Anaerobic digestion: conventional vs. advanced with thermal hydrolysis

Anaerobic digestion is a biological process that, in the absence of oxygen, transforms organic waste into biogas and stabilized biosolids.

In its conventional configuration, anaerobic digestion is limited by the hydrolysis or solubilisation of particulate organic matter, which affects biogas production, digester size, and process stability.

Advanced anaerobic digestion, which incorporates thermal hydrolysis pretreatment, eliminates this limitation by solubilising the sludge before the digester to increase biogas yield, reduce biosolids volume, and minimise the required digestion capacity and its associated investment.
Diagram of the operating principle of continuous thermal hydrolysis in the Polanco Process: reactor, flash mechanism and anaerobic digestion

Conventional anaerobic digestion: operation and limitations

Conventional anaerobic digestion is a biological process in which bacteria and archaea convert organic waste into biogas through four sequential stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

In WWTP sludge digestion, hydrolysis is the rate-limiting step, as most organic matter is in particulate form and its solubilisation is a slow enzymatic process. As a direct consequence:

These limitations do not depend on the infrastructure or operational control, but on the low waste biodegradability.

Conventional anaerobic digestion

Advanced anaerobic digestion: overcoming the limiting stage

Polanco Process patents: continuous thermal hydrolysis with in-line steam injection, registered in Spain, Europe, the USA, and Japan

Advanced anaerobic digestion incorporates a pretreatment designed to overcome the limitation of conventional enzymatic hydrolysis. Among the various available pretreatments, thermal hydrolysis offers the greatest technical and economic benefits.

Thermal hydrolysis subjects sludge to high pressures and temperatures, causing the rupture of cell walls and flocs, the solubilisation of particulate organic matter, improved biodegradability, and biosolids hygienisation.

As a result, the digester receives easy to degrade, solubilised sludge and operates in a more efficient and stable regime.

The Polanco Process transforms conventional anaerobic digestion into advanced anaerobic digestion.

THE POLANCO PROCESS

Conventional vs. advanced anaerobic digestion with thermal hydrolysis

Parameter Conventional anaerobic digestion (without thermal hydrolysis) Advanced anaerobic digestion (with thermal hydrolysis)
Main organic matter state Particulate Soluble
Limiting stage Hydrolysis
(enzymatic hydrolysis in the digester) 		None
(thermal hydrolysis before the digester)
Biogas production 1001 135
Hydraulic retention time (HRT) > 20 days < 14 days
Organic loading rate < 2 kg VS/m³·d > 4 kg VS/m³·d
Anaerobic digestion volume 1001 < 50
Biosolids production 1001 < 50
Hygienisation No Yes
1 Base 100 for conventional anaerobic digestion
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The Polanco Process as an enabler of advanced anaerobic digestion

Polanco Process patents: continuous thermal hydrolysis with in-line steam injection, registered in Spain, Europe, the USA, and Japan

When continuous, energy-efficient thermal hydrolysis technology is implemented, anaerobic digestion reaches its maximum performance.

The Polanco Process, specifically designed to optimise anaerobic digestion, operates in steady-state mode with a double flash mechanism and in-line steam injection to:

Integrating the Polanco Process into the sludge line transforms conventional anaerobic digestion into an advanced anaerobic digestion system, supported by verified results from commercial plants in operation.

The plant-verified results, engineering, and history behind the Polanco Process.

continuous thermal hydrolysis technology by teCH4+

Frequently asked questions about anaerobic digestion

What is anaerobic digestion?

It is an environmental technology used in the treatment of WWTP sludge and other organic waste in which bacteria and archaea, through a biological process in the absence of oxygen, transform organic matter into biogas and stabilized biosolids.

Why does conventional anaerobic digestion have limited performance?

Because the hydrolysis or solubilisation stage of particulate organic matter limits the process kinetics, reducing the rate of conversion to biogas and requiring operation with large digestion volumes.

What is meant by advanced anaerobic digestion?

It is anaerobic digestion that incorporates a pretreatment, such as thermal hydrolysis, to solubilise organic matter before the digester and eliminate the limitation imposed by conventional enzymatic hydrolysis.

Where can the Polanco Process be applied to achieve advanced anaerobic digestion?

teCH4+ continuous thermal hydrolysis, with proven performance in commercial plants, acts as a pretreatment that converts conventional anaerobic digestion into advanced anaerobic digestion and can be integrated into new or existing plants, both in municipal WWTPs and in biogas plants that manage other organic waste.

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