ORP vs dissolved ozone

A common misunderstanding in ozone systems

One of the most common assumptions in ozone water treatment is:

“My ORP is high, therefore I have enough ozone.”

While this may seem logical, it is often inaccurate.

Many industrial facilities, food processors, aquaculture farms and wastewater treatment plants still rely solely on ORP (Oxidation Reduction Potential) measurements to control ozone dosing. However, ORP does not directly measure ozone concentration.

As a result, operators may experience:

• Inconsistent treatment performance
• Excessive ozone consumption
• Higher operating costs
• Unstable process control
• Difficulty validating treatment effectiveness

At OXYTRADING UK, we frequently encounter installations where process monitoring and control have a greater impact on performance than the ozone generator itself.

The key question is not how much ozone is produced, but how much ozone is actually dissolved and available to perform the treatment.

What Is ORP?

ORP (Oxidation Reduction Potential) is measured in millivolts (mV) and indicates the overall oxidising capacity of water.

In simple terms, ORP measures how oxidising or reducing a water environment is at a given moment.

ORP values are influenced by many factors, including:

• Ozone
• Chlorine
• Hydrogen peroxide
• Dissolved oxygen
• Metal ions
• pH
• Temperature
• Organic matter
• Various chemical reactions occurring in the water

A high ORP value indicates that the water has strong oxidising characteristics, but it does not tell you:

• How much ozone is dissolved in the water
• How much ozone remains available for oxidation
• Whether an effective ozone residual is present

This distinction is critical when optimising an ozone treatment process.

What is dissolved ozone?

Dissolved ozone refers to the actual concentration of ozone present in water.

It is typically measured in:

• mg/L
• ppm
• µg/L

Unlike ORP, dissolved ozone provides a direct measurement of the ozone available for treatment.

This parameter directly reflects:

• Oxidation potential
• Disinfection capability
• Process effectiveness
• Ozone transfer efficiency

If the objective is to understand whether an ozone system is performing correctly, dissolved ozone measurement is generally the most reliable indicator.

Why high ORP does not necessarily mean ozone is present

This is one of the most misunderstood aspects of ozone treatment.

Water may display:

• 750 mV
• 850 mV
• 900 mV or more

while containing little or no measurable dissolved ozone.

The reason is simple: ORP measures the overall electrochemical state of the water, not a specific chemical compound.

Several factors can increase ORP values without indicating the presence of ozone:

• Residual disinfectants
• Chemical additives
• Changes in pH
• Low organic loading
• Intermediate oxidation products

Conversely, water may contain an effective dissolved ozone concentration while displaying a relatively moderate ORP value.

The limitations of ORP-based control

1. Ozone overdosing

When ozone injection is controlled solely through ORP, systems may continue generating ozone after treatment objectives have already been achieved.

This can lead to:

• Increased electricity consumption
• Higher oxygen demand
• Greater ozone destruction requirements
• Unnecessary operating costs

2. Process instability

ORP can respond slowly to changes in water quality.

As a result, control systems may:

• Overcorrect
• Undercorrect
• Oscillate around the setpoint
• Deliver inconsistent treatment performance

3. Difficulty demonstrating performance

In sectors such as:

• Food and beverage processing
• Aquaculture
• Pharmaceutical manufacturing
• Semiconductor production
• Industrial wastewater treatment

it is increasingly important to verify actual dissolved ozone concentrations rather than relying solely on indirect indicators.

Why ORP still remains valuable

Despite its limitations, ORP remains an extremely useful operational parameter.

It can provide valuable information about:

• Process trends
• Changes in water quality
• Variations in organic loading
• Overall treatment stability

ORP is therefore an excellent monitoring tool, but it should not be considered a direct measurement of ozone concentration.

Why dissolved ozone monitoring is becoming essential

Modern ozone treatment systems are increasingly focused on:

• Reducing operating costs
• Improving energy efficiency
• Achieving repeatable treatment performance
• Generating measurable process data
• Optimising ozone consumption

For this reason, many facilities are now integrating:

• Online dissolved ozone analysers
• Continuous monitoring systems
• Advanced automated control loops
• Real-time process optimisation

The objective is straightforward:

Inject only the amount of ozone that is actually required.

The best strategy: ORP and dissolved ozone together

For most industrial applications, the best approach is not choosing one measurement over the other.

It is using both.

ORP provides:

• Process trends
• Overall oxidation conditions
• Early warning of changes
• Operational visibility

Dissolved ozone provides:

• Actual ozone concentration
• Treatment verification
• Precise process control
• Energy optimisation

Together, these measurements offer a far more complete understanding of system performance.

Industries where dissolved ozone measurement is critical

Food and Beverage processing

• Fruit and vegetable washing
• Process water treatment
• Bottling operations
• Shelf-life optimisation

Aquaculture

• Recirculating Aquaculture Systems (RAS)
• Biosecurity management
• Water quality optimisation
• Fish health protection

Industrial water treatment

• Process water treatment
• Cooling towers
• Water reuse systems
• Advanced oxidation processes

Wastewater treatment

• Micropollutant removal
• Odour reduction
• Colour removal
• Tertiary and quaternary treatment

The reality of ozone performance

Two facilities equipped with identical ozone generators can achieve completely different results.

Why?

Because ozone treatment performance depends on much more than generator capacity.

Key factors include:

• Gas-to-liquid transfer efficiency
• Hydraulic design
• Contact time
• Water quality
• Control strategy
• Actual dissolved ozone concentration

This is why process engineering and instrumentation are just as important as ozone production itself.

How can we help you?

ORP is a valuable operational indicator, but it should not be confused with dissolved ozone measurement.

When accurate process control, performance validation and optimisation are required, dissolved ozone monitoring becomes essential.

The most effective ozone treatment systems combine both measurements to maximise efficiency, reduce operating costs and improve treatment consistency.

At OXYTRADING UK, we support industrial clients, utilities, food processors and aquaculture operators with:

• Ozone system design
• Process audits
• Pilot testing
• Dissolved ozone monitoring
• Instrumentation and automation
• Safety and compliance
• Process optimisation

Because successful ozonation is not defined by how much ozone you produce. It is defined by how much ozone is effectively available where treatment is needed.