxygen, loading, retention time or treatment efficiency.

Process position

Where Does A/O Treatment Fit in a Wastewater Treatment Plant?

In the W1–W8 treatment architecture, A/O is the W4 biological-conversion stage. W5 then separates treated water from biomass and returns or wastes solids according to the selected process.

WATER LINE / W4–W5ILLUSTRATIVE PROCESS POSITION

W1PretreatmentScreening and grit removal
W2EqualizationBalance flow and feed condition
W4A/O biological treatmentBiological conversion
W5Secondary separationClarify, return and waste biomass
W6Optional polishingWhen final water quality requires it
W7DisinfectionMethod and contact are project-specific

Recycle and sludge interfaces

Nitrate-bearing mixed liquor commonly returns from the aerobic zone to the anoxic zone. Where a secondary clarifier is used, settled biomass can return to the biological stage while excess solids leave through the confirmed wasting route.

This page expands W4 biological conversion and its W5 separation interface; it does not replace the complete W1–W8 process overview.

Working principle

How Does Anoxic–Aerobic Treatment Work?

The process links influent carbon, anoxic mixing, aerobic oxidation, nitrification, internal recycle and downstream solids separation. Each function must be evaluated on the same hydraulic and mass-loading basis.

01
Influent distribution and conditioning
Pretreatment and equalization protect the biological system and establish a feed condition that can be distributed through the selected reactor arrangement.
02
Anoxic biological reactions
Mixing maintains contact between wastewater, biomass and recycled nitrate while intentional aeration is limited so denitrification can proceed when usable carbon is available.
03
Aerobic organic oxidation
Aeration supplies oxygen and mixing for biological conversion of biodegradable organic matter in the aerobic zone.
04
Biological nitrification
Ammonia-oxidizing and nitrite-oxidizing organisms convert ammonia toward nitrate when oxygen, alkalinity, temperature and biomass retention are adequate.
05
Internal recycle and denitrification
A controlled recycle carries nitrate-bearing mixed liquor back to the anoxic zone, where denitrifiers can reduce oxidized nitrogen using an available electron donor.
06
Solids separation and biomass management
Clarification, membranes or timed settling separate treated water from biomass; return and wasting practices maintain the selected solids inventory.

InputConditioned influent

Stage 01Anoxic conversion

Stage 02Aerobic oxidation

OutputsMixed liquor to separation

Nitrogen-removal fundamentals

Understanding Nitrification and Denitrification

Biological nitrogen removal depends on two linked reactions operating under different electron-acceptor conditions. Carbon, alkalinity and retained biomass connect those reactions to stable plant performance.

01
Aerobic nitrification
Nitrifying organisms oxidize ammonia first to nitrite and then toward nitrate in an oxygenated biological environment.
Requires oxygen + retained nitrifiers
02
Anoxic denitrification
Denitrifying organisms reduce nitrate or nitrite toward nitrogen gas while oxidizing a suitable electron donor.
Requires nitrate + usable carbon
03
Carbon and alkalinity
Readily biodegradable carbon supports denitrification, while alkalinity and pH buffering are important to the nitrification reaction.
Balance across both zones
04
Biomass retention
Suspended solids, attached biofilm or a hybrid arrangement must retain enough active organisms for the actual load and temperature.
Confirm SRT or effective retention

REACTION VIEW / AO-02CONCEPTUAL

Aerobic nitrification
Anoxic denitrification
Carbon and alkalinity
Biomass retention

The diagram groups the reaction conditions conceptually; particle sizes and bars do not represent kinetics, concentrations or removal efficiency.

Design factors

What Affects A/O Treatment Performance?

A/O response is governed by wastewater characteristics, biological loading, retention, oxygen, recycle, carbon, alkalinity, temperature and solids separation—not by reactor volume alone.

01
Influent BOD and COD
Biodegradable carbon defines the organic load and may provide the electron donor needed for denitrification; total COD alone does not show how much is readily usable.
02
Ammonia and total nitrogen
Average, peak and seasonal nitrogen loads establish the nitrification and denitrification duties rather than a flow value alone.
03
Carbon-to-nitrogen balance
Insufficient usable carbon can limit denitrification even when the anoxic volume and nitrate recycle are available.
04
Hydraulic retention
Reactor volume and the actual flow profile determine contact time, but an HRT should not be transferred between projects without a mass-loading basis.
05
Solids or biomass retention
Suspended-growth SRT, fixed-film inventory and hybrid retention affect the ability to maintain slow-growing nitrifying organisms.
06
Dissolved oxygen and aeration
The aerobic zone needs adequate oxygen transfer, while oxygen carried into the anoxic zone can consume carbon without producing the intended denitrification pathway.
07
Internal recycle strategy
Recycle location, rate, pumping or air-lift duty and control determine how nitrate and dissolved oxygen return to the anoxic reactor.
08
Temperature
Biological reaction rates change with wastewater temperature, making the minimum design temperature important for volume and retention decisions.
09
pH and alkalinity
Nitrification consumes alkalinity and can depress pH; the buffering basis must be checked across expected ammonia loads and recycle streams.
Review chemical dosing and control
10
Shock and inhibitory loads
Hydraulic peaks, organic shocks, salinity, toxic compounds and abrupt pH changes can disturb different biomass groups in different ways.
11
Biological media
Media type, retained surface, wetting, mixing, oxygen transfer and solids control matter where attached or carrier-assisted growth is used.
12
Secondary solids separation
Poor settling, solids loss or an unsuitable return-sludge strategy can reduce overall effluent quality even when the biological reactions are functioning.

Performance

How Is A/O Treatment Performance Evaluated?

Monitor organic conversion, nitrogen species, operating conditions, biomass and final solids separation on the same time and loading basis.

Indicator	What it means	Why it matters
BOD and biodegradable COD	Organic matter entering and leaving the biological stage, interpreted with the selected analytical method.	Shows the carbon-conversion duty and the potential carbon available to anoxic reactions.
Ammonia nitrogen	Reduced nitrogen remaining after the aerobic treatment duty.	A primary indicator of nitrification response, interpreted with load, temperature, pH and oxygen data.
Nitrate and nitrite	Oxidized nitrogen species formed by nitrification and consumed during denitrification.	Helps distinguish incomplete nitrification, recycle limitations and incomplete denitrification.
Total nitrogen	The combined nitrogen result across relevant dissolved and particulate forms.	Connects individual reactions and solids separation to the overall nitrogen objective.
Dissolved oxygen	The oxygen condition measured at representative points in the aerobic and anoxic system.	Supports aeration control and identifies oxygen carryover that can compete with anoxic denitrification.
pH and alkalinity	Acid-base condition and buffering capacity through the biological process.	Provides evidence that nitrification can proceed without exhausting the required buffer.
Temperature	Actual wastewater temperature, including seasonal minimum conditions.	Needed to interpret biological rates and the biomass-retention requirement.
Biomass and solids condition	Suspended-solids inventory, sludge age or effective attached-biomass condition, as applicable.	Shows whether the process is retaining active biomass and whether wasting matches the design strategy.
Internal recycle flow	Measured or verified nitrate-recycle flow and its operating availability.	Connects aerobic nitrate production with anoxic nitrate loading and hydraulic effects.
Effluent suspended solids	Biological solids leaving the final separation step with treated water.	Separates clarification or membrane performance from dissolved biological-conversion performance.

Technology options

Common Anoxic–Aerobic Treatment Configurations

Suspended, attached and hybrid biomass arrangements use the same linked biological principles but differ in retention, reactor, mixing, aeration and separation details.

Suspended-growth A/O

Working principle

Mixed liquor passes through an anoxic zone and an aerobic zone, with internal nitrate recycle and a return-sludge loop where a separate clarifier is used.

Typical characteristics

Continuous-flow reactor train
Suspended biomass inventory
Internal mixed-liquor recycle
Clarifier-based return and wasting

Suitable conditions

Municipal or industrial wastewater with suitable biodegradability where aeration, mixing, recycle and clarifier operation can be managed as one activated-sludge system.

Main trade-off

Nitrification and solids retention depend on the sludge-age and separation strategy, while anoxic performance depends on nitrate delivery and usable carbon.

Attached-growth or contact-oxidation A/O

Working principle

Biomass grows on fixed or retained media in the anoxic and/or aerobic biological zones while water and recycle flows contact the biofilm.

Typical characteristics

Retained biofilm surface
Media-specific mixing and aeration
Suspended solids may still be present
Downstream separation remains configuration-specific

Suitable conditions

Compact and packaged duties where the selected media, reactor geometry and operating plan provide effective contact and maintainable biomass retention.

Main trade-off

Media attachment does not remove the need to evaluate oxygen transfer, excess solids, hydraulic distribution, clogging risk and final solids separation.

Hybrid or carrier-assisted A/O

Working principle

Suspended biomass and an engineered attached-growth fraction operate together to increase or stabilize the effective biomass inventory for selected duties.

Typical characteristics

Combined suspended and attached growth
Carrier or fixed-media retention
Project-specific screens or retention devices
Existing-basin retrofit potential in some cases

Suitable conditions

Applications that need additional biomass retention or a retrofit path after the carrier, reactor and separation interfaces have been confirmed.

Main trade-off

A carrier-assisted reactor is not automatically an MBBR. IFAS, fixed contact media and true moving-bed systems have different media motion, retention and solids-management requirements.

Technology comparison

Comparing Biological Nitrogen Removal Configurations

This matrix distinguishes biological arrangement, biomass retention and solids separation without assigning universal performance rankings.

Configuration	Biological arrangement	Biomass retention	Nitrogen-removal path	Solids separation	Key distinction
Suspended-growth anoxic–aerobic	Continuous anoxic zone before an aerobic zone	Suspended mixed liquor with return and wasting control	Aerobic nitrification plus internal nitrate recycle to anoxic denitrification	Normally a downstream secondary clarifier	Carbon, SRT, oxygen carryover, recycle and settleability must be coordinated
A²/O	Anaerobic, anoxic and oxic zones in sequence	Usually suspended growth with return sludge	Adds an anoxic nitrogen-removal step to a process that also targets biological phosphorus removal	Normally a downstream secondary clarifier	The added anaerobic zone has a different purpose and must be protected from nitrate intrusion
Carrier-assisted A/O	Anoxic and aerobic zones with fixed, retained or moving media as designed	Attached growth alone or combined with suspended growth	Reactions remain linked by nitrate transport and available carbon	Depends on suspended solids and the selected media/reactor system	Media presence alone does not define the process as MBBR
Anoxic–aerobic MBR	Anoxic and aerobic biology combined with membrane separation	Suspended or hybrid biomass retained by the membrane system	Can use the same nitrification, recycle and denitrification principles	Membranes replace conventional secondary clarification	MBR describes the solids-separation configuration, not a separate biological reaction
SBR with anoxic/aerobic periods	Conditions are created in timed fill, react, settle and decant phases	Suspended biomass retained through the batch cycle	Anoxic and aerobic periods are sequenced in time rather than only in separate continuous-flow zones	Settling and decant occur in the reactor basin	SBR is an operating configuration, not simply another two-zone A/O reactor

Technology selection

How to Evaluate an A/O Treatment Process

Start with the wastewater and governing treatment objectives, then define reaction, retention, recycle, separation and operating requirements from project-specific evidence.

01
Wastewater source
Is the feed municipal, decentralized domestic or industrial wastewater, and which compounds could inhibit biological treatment?
02
Flow profile
What are the average, peak, minimum, seasonal and operating-hour flows?
03
Organic load
How much biodegradable BOD or COD is available, and how variable is the load?
04
Nitrogen objective
Which ammonia, nitrate and total-nitrogen requirements govern the biological duty?
05
Temperature and climate
What minimum wastewater temperature and external climate conditions must the system accommodate?
06
Carbon and alkalinity
Is influent carbon usable for denitrification, and is buffering adequate for the nitrification load?
07
Available space
What footprint, access, tank geometry, headroom and future-expansion limits apply?
08
Aeration, mixing and recycle
How will oxygen transfer, anoxic mixing, internal recycle and measurement be provided and controlled?
09
Solids management
How will biomass be separated, returned, wasted, stored and treated downstream?
10
Operations and control
Which instruments, operator coverage, maintenance resources, alarms and process-response checks are available?

A/O selection is application-specific. Confirm the mass-loading basis, minimum temperature, biological retention, recycle, oxygen and separation duties together rather than transferring a standard HRT, recycle ratio or efficiency from another installation.

Relevant equipment

Applying A/O Treatment in Packaged Wastewater Systems

An anoxic–aerobic process can be incorporated into a containerized wastewater treatment plant, but the package is a physical implementation of a wider treatment train—not the biological process itself. Clarification, filtration, disinfection, controls and external headworks remain distinct treatment or equipment functions.

The wastewater is sufficiently biodegradable and free from unaddressed inhibitory loads.
Organic and nitrogen loads are confirmed across the expected flow and temperature range.
Anoxic mixing, aerobic oxygen transfer and nitrate recycle can be provided and controlled.
Clarification, sludge return and excess-solids handling are defined for the selected package.
Required polishing, disinfection and external interfaces are identified separately from the A/O reactors.

Explore KUOSI Containerized Wastewater Treatment Plant

Two green containerized wastewater treatment plant units with roof access and service doors — Relevant equipmentContainerized Wastewater Treatment Plant
The packaged plant is one equipment implementation of the process; final configuration requires confirmed project data.

System integration

Typical A/O Wastewater Treatment Process Configuration

A typical continuous-flow A/O system connects upstream protection, equalization, biological conversion, nitrate recycle, solids separation and selected final-treatment steps. The actual arrangement depends on the process type and design basis.

SYSTEM / AO-03TYPICAL CONFIGURATION

P1Pretreatment
E1Equalization
A1Anoxic biological reactor
O1Aerobic biological reactor
C1Secondary clarification
T1Optional tertiary treatment
D1Optional disinfection

Internal recycle · RAS · WAS

Internal nitrate recycle returns mixed liquor from the aerobic zone to the anoxic zone. In clarifier-based systems, return activated sludge supports biomass inventory and waste activated sludge leaves for the confirmed solids-management route.

A complete system may also include

Screening and grit removal
Equalization and feed pumping
Anoxic mixing
Aeration and blowers
Internal recycle
Sludge return and wasting
Process instrumentation
Optional chemical addition

Process integration

How Does A/O Treatment Integrate with the Complete Plant?

Upstream units establish a treatable feed; downstream separation and final treatment receive the biological process output and residual solids.

Upstream interfaces

Upstream units protect biology and establish the hydraulic, organic, nutrient, pH and temperature conditions entering the anoxic reactor.

Screening
Grit and grease control
Equalization
Primary or physicochemical separation
pH / alkalinity management

Downstream interfaces

Downstream units separate biomass, manage residual solids and apply polishing or disinfection only where the final water objective requires them.

Secondary clarification or membranes
Sludge return and wasting
Optional filtration
Optional phosphorus removal
Disinfection and verification

W4 biological conversion and W5 secondary separation are connected but distinct duties. Filtration and disinfection remain downstream processes and should not be described as biological conversion.

Operation

Common A/O Treatment Problems

These are diagnostic starting points, not universal corrective setpoints. Preserve as-found data and distinguish biological conversion from solids-separation symptoms.

Poor ammonia removal

Check actual ammonia load, minimum temperature, aerobic dissolved oxygen, pH and alkalinity, biomass retention, inhibitory inputs and solids loss before changing operating targets.

High effluent total nitrogen

Separate ammonia, nitrate, nitrite, organic nitrogen and suspended-solids contributions; then review nitrate recycle, anoxic volume, usable carbon and oxygen carryover.

Insufficient carbon for denitrification

Confirm readily biodegradable carbon through representative data or testing, review upstream carbon removal and assess any external carbon strategy as a controlled chemical-dosing duty.

Excess oxygen in the anoxic zone

Review recycle dissolved oxygen, upstream aeration, mixer operation, flow distribution and the location of the anoxic monitoring point.

Unstable biological activity

Investigate load swings, pH, temperature, salinity, toxicity, nutrient balance, aeration, wasting and media or solids loss using time-aligned operating data.

Cold-weather performance loss

Compare the actual wastewater temperature with the design basis and review biomass retention, loading, aeration capacity and seasonal operating strategy.

Variable hydraulic loading

Check equalization, peak flow, short-circuiting, recycle contribution, clarifier loading and whether control sequences remain valid at minimum and maximum flow.

Secondary clarification problems

Evaluate settleability, sludge blanket, hydraulic and solids loading, return and wasting rates, filamentous growth and solids carryover separately from dissolved biological performance.

Applications

Where Is A/O Biological Treatment Used?

A/O is applicable where wastewater biodegradability, nitrogen objectives, temperature, site conditions and operating resources support the selected biological configuration.

Municipal wastewater treatment

Continuous-flow anoxic and aerobic zones can support carbon and nitrogen duties where the influent, temperature, recycle and solids-separation basis are defined.

Rural and decentralized sewage

A/O can be incorporated into compact or modular systems when variable occupancy, equalization, operator coverage and sludge handling are addressed.

Residential communities

Community treatment can use biological nitrogen removal where the actual population, flow pattern, influent data and governing discharge requirement justify it.

Hotels and institutions

Sites with occupancy-driven peaks need equalization and an operating strategy that protects biomass during both high-load and low-flow periods.

Biodegradable industrial wastewater

Industrial application depends on representative biodegradability, nutrient balance, salinity, inhibitory compounds and load variability rather than industry name alone.

Packaged and modular treatment

Anoxic and aerobic reactors, recycle and clarification can be integrated into transportable equipment while external headworks and final-treatment interfaces remain project-specific.

Technical FAQ

Anoxic–Aerobic Treatment Frequently Asked Questions

01What is A/O biological treatment?

On this page, A/O is an anoxic–aerobic biological process: an anoxic zone supports denitrification and an aerobic zone supports organic oxidation and nitrification. Because A/O can mean anaerobic/oxic in other references, the process zones should always be defined explicitly.

02What is the difference between anoxic and anaerobic conditions?

An anoxic biological zone has little free dissolved oxygen but contains nitrate or nitrite that can act as an electron acceptor. An anaerobic zone has neither free oxygen nor nitrate/nitrite available for respiration and is used for different biological purposes, including the anaerobic phase of some phosphorus-removal processes.

03How do nitrification and denitrification work together?

Aerobic nitrification converts ammonia toward nitrate. Internal recycle can carry that oxidized nitrogen to an anoxic zone, where denitrifying organisms reduce nitrate or nitrite toward nitrogen gas while using a suitable electron donor.

04Why is internal recirculation needed?

In a pre-denitrification arrangement, the anoxic zone is upstream of the aerobic zone. Internal recycle transfers nitrate-bearing mixed liquor from the aerobic section back to the anoxic section so denitrification can occur. The required rate is project-specific.

05What affects nitrogen removal?

Important factors include biodegradable carbon, ammonia and total-nitrogen load, temperature, dissolved oxygen, pH, alkalinity, biomass retention, internal recycle, mixing, inhibitory compounds and final solids separation.

06What is the difference between A/O and A²/O?

The anoxic–aerobic arrangement discussed here uses an anoxic zone followed by an aerobic zone for nitrogen-removal duties. A²/O adds a true anaerobic zone ahead of the anoxic and oxic zones to support combined biological phosphorus and nitrogen removal.

07Can A/O use attached-growth biological media?

Yes. Fixed contact media, retained carriers or hybrid biomass can be used when their hydraulic, oxygen-transfer, media-retention and solids-management requirements are engineered. Media use alone does not automatically make every reactor an MBBR.

08Is secondary clarification always required?

A separate secondary clarifier is common in continuous-flow suspended-growth and many contact-oxidation systems, but it is not universal. Membrane systems use membrane separation, while SBRs can settle and decant within the batch reactor.

09How does temperature affect A/O treatment?

Lower wastewater temperature generally slows biological reaction rates, especially nitrification. The design should use the expected minimum water temperature to confirm biomass retention, reactor duty and seasonal operating strategy.

10Can A/O be integrated into containerized treatment plants?

Yes. A containerized plant can house anoxic and aerobic reactors, recycle, clarification and selected auxiliary functions. The package also includes physical equipment and may include filtration or disinfection, so it should not be treated as equivalent to the A/O biological process alone.

Application discussion

Evaluating an Anoxic–Aerobic Treatment Process?

Share the wastewater source, flow profile, representative organic and nitrogen data, temperature range, treatment objective, site constraints and operating resources. KUOSI can review the application and define the information needed for a process and equipment evaluation.

Useful inputs: source · flow profile · BOD/COD · ammonia/TN · temperature · alkalinity · effluent objective · site and operating constraints

Discuss Your Biological Treatment Duty View Containerized Treatment Equipment

Anoxic–Aerobic Biological Treatment

What Is Anoxic–Aerobic Biological Treatment?

Reduce biodegradable organic load

Oxidize ammonia

Reduce oxidized nitrogen

Coordinate biomass retention

Support decentralized treatment

Create a controllable biological stage

Influent distribution and conditioning

Anoxic biological reactions

Aerobic organic oxidation

Biological nitrification

Internal recycle and denitrification

Solids separation and biomass management

Understanding Nitrification and Denitrification

Aerobic nitrification

Anoxic denitrification

Carbon and alkalinity

Biomass retention

Influent BOD and COD

Ammonia and total nitrogen

Carbon-to-nitrogen balance

Hydraulic retention

Solids or biomass retention

Dissolved oxygen and aeration

Internal recycle strategy

Temperature

pH and alkalinity

Shock and inhibitory loads

Biological media

Secondary solids separation

Typical characteristics

Suitable conditions

Main trade-off

Typical characteristics

Suitable conditions

Main trade-off

Typical characteristics

Suitable conditions

Main trade-off

Wastewater source

Flow profile

Organic load

Nitrogen objective

Temperature and climate

Carbon and alkalinity

Available space

Aeration, mixing and recycle

Solids management

Operations and control

Applying A/O Treatment in Packaged Wastewater Systems

Poor ammonia removal

High effluent total nitrogen

Insufficient carbon for denitrification

Excess oxygen in the anoxic zone

Unstable biological activity

Cold-weather performance loss

Variable hydraulic loading

Secondary clarification problems

Municipal wastewater treatment

Rural and decentralized sewage

Residential communities

Hotels and institutions

Biodegradable industrial wastewater

Packaged and modular treatment

Anoxic–Aerobic Treatment Frequently Asked Questions

Evaluating an Anoxic–Aerobic Treatment Process?