How to remove refractory organic compounds after biological wastewater treatment

Biological wastewater treatment is widely used across UK industry to reduce organic pollution before discharge. Activated sludge systems, membrane bioreactors (MBRs), sequencing batch reactors and biological filtration processes can achieve substantial reductions in biodegradable organic matter.

However, many industrial effluent treatment plants continue to experience residual Chemical Oxygen Demand (COD), persistent organic pollutants and poorly biodegradable compounds after biological treatment.

This can create significant challenges for industrial operators attempting to meet discharge requirements, improve effluent quality or develop water reuse schemes.

When conventional biological treatment has reached its practical limits, tertiary ozonation can provide an additional oxidation stage capable of transforming certain refractory organic compounds.

The key question is not simply whether ozone can oxidise these contaminants, but whether the proposed process can deliver measurable treatment improvements at an acceptable operating cost.

Why do refractory organic compounds remain after biological treatment?

Biological treatment relies on microorganisms to degrade organic substances under controlled operating conditions.

In a conventional activated sludge plant, readily biodegradable compounds are consumed and converted into biomass, carbon dioxide and other metabolic products.

However, not all industrial pollutants are equally biodegradable.

Certain substances contain molecular structures that resist microbial degradation or require treatment conditions that are not available within a conventional effluent treatment plant.

These substances are commonly described as refractory or recalcitrant organic compounds.

Their persistence may be associated with:

• Complex aromatic molecular structures.
• Limited susceptibility to microbial enzymes.
• Low bioavailability.
• Toxic or inhibitory effects on biological treatment organisms.
• Insufficient biological retention time.
• Formation of persistent metabolites.
• Variations in industrial wastewater composition.

It is important to establish whether the residual organic matter is genuinely refractory or whether its persistence results from an underperforming biological process.

An assessment of the existing plant should therefore precede any decision to introduce tertiary oxidation.

Understanding residual COD after biological treatment

Chemical Oxygen Demand (COD) is commonly used by industrial wastewater operators to assess the overall quantity of chemically oxidisable substances present in an effluent.

A biological treatment plant may achieve a substantial reduction in Biological Oxygen Demand (BOD5) while continuing to discharge water containing measurable COD.

For example, an industrial ETP may achieve good biological treatment performance but still struggle to meet a site-specific final effluent COD limit.

Residual COD can originate from several different sources, including dissolved refractory organic compounds, residual biodegradable material, particulate substances and inorganic reducing agents.

A high residual COD result does not automatically demonstrate that ozonation is the appropriate solution.

The chemical composition of the remaining pollution must first be understood.

For a broader discussion of COD treatment, see our article on COD reduction in wastewater using ozonation.

Which industrial pollutants can resist biological treatment?

The composition of refractory wastewater depends on the industrial activity involved.

Chemical and pharmaceutical mManufacturing

Chemical and pharmaceutical effluents may contain complex organic molecules, chemical intermediates and residual active substances that are insufficiently removed by conventional biological treatment.

Certain pharmaceuticals and aromatic compounds react readily with ozone, making ozonation a potential tertiary treatment option.

However, oxidation performance varies considerably between individual substances.

Textile and dye manufacturing

Textile wastewater can contain dyes and auxiliary chemicals with relatively low biodegradability.

Some colour-producing molecular structures are particularly susceptible to ozone attack.

Consequently, substantial decolourisation may occur even when the overall COD reduction remains modest.

Ozonation can therefore be useful when colour removal is the primary treatment objective.

Petrochemical and chemical processing

Petrochemical wastewater may contain phenolic substances, aromatic compounds and other complex organic contaminants.

Some of these compounds can be transformed by ozone, although the treatment performance depends on molecular structure, wastewater chemistry and competing ozone demand.

Learn more about ozonation for petrochemical wastewater treatment.

Landfill leachate and waste management

Biologically treated landfill leachate may contain persistent dissolved organic compounds that remain after conventional treatment.

Ozonation can be considered as part of an advanced treatment process, potentially combined with biological or adsorption-based polishing.

Read our article about ozone treatment for landfill leachate.

How does ozone treat refractory organic compounds?

Ozone (O₃) is a powerful oxidising agent used in advanced water and wastewater treatment.

Its reactions in water occur through two principal mechanisms.

Direct oxidation by molecular ozone

Molecular ozone reacts selectively with certain chemical structures, particularly some electron-rich aromatic compounds, unsaturated bonds and functional groups.

These reactions can alter the original molecules, producing compounds with different chemical properties.

In some applications, this transformation improves the subsequent biodegradability of the wastewater.

Indirect oxidation through hydroxyl radicals

Ozone decomposition in water can generate hydroxyl radicals (•OH).

These highly reactive species can oxidise a broader range of organic compounds.

The extent of radical oxidation depends on pH, dissolved organic matter, alkalinity and other wastewater characteristics.

Advanced oxidation processes (AOPs), including ozone combined with hydrogen peroxide, can be investigated where molecular ozone alone provides insufficient pollutant removal.

However, more intensive oxidation is not always economically advantageous.

The optimum treatment route must be determined from the characteristics of the wastewater and the required final effluent quality.

Does ozonation completely remove refractory COD?

Not necessarily.

This is an important distinction when evaluating an ozone treatment system.

Ozonation may significantly reduce the concentration of a specific organic pollutant without achieving an equivalent reduction in overall COD or Total Organic Carbon (TOC).

An organic compound may be transformed into smaller molecules that remain present in the treated water.

The following treatment outcomes should therefore be assessed separately:

• Removal of individual target pollutants.
• Overall COD reduction.
• TOC reduction and mineralisation.
• Improvement in biodegradability.
• Reduction in colour or UV absorbance.
• Changes in effluent toxicity.
• Formation of oxidation transformation products.

For some applications, partial oxidation followed by biological treatment may be more effective than attempting complete mineralisation by ozone alone.

This distinction is particularly relevant for industrial operators seeking to reduce treatment costs.

Where should ozone be installed in an industrial effluent treatment plant?

Several process configurations are possible.

Option 1 — Biological treatment followed by ozonation

Process sequence:

Biological ETP → Ozone Contactor → Final Effluent

This arrangement may be suitable where ozone provides sufficient treatment of identified contaminants and the final effluent meets the required quality criteria.

The design must account for potential transformation products, residual oxidants and any necessary additional treatment.

Option 2 — Biological treatment, ozonation and biofiltration

Process sequence:

Biological ETP → Ozone Contactor → Biological Filtration → Final Effluent

This arrangement is particularly interesting when ozonation produces more biodegradable compounds.

The biological filtration stage can remove a proportion of the biodegradable organic matter formed during oxidation.

Depending on the wastewater, this may improve overall treatment efficiency.

Option 3 — Biological treatment, ozonation and activated carbon

Process sequence:

Biological ETP → Ozone Contactor → Activated Carbon → Final Effluent

Activated carbon adsorption can complement ozonation by removing certain residual contaminants and oxidation products.

Biological activity within granular activated carbon filters may also contribute to the removal of biodegradable compounds.

The suitability of this configuration depends on the target contaminants, competing dissolved organic matter and activated carbon operating conditions.

Option 4 — Advanced oxidation followed by polishing

For particularly challenging wastewater, ozone may be combined with another oxidation technology, such as hydrogen peroxide.

The resulting process can then be followed by biological filtration or another polishing stage.

The choice between conventional ozonation and an AOP should be supported by comparative pilot trials.

How to assess an existing UK industrial ETP

Before specifying an ozone generator, a technical assessment should establish the principal causes of the residual pollution.

Useful information includes:

• Average and peak wastewater flow rates, in m³/h.
• Influent and final effluent COD.
• BOD5 and BOD5/COD ratio.
• TOC and dissolved organic carbon (DOC).
• pH and operating temperature.
• Suspended solids and turbidity.
• Nitrite and bromide concentrations.
• Identified organic micropollutants.
• Existing biological treatment configuration.
• Required final effluent discharge standards.
• Available space, utilities and installation constraints.

Where appropriate, UV254 absorbance, toxicity tests and targeted chemical analyses may provide additional information.

These measurements can help distinguish readily oxidisable contaminants from compounds that require alternative treatment methods.

How is an industrial ozone treatment system sized?

Ozone generator sizing depends on wastewater flow, target pollutant removal, ozone demand and gas-to-liquid transfer efficiency.

The theoretical mass of ozone applied can be calculated as follows:

Applied ozone (g/h) = Wastewater flow (m³/h) × Applied ozone dose (mg/L)

Example: industrial ETP treating 25 m³/h

Consider an existing industrial effluent treatment plant with the following characteristics:

Parameter	Illustrative value
Final biological effluent flow	25 m³/h
Residual COD	110 mg/L
Trial ozone dose	8 mg/L
Applied ozone mass	200 g/h

The calculation is:

25 × 8 = 200 g O₃/h

This example illustrates the mass balance only.

The 8 mg/L value is an assumed trial dose and must not be interpreted as a recommended operating dose for this wastewater.

The required generator capacity will depend on actual ozone transfer efficiency, competing oxidant demand, operating conditions and treatment objectives.

An industrial design must also consider the oxygen supply, gas injection system, contactor, cooling, instrumentation, off-gas destruction and safety provisions.

Two wastewater streams with identical COD values may require substantially different ozone doses.

COD alone is not a reliable basis for specifying an industrial ozone generator.

Can ozone improve wastewater biodegradability?

In certain applications, yes.

Partial oxidation may convert complex organic compounds into smaller or more oxygenated molecules that are more accessible to microorganisms.

This can support a downstream biological polishing stage.

However, an improvement in the BOD5/COD ratio does not necessarily demonstrate that all potentially harmful substances have been removed.

The actual performance should be confirmed through laboratory testing and, where appropriate, pilot-scale biological treatment.

It is also important to consider that some oxidation products may remain persistent or exhibit undesirable toxicological properties.

UK Environmental permits and trade effluent consent

Industrial wastewater treatment projects in the UK must be designed around the site’s actual discharge requirements.

In England, direct discharges of wastewater to surface water or groundwater may require an environmental permit from the Environment Agency.

Discharges of trade effluent into a public sewer generally require consent or an agreement with the relevant sewerage undertaker.

Scotland, Wales and Northern Ireland have their own regulatory authorities and applicable requirements.

Operators should review their existing discharge conditions before selecting an advanced treatment technology.

Relevant parameters may include COD, BOD, suspended solids, specific chemical pollutants and other site-specific limits.

Installing an ozone treatment system does not itself establish compliance with an environmental permit or trade effluent consent. Compliance must be demonstrated against the applicable conditions.

Official UK guidance:

Environment Agency — Discharges to Surface Water and Groundwater

Why are pilot trials important?

For industrial wastewater containing refractory organic compounds, pilot testing is often the most reliable way to establish the feasibility of ozone treatment.

A structured trial can assess:

1. Ozone demand and reaction behaviour.
2. Pollutant removal at different applied doses.
3. Changes in COD, TOC and UV254.
4. Formation of relevant transformation products.
5. Changes in biodegradability.
6. Ozone transfer efficiency.
7. Potential requirements for downstream treatment.
8. Estimated energy consumption and operating costs.

Pilot trials can also help determine whether an existing treatment plant should be upgraded, modified or combined with an alternative technology.

The objective is to avoid oversizing the ozone generator and to select a treatment process supported by measurable results.

OXYTRADING UK — engineering-led ozone treatment solutions

OXYTRADING UK supports industrial clients and engineering consultancies seeking to improve wastewater treatment performance through appropriate ozone technology.

Our approach is based on process assessment and system integration rather than the supply of stand-alone ozone generators without application-specific validation.

Depending on the project, our services can include:

• Preliminary technical assessment.
• Wastewater characterisation and treatment feasibility.
• Laboratory and pilot-scale evaluations.
• Ozone generator selection and sizing.
• Oxygen generation and ozone injection systems.
• Contact tank and gas–liquid transfer design.
• Instrumentation, control and off-gas destruction.
• Installation and commissioning support.
• Operator training and maintenance arrangements.

We support industrial wastewater projects across the UK, including applications involving chemicals, pharmaceuticals, textile manufacturing, petrochemicals, food processing and advanced effluent treatment.

Further information about our capabilities is available on our industrial ozone engineering website.

Request a technical assessment of your industrial effluent

Is your biological wastewater treatment plant operating correctly but still leaving excessive residual COD or persistent organic contaminants?

OXYTRADING UK can help assess whether tertiary ozonation offers a technically appropriate solution.

For an initial review, the most useful information includes your wastewater flow rate, recent analytical results, identified target contaminants and required discharge standards.

Our objective is to establish whether ozone can deliver a measurable improvement before recommending full-scale equipment.

Contact OXYTRADING UK for a preliminary wastewater treatment assessment →