Dissolved Oxygen and Biological Activity in Wastewater Treatment

Operators of animal, industrial, and municipal wastewater systems have sometimes reported increases in dissolved oxygen (DO) after applying BioLynceus® products, along with reductions in odors and accumulated sludge.

At first, this may seem unexpected. Increased microbial activity generally creates greater oxygen demand, so operators may wonder how biological activity can increase while dissolved oxygen concentrations also improve.

Several biological and environmental processes may contribute to this response.

How Dissolved Oxygen Can Increase in Wastewater Lagoons

One possible contributor is photosynthetic activity within the wastewater system.

Algae and certain photosynthetic microorganisms use sunlight to convert carbon dioxide and water into organic compounds while releasing oxygen.

When photosynthetic oxygen production exceeds biological oxygen consumption during daylight hours, dissolved oxygen concentrations in the upper portion of a lagoon may increase.

Other factors may also influence DO levels, including:

• Water temperature

• Sunlight intensity

• Algae and photosynthetic microbial populations

• Organic loading

• Mixing and circulation

• Surface aeration

• Microbial community structure

• Wastewater detention time

For this reason, an increase in dissolved oxygen should be interpreted as the result of the overall biological and physical conditions within the treatment system rather than a single mechanism.

Photosynthesis and Oxygen Production

The biological production of oxygen occurs through oxygenic photosynthesis.

During this process, photosynthetic organisms use solar energy to split water molecules, releasing molecular oxygen.

Research into the biochemical mechanisms of photosynthesis has helped explain how plants, algae, and certain bacteria are able to produce oxygen from water using light energy.

In wastewater lagoons, photosynthetic organisms can therefore contribute oxygen to surface waters during periods of sufficient sunlight.

Dissolved Oxygen and Organic Matter Degradation

Higher dissolved oxygen concentrations can support aerobic biodegradation, allowing aerobic microorganisms to metabolize biodegradable organic material more efficiently.

As microorganisms break down organic matter, carbon-containing compounds may ultimately be converted into:

• Carbon dioxide

• Water

• New microbial biomass

• More stable organic compounds

Improved aerobic conditions can also reduce the amount of lagoon volume experiencing strongly anaerobic conditions.

This shift may help reduce the production of compounds associated with wastewater odors.

Oxygen Zones in Wastewater Lagoons

Many wastewater lagoons contain overlapping biological zones based on oxygen availability.

These zones generally include:

1. Aerobic zone

2. Facultative zone

3. Anaerobic zone

The size and depth of each zone can change throughout the day and with variations in temperature, loading, mixing, and sunlight.

Aerobic Zone

The upper portion of a lagoon usually contains the greatest concentration of dissolved oxygen.

Aerobic microorganisms require available oxygen to metabolize organic material.

Oxygen in this zone may come from:

• Atmospheric transfer

• Mechanical aeration

• Wind-driven mixing

• Photosynthesis

In heavily loaded lagoons, oxygen can be consumed rapidly, causing the aerobic layer to become relatively shallow.

Facultative Zone

Below the aerobic surface layer is often a facultative zone.

Facultative microorganisms can adjust their metabolism based on available oxygen and other electron acceptors.

This flexibility allows biological treatment to continue as oxygen concentrations fluctuate.

Anaerobic Zone

Near the lagoon bottom, accumulated sludge and high organic loading can create oxygen-depleted conditions.

Under these conditions, anaerobic microorganisms break down organic matter through pathways that do not rely on dissolved oxygen.

Anaerobic treatment is an important natural process, but some anaerobic reactions can produce compounds associated with strong wastewater odors.

Hydrogen Sulfide and Wastewater Odors

One of the most recognizable wastewater odor compounds is hydrogen sulfide (H₂S).

Hydrogen sulfide can be produced by sulfate-reducing bacteria under anaerobic conditions when sulfate and biodegradable organic material are available.

Other odor-producing compounds may include:

• Mercaptans and other reduced sulfur compounds

• Volatile fatty acids

• Ammonia

• Other volatile organic compounds

These compounds can create odor complaints when they escape from wastewater and enter the surrounding atmosphere.

How Aerobic Conditions Can Help Reduce Odors

Improving oxygen availability can reduce conditions that favor the formation of certain anaerobic odor compounds.

As gases produced in deeper portions of a lagoon move upward, some reduced compounds may also be biologically or chemically oxidized within oxygen-rich zones before reaching the atmosphere.

Maintaining a healthier aerobic layer may therefore help support:

• Reduced hydrogen sulfide formation

• Lower concentrations of reduced sulfur compounds

• Improved organic matter degradation

• More stable biological treatment

• Reduced wastewater odors

Supporting Aerobic Biological Treatment

BioLynceus® products are designed to support microbial communities involved in biological wastewater treatment and organic solids degradation.

When treatment conditions favor aerobic biological activity, microorganisms can more efficiently process biodegradable organic material while reducing conditions associated with excessive anaerobic odor production.

The effectiveness of any biological treatment program depends on factors such as:

• Organic loading

• Dissolved oxygen

• Temperature

• pH

• Nutrient availability

• Microbial populations

• Lagoon design

• Treatment time

Monitoring these conditions can help wastewater operators understand changes in dissolved oxygen, odors, sludge, and overall treatment performance.