Dissolved Oxygen & Biological Chemistry

Learn dissolved oxygen and biological chemistry fundamentals, including oxygen demand, aerobic, anoxic, and anaerobic conditions, nitrification, denitrification, ORP, temperature effects, and operator interpretation.

Dissolved oxygen, commonly called DO, is one of the most important chemical measurements in biological wastewater treatment. It indicates how much oxygen is dissolved in the liquid and strongly affects microbial activity, nitrification, odor formation, and treatment stability.

Operators should understand that DO is not simply a blower or aeration measurement. It reflects the balance between oxygen supplied to the process and oxygen consumed by microorganisms and other reactions.

What Is Dissolved Oxygen?

Dissolved oxygen is molecular oxygen dissolved in water.

It is commonly reported as:

mg/L

DO is important because many treatment microorganisms require oxygen to carry out aerobic biological reactions.

Where Dissolved Oxygen Comes From

Oxygen can enter water through:

diffused aeration;
mechanical aeration;
surface contact with the atmosphere;
mixing and turbulence.

Oxygen Transfer

Oxygen transfer is the movement of oxygen from air or another gas phase into the liquid.

Transfer efficiency depends on factors such as:

bubble size;
water depth;
mixing;
temperature;
wastewater characteristics;
existing DO concentration;
diffuser condition.

Fine-Bubble Aeration

Fine bubbles generally provide more gas-liquid contact area than large bubbles.

This can improve oxygen-transfer efficiency when diffuser condition and process conditions are suitable.

DO Is a Balance

The measured DO concentration reflects the balance between:

oxygen supplied;
oxygen transferred into the liquid;
oxygen consumed by microorganisms;
oxygen consumed by chemical reactions.

Oxygen Demand

Oxygen demand is the amount of oxygen consumed by biological and chemical reactions.

Demand may increase when:

organic loading increases;
ammonia loading increases;
temperature changes biological activity;
more reduced compounds enter the process.

Organic Matter and Oxygen Demand

Heterotrophic microorganisms consume biodegradable organic matter and use oxygen under aerobic conditions.

As organic loading increases, oxygen demand usually increases.

Biochemical Oxygen Demand

Biochemical Oxygen Demand, or BOD, is a laboratory measure related to the amount of oxygen microorganisms use while degrading biodegradable material under defined test conditions.

BOD is not the same as DO.

DO is the oxygen currently present in the sample.

BOD represents oxygen demand associated with biodegradable material.

Chemical Oxygen Demand

Chemical Oxygen Demand, or COD, measures oxygen equivalent associated with chemically oxidizable material using a laboratory chemical method.

COD and BOD are different tests.

COD results are often available faster and may be higher than BOD because COD can measure material not biologically degraded during the BOD test.

DO Versus BOD

A wastewater can have:

high BOD and low DO;
high BOD and adequate DO if aeration is sufficient;
low BOD and high DO.

The measurements describe different aspects of the process.

Aerobic Conditions

Aerobic conditions have dissolved molecular oxygen available for microorganisms.

Aerobic processes include:

carbonaceous organic removal;
nitrification;
many activated-sludge reactions.

Anoxic Conditions

Anoxic conditions have little or no dissolved molecular oxygen but may contain oxidized nitrogen forms such as nitrate.

Under suitable conditions, microorganisms can use nitrate instead of dissolved oxygen as an electron acceptor.

This is important in denitrification.

Anaerobic Conditions

Anaerobic conditions lack both dissolved oxygen and significant oxidized electron acceptors such as nitrate.

Anaerobic processes can produce compounds such as:

methane;
carbon dioxide;
hydrogen sulfide under suitable sulfur conditions.

Aerobic, Anoxic, and Anaerobic Are Different

These terms should not be used interchangeably.

Aerobic means dissolved oxygen is available.
Anoxic means DO is absent or very low but nitrate or similar oxidized forms may be available.
Anaerobic means strongly reduced conditions without dissolved oxygen and generally without nitrate as an electron acceptor.

Nitrification

Nitrification is the biological oxidation of ammonia to nitrate through intermediate nitrogen forms.

It requires:

appropriate nitrifying microorganisms;
oxygen;
sufficient alkalinity;
suitable pH;
suitable temperature;
adequate solids retention time.

Oxygen Demand of Nitrification

Nitrification requires substantial oxygen.

A commonly used operator approximation is:

About 4.6 lb of oxygen are required per lb of ammonia-nitrogen oxidized.

This helps explain why ammonia loading can significantly increase aeration demand.

Nitrification Example

If the process oxidizes 100 lb/day of ammonia-nitrogen, the approximate oxygen requirement associated with nitrification is:

100 lb/day × 4.6 = 460 lb O2/day

This is a simplified process-control estimate and does not include all other oxygen demands in the treatment system.

Nitrification Also Consumes Alkalinity

In addition to oxygen, nitrification consumes alkalinity.

A commonly used operator relationship is:

Approximately 7.14 mg/L of alkalinity as CaCO3 is consumed for each 1 mg/L of ammonia-nitrogen nitrified.

Low DO and low alkalinity can therefore both contribute to poor nitrification.

Denitrification

Denitrification is the biological reduction of nitrate to nitrogen gas under anoxic conditions.

Denitrifying microorganisms typically need:

nitrate;
low dissolved oxygen;
a usable carbon source;
suitable environmental conditions.

Why Low DO Is Important for Denitrification

If significant dissolved oxygen is present, microorganisms generally use oxygen before nitrate.

Too much oxygen entering an anoxic zone can therefore reduce denitrification efficiency.

Denitrification and Alkalinity

Denitrification can recover some alkalinity that was consumed during nitrification.

This can help support process pH and buffering.

Nitrogen Gas Formation

Successful denitrification converts nitrate-nitrogen toward nitrogen gas.

Nitrogen gas can leave the liquid and return to the atmosphere.

Denitrification in Secondary Clarifiers

Unwanted denitrification can occur in settled sludge if nitrate is present and the sludge becomes anoxic.

Nitrogen gas bubbles can attach to sludge and cause it to rise.

This condition may contribute to floating sludge or rising sludge in secondary clarifiers.

Biological Oxygen Demand Changes Through Treatment

As biodegradable organic matter is removed, oxygen demand generally decreases.

Operators may therefore observe changing aeration demand as influent loading and treatment conditions change.

DO and Process Loading

If influent loading increases while airflow remains unchanged, DO may decrease because microorganisms consume oxygen faster.

This does not necessarily mean the blower has failed.

Low DO

Low DO can result from:

high organic loading;
high ammonia loading;
insufficient airflow;
diffuser fouling;
blower problems;
poor mixing;
incorrect DO measurement.

Effects of Low DO

Depending on the process, low DO can contribute to:

poor organic removal;
poor nitrification;
odor formation;
filamentous growth under some conditions;
reduced process stability.

High DO

High DO may indicate that oxygen supply is greater than current process demand.

Possible causes include:

excessive airflow;
low process loading;
incorrect DO setpoint;
bad DO sensor.

High DO Can Waste Energy

Aeration is often one of the largest energy loads at a wastewater treatment plant.

Supplying much more air than the process requires can increase energy use without improving treatment.

More DO Is Not Always Better

DO should be maintained within the range needed for the specific process.

Excessive DO can:

waste energy;
carry oxygen into intended anoxic zones;
affect biological nutrient-removal performance.

Temperature and DO Solubility

As water temperature increases, the amount of oxygen that can remain dissolved generally decreases.

Warm water therefore has a lower oxygen saturation concentration than cold water.

Temperature and Biological Activity

Temperature also affects biological reaction rates.

Warmer conditions can increase biological activity within an appropriate range, increasing oxygen demand.

Very high or very low temperatures can reduce process performance.

Why Summer Can Be Challenging

During warm weather:

oxygen solubility decreases;
biological oxygen demand may increase;
aeration systems may need to work harder.

DO Saturation

Saturation is the approximate maximum equilibrium concentration of dissolved oxygen under specific temperature, pressure, and water-quality conditions.

DO saturation depends strongly on:

temperature;
atmospheric pressure;
salinity.

Elevation and DO

At higher elevations, atmospheric pressure is lower.

This reduces the equilibrium amount of oxygen that can dissolve in water compared with sea-level conditions at the same temperature.

Mixing and DO

Good mixing distributes oxygen throughout the basin.

Poor mixing can create:

high-DO zones;
low-DO zones;
solids deposition;
unrepresentative sensor readings.

DO Sensor Location

A DO sensor should represent the process condition being controlled.

A poorly located sensor may read:

too close to an air source;
in a stagnant zone;
in a location unrepresentative of the basin.

DO Instrument Maintenance

DO sensors may require:

cleaning;
calibration or verification;
membrane or cap replacement depending on technology;
inspection for fouling.

Fouled DO Sensor

A fouled sensor can respond slowly or incorrectly.

If DO changes suddenly without matching process behavior, verify the measurement before making large airflow adjustments.

Online DO Versus Portable DO

Operators can compare an online DO reading with a calibrated portable instrument when troubleshooting questionable measurements.

Differences may indicate:

sensor fouling;
calibration problems;
poor sensor location;
real DO variation within the basin.

ORP

Oxidation-Reduction Potential, or ORP, is an electrical measurement related to the oxidation-reduction condition of a liquid.

ORP is commonly reported in millivolts.

ORP as a Process Indicator

ORP can help indicate whether conditions are more:

oxidizing;
reducing.

It may be used with other measurements to evaluate:

aerobic zones;
anoxic zones;
anaerobic zones;
biological nutrient-removal processes.

ORP Is Not a Direct DO Measurement

DO and ORP are related to process chemistry but are not interchangeable.

ORP responds to the combined oxidation-reduction chemistry of the liquid, not only dissolved oxygen.

Use ORP Trends Carefully

Absolute ORP values can vary with:

wastewater chemistry;
sensor condition;
process location;
reference electrode condition.

Process trends may be more useful than applying one universal ORP target to every plant.

Electron Acceptors

Microorganisms use different electron acceptors depending on process conditions.

A simplified sequence may include:

oxygen under aerobic conditions;
nitrate under anoxic conditions;
other compounds under more strongly reducing conditions.

Reduced Conditions

Strongly reduced conditions can encourage formation of compounds associated with:

odor;
corrosion;
septic wastewater.

Hydrogen Sulfide

Hydrogen sulfide can form under reducing conditions when sulfate is biologically reduced.

It can create:

rotten-egg odor;
worker-safety hazards;
severe corrosion.

Septic Conditions

Wastewater may become septic when oxygen and other favorable electron acceptors are depleted during long detention.

Possible indicators include:

strong odors;
dark wastewater;
hydrogen sulfide;
low ORP.

DO in Collection Systems

Long detention times in force mains or collection systems can contribute to oxygen depletion.

This can increase the risk of:

septicity;
odor;
hydrogen sulfide formation.

DO and Activated Sludge

In activated sludge systems, DO influences:

organic removal;
nitrification;
floc characteristics;
energy use;
nutrient-removal conditions.

DO Should Be Interpreted with Other Data

Important related data include:

ammonia;
nitrate;
BOD or COD;
airflow;
blower pressure;
temperature;
pH;
alkalinity;
mixed-liquor condition.

Example: Low DO with High Ammonia

If DO is low and effluent ammonia is increasing, inadequate oxygen may be contributing to poor nitrification.

However, operators should also check:

alkalinity;
pH;
temperature;
solids retention time;
nitrifier population.

Example: High DO with High Ammonia

If DO is high but ammonia removal is poor, lack of oxygen is less likely to be the main cause.

Other possible causes include:

low temperature;
low pH;
insufficient alkalinity;
low solids retention time;
toxic inhibition.

Example: High Airflow but Low DO

This pattern can indicate:

very high oxygen demand;
poor oxygen-transfer efficiency;
diffuser fouling;
poor mixing;
bad DO measurement.

Example: Low Airflow but Adequate DO

This may simply indicate low process demand.

Operators should avoid increasing airflow automatically if biological performance and DO are already satisfactory.

Biological Reaction Rates

Biological treatment depends on living microorganisms.

Reaction rates can be affected by:

temperature;
pH;
DO;
food availability;
toxicity;
solids retention time.

Toxicity

Toxic compounds can inhibit biological activity even when DO is adequate.

A sudden process upset with normal aeration should prompt consideration of:

industrial discharge;
chemical spill;
extreme pH;
toxic contaminants.

Oxygen Uptake

Active biomass consumes oxygen.

Changes in oxygen uptake can provide information about:

biological activity;
organic loading;
toxicity;
endogenous conditions.

Respiration

Respiration is the biological process in which microorganisms use available electron acceptors to obtain energy.

Under aerobic conditions, oxygen is the primary electron acceptor.

Endogenous Respiration

When readily available food becomes limited, microorganisms begin consuming stored material and cell mass for maintenance.

This condition is called endogenous respiration.

DO Control Systems

Automatic DO control may use:

DO sensor;
PLC or controller;
airflow valve;
blower VFD;
airflow meter.

DO Control Loop Example

DO falls below setpoint.
Controller increases airflow command.
Blower speed or valve position increases.
More oxygen is transferred.
DO rises toward the setpoint.

Control Delay

DO does not always respond instantly to airflow changes.

Operators should consider:

basin volume;
mixing time;
biological demand;
sensor response time.

Overcorrecting DO

Repeated large airflow changes before the process responds can cause:

control instability;
high energy use;
DO oscillation.

Common DO Mistakes

Confusing DO with BOD.
Assuming low DO always means blower failure.
Assuming more air is always better.
Ignoring high biological loading when DO falls.
Ignoring temperature effects on oxygen solubility and demand.
Adjusting airflow based on one questionable DO sensor.
Ignoring alkalinity and pH when nitrification fails.
Allowing excessive DO to enter intended anoxic zones.
Confusing anoxic and anaerobic conditions.
Using ORP as if it were a direct DO measurement.

A Practical Low-DO Troubleshooting Sequence

Verify the DO measurement.
Check sensor cleanliness and calibration status.
Review airflow.
Review blower discharge pressure.
Check diffuser condition and air distribution.
Review organic and ammonia loading.
Review temperature.
Check mixing.
Review pH and alkalinity if nitrification is affected.
Make controlled airflow adjustments according to approved procedures.
Allow adequate response time.

A Practical Nitrification Review

Review ammonia trend.
Review DO.
Review pH.
Review alkalinity.
Review temperature.
Review solids retention time.
Review recent toxic or unusual influent conditions.
Estimate oxygen and alkalinity demand where useful.
Verify laboratory results if the trend is unexpected.

What to Remember for the Exam

Dissolved oxygen is molecular oxygen dissolved in water and is commonly reported in mg/L.
DO reflects the balance between oxygen supply, transfer, and process oxygen demand.
BOD describes biological oxygen demand and is not the same as DO.
COD measures chemically oxidizable material and is different from BOD.
Aerobic conditions contain dissolved oxygen.
Anoxic conditions have little or no DO but may contain nitrate for biological use.
Anaerobic conditions lack dissolved oxygen and are more strongly reducing.
Nitrification requires oxygen, alkalinity, suitable pH, temperature, and adequate biological solids retention.
Approximately 4.6 lb of oxygen are required per lb of ammonia-nitrogen nitrified.
Nitrification also consumes approximately 7.14 mg/L of alkalinity as CaCO3 per 1 mg/L of ammonia-nitrogen oxidized.
Denitrification occurs under anoxic conditions and converts nitrate toward nitrogen gas.
Too much DO in an anoxic zone can reduce denitrification.
Water holds less dissolved oxygen as temperature increases.
Warm weather can increase biological oxygen demand while reducing oxygen solubility.
Low DO can result from high loading, poor aeration, diffuser fouling, mixing problems, or bad instrumentation.
High DO can indicate excess aeration and unnecessary energy use.
ORP indicates overall oxidation-reduction conditions and is not a direct DO measurement.
Strongly reduced wastewater conditions can contribute to hydrogen sulfide, odor, and corrosion.
DO should be interpreted together with ammonia, nitrate, airflow, temperature, pH, alkalinity, and biological process data.
Questionable DO measurements should be verified before major aeration adjustments.