Heat exchangers for wastewater treatment

Heat exchangers in wastewater treatment plants (WWTPs) and recycling facilities are key equipment for optimizing biological processes, stabilizing sludge, recovering thermal energy, and reducing the overall plant energy consumption. Proper thermal engineering improves reactor performance, reduces operating costs (kWh/m³ treated), and contributes to energy-efficient treatment models.

Why is heat transfer strategic in a WWTP?

Although treatment is mainly a physical and biological process, temperature directly affects:

  • Microbiological activity

  • Rate of organic matter degradation

  • Efficiency of anaerobic digestion

  • Biogas production

  • Sludge viscosity

  • Stabilization and dewatering

In modern plants, energy recovery is a key driver of sustainability and OPEX reduction.

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Architecture of a wastewater treatment plant

A typical WWTP includes two main lines:

Water line

  • Pretreatment (screening, grit removal, grease removal)

  • Primary clarification

  • Biological reactor (activated sludge, MBR, biodiscs, etc.)

  • Secondary clarification

  • Tertiary treatment (if applicable)

  • Reuse or discharge

Sludge line

  • Thickening

  • Anaerobic or aerobic digestion

  • Dewatering

  • Valorization or final disposal

Heat transfer is especially critical in the sludge line and in energy recovery systems.

Classification of heat exchangers in water treatment and purification

Stainless steel BOIXAC pillow plate featuring enhanced thermal channels for maximum heat transfer efficiency, superior cleanability and reliable operation in harsh industrial environments.

Pillow plate heat exchanger (dimple plate) 

Especially suitable for:

  • Sludge with high solids content

  • Media with high sedimentation

  • Viscous or abrasive fluids

Technical advantages:

  • High mechanical strength

  • Ease of inspection

  • Fouling tolerance

  • Robust design for aggressive environments

Typical applications:

  • Heating of anaerobic digesters

  • Thermal stabilization of sludge

The BOIXAC finned tube heat exchanger delivers high thermal performance for air treatment, industrial gases and thermal fluids. Robust construction, advanced conductivity and fully customizable configurations for water, steam, thermal oil and diathermic fluids. Engineered for HVAC, refrigeration and industrial heat-recovery applications.

Flat and finned tube heat exchanger

Applications:

  • Heat recovery in gas streams

  • Air treatment in deodorization processes

  • Cooling or heating of fluids with organic load

Characteristics:

  • High overall heat transfer coefficient

  • Possibility of immersion in liquids

  • Adaptability to highly contaminated environments

Industrial heat exchanger for wastewater treatment featuring removable tubes, enabling easy cleaning and reliable long-term operation in harsh conditions.

Double pipe heat exchanger (concentric)

Suitable for:

  • Heat recovery in liquid streams with suspended solids

  • Industrial wastewater

  • Systems with high thermal gradients

Advantages:

  • Modular design

  • Resistance to sedimentation

  • Ease of maintenance

Image of a custom-designed industrial economizer conceived by BOIXAC, available in stainless steel or carbon steel. Engineered for heat recovery from clean or dirty flue gases generated by boilers operating on natural gas, biomass, diesel, fuel oil and other fuels. The system is suitable for superheated water, steam, thermal oil and a wide range of process fluids, maximizing energy efficiency, reducing fuel consumption and lowering emissions in industrial applications.

Heat recovery unit and economizer

In plants with:

  • Anaerobic digesters

  • Biogas production

  • Auxiliary boilers

Objective:

  • Recover heat from exhaust gases

  • Preheat process water

  • Improve overall efficiency of the energy system

Thermally critical applications in treatment

Anaerobic digestion

Transforms organic matter into biogas

Typical ranges:

  • Mesophilic: 35–38 °C

  • Thermophilic: 50–55 °C

Stable temperature is critical for:

  • Methane production

  • Pathogen reduction

  • Process stability

Sludge heating

Efficient sludge conditioning

Before digestion or dewatering:

  • Reduction of viscosity

  • Improved pumping

  • Optimization of centrifugation

Recovery in effluents

Heat recovery from sludge and water

Recovery allows:

  • Preheating incoming streams

  • Reducing external energy demand

  • Improving the plant’s energy balance

Relevant physicochemical parameters

Wastewater typically contains:

  • Suspended solids

  • Organic matter (COD, BOD)

  • Nitrogen (NH₄⁺, NO₂⁻, NO₃⁻)

  • Phosphorus

  • Fats

  • Abrasive compounds

Typical pH range: 6.5–8 (depending on origin)

Heat exchangers must consider:

  • Corrosion resistance

  • Fouling

  • Abrasion

  • Ease of cleaning

Economic and energy impact

A well-designed thermal strategy can:

  • Increase biogas production

  • Reduce external energy consumption

  • Improve biological stability

  • Reduce indirect emissions

  • Optimize sludge dewatering

In plants with anaerobic digestion, thermal recovery can make the facility partially energy self-sufficient.

Critical engineering variables

  • Overall heat transfer coefficient (U)

  • LMTD

  • Solids content (%)

  • Viscosity

  • Flow regime

  • Fouling resistance

  • Ease of inspection

The design must account for highly contaminated environments and demanding maintenance conditions.

Sustainability and circular economy

Modern treatment integrates:

  • Energy recovery

  • Sludge valorization

  • Biogas production

  • Reuse of reclaimed water

Heat transfer is a structural element for advancing toward circular economy models in water management.

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FAQs

Why is it necessary to control temperature in anaerobic digesters?

To maintain stable microbiological activity and maximize biogas production.

Methanogenic microorganisms operate within specific thermal ranges; deviations affect degradation kinetics and methane production.

Where is heat recovery mainly applied?

In exhaust gases, hot sludge, and effluents.

Residual heat from thermal streams can be reused to preheat processes, reducing external energy consumption.

What is the main challenge for heat exchangers in this sector?

The presence of solids and fouling.

Equipment must tolerate fluids with organic, abrasive, or sedimentable loads while maintaining performance and accessibility for cleaning.

Value proposition for technical management and operations

In modern treatment, the objective is not only to meet discharge parameters but to optimize energy and sustainability.

An efficient thermal architecture enables:

  • Higher biogas production

  • Reduced energy costs

  • Improved biological performance

  • Adaptation to environmental objectives

  • Greater operational robustness

We optimize the thermodynamics of WWTPs