Continuous thermal hydrolysis: Polanco Process technical definition and operating principle

Diagram of the operating principle of continuous thermal hydrolysis in the Polanco Process: reactor, flash mechanism and anaerobic digestion

Continuous thermal hydrolysis is a process that subjects sludge to high pressure and temperature to break bacteria cell walls, solubilise organic matter and accelerate the anaerobic digestion limiting stage. The Polanco Process, developed by Fernando Fernández-Polanco, Professor of Chemical Engineering at the University of Valladolid, implements it by operating in steady-state regime, with a double flash or steam explosion and with in-line steam injection, achieving complete hygienisation, increasing biogas yields by 35%, reducing biosolids volume by 50% and doubling the digesters organic loading rates. Performance has been verified in commercial plants, such as the Copero WWTP, in continuous operation since 2020.

What physically happens during thermal hydrolysis? The "pressure cooker" principle

The science behind thermal hydrolysis is that anaerobic digestion is limited by the hydrolysis or solubilisation of organic matter. To overcome this limitation, accelerating the process kinetics, different pretreatments have been proposed (e.g. biological, physical, chemical). Among them, due to its technical and economic benefits, thermal hydrolysis stands out by combining two solubilisation mechanisms. These two mechanisms convert particulate organic matter into soluble, readily biodegradable compounds.

The first one is a thermal mechanism, equivalent to “cooking” the sludge in a pressure cooker at 140-160ºC and 3-5 barg, for a predetermined retention time of 15-30 minutes.

The second mechanism, flash or steam explosion, occurs when the sludge is suddenly depressurised and, as it partially vaporises, its volume increases fracturing cell membranes and physical structures.

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Technical differentiators of the Polanco Process

The main differences between continuous thermal hydrolysis and other technologies such as batch are evaluated not only per process type, but also according to heating method, hydrolysis mechanisms and operating conditions, non-condensable gases management and overall maintenance strategy.

Polanco Process Other technologies Benefits
Process type Continuous and steady-state
1 hydrolysis reactor 		Batch
2, 3 or 4 reactors
  • Fewer units: lower cost and smaller footprint
  • Controllability
Heating method In-line steam injection Tanks steam injection
or
Heat exchangers 		Lower maintenance cost
Non-condensables evacuation Direct (pressurised non-condensables)

 Via mechanical equipment Less equipment: lower operating and investment cost
Hydrolysis mechanisms Thermal + double flash
< 0.15 kg steam/kg sludge 		Thermal only
or
Thermal + flash
  • More biogas
  • Lower energy consumption
Operating conditions Optimised and configurable Fixed and predetermined Lower recalcitrant compounds formation
Annual maintenance shutdown Partial plant shutdown (hygienisation module in operation) Complete plant shutdown Greater stability during maintenance: partially hydrolysed and hygienised sludge

Verified performance: impact on biogas, biosolids and digesters

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Polanco Process installation for continuous thermal hydrolysis in a WWTP sludge line

Operating conditions: temperature, pressure and retention time

Polanco Process operating parameters, corresponding to commercial plants in operation.

Fluid Sludge Steam
Location Thermal Hydrolysis inlet Hydrolysis reactor Thermal Hydrolysis outlet Boiler outlet
Temperature Ambient 140-160ºC * 102-106ºC 204ºC
Pressure 2-4 barg 3-5 barg 2-4 barg 16 barg
Hydraulic retention time (HRT) - 15-30* minutos - -
Concentration 14-16%MS - 12-14%MS Saturated
Flow rate Between min. and max.* - - < 0.15 kg steam/kg sludge
* Configurable by setpoint

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Frequently asked questions about continuous thermal hydrolysis

What is continuous thermal hydrolysis in sludge treatment?

It is a thermal pretreatment that subjects sludge to high pressures and temperatures to solubilise particulate organic matter and improve its biodegradability in anaerobic digestion, increasing biogas production, reducing the biosolids volume and hygienising them.

What is in-line steam injection in the Polanco Process?

It is the proprietary technology that enables live steam to be condensed, preheating the sludge, in equipment that is directly installed in the pipeline, reducing maintenance costs and improving the process energy efficiency.

Why does continuous thermal hydrolysis boost biogas yield when compared to conventional anaerobic digestion?

Because thermal hydrolysis solubilises particulate organic matter before it feds the digester, overcoming its limiting stage and generating a sludge that is easier to biodegrade.

What are non-condensables and why does the Polanco Process manage them pressurised?

They are Volatile Organic Compounds (VOCs), gases generated during the thermal process which don’t condense and must be evacuated from the system. The Polanco Process extracts them from a pressurised degasifier, evacuating them directly without the need for additional mechanical equipment.

Can thermal hydrolysis be installed in an existing WWTP?

Yes. The Polanco Process can be implemented in different configurations to optimise its integration into the WWTP and enables a treatment capacity increase without replacing the existing anaerobic digestion infrastructure.

What sets the Polanco Process apart from other thermal hydrolysis systems on the market?

Its operation in continuous and steady-state regime as opposed to batch systems, the double flash mechanism, in-line steam injection and pressurised non-condensables management are the technical differentiators that set it apart in terms of operational efficiency and process stability.

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