Advanced Anaerobic Digestion: A Guide to Key Technologies

Anaerobic digestion (AD) has been a trusted process in wastewater management for decades, converting sludge into valuable biogas while mitigating environmental impacts. However, as treatment facilities face growing challenges—ranging from stricter biosolids standards in some regions to the demand for higher biogas yields—traditional AD processes alone may not suffice. This primer explores advanced anaerobic digestion technologies, showcasing innovations designed to enhance efficiency, meet regulatory requirements, and support sustainability goals in sludge treatment.

Anaerobic Digestion: Why the Need for Enhanced Treatment?

Anaerobic digestion is a well-established biological process used in waste treatment to break down organic matter such as wastewater sludge, food waste, or animal manure. In the wastewater industry, this method is particularly valued for its role in sludge stabilisation, producing biogas for energy recovery and biosolids that, when meeting regulatory standards, can be repurposed as a soil product or fertiliser.

Traditional anaerobic digestion systems have long been a cornerstone of wastewater sludge treatment, offering a reliable solution for sustainable waste management. However, the growing demand for improved performance and the need to address modern challenges have driven the development of advanced anaerobic digestion (AAD) technologies.

AAD refers to a range of innovations that enhance conventional AD systems, tackling limitations in pathogen reduction, biogas yield, and system capacity. These technologies aim to achieve one or more key improvements, with varying levels of effectiveness depending on the specific application:

Enhanced Pathogen Reduction

AAD technologies aim to significantly reduce harmful microorganisms in the biosolids or final sludge, to meet stringent regulatory standards for land application. Though anaerobic digestion achieves some level of pathogen removal, it often falls short of achieving the higher quality required for advanced classifications.

To see the standards set by leaders in this space, check the United States’ requirements for Class A biosolids and the United Kingdom’s enhanced Biosolids A. It’s important to note that many AAD technologies still face challenges in meeting these standards due to issues such as pathogen regrowth.

Reduced Biosolids Volume

AAD minimises the quantity of final sludge or biosolids, which translates to lower costs for dewatering, transportation, and disposal.

 Increased Biogas Production

Advanced digestion technologies maximise energy recovery by improving the availability of organic material for microbial degradation.

Optimised Digester Capacity

These technologies increase the throughput of existing facilities in space-constrained sites or enable the design of smaller, more cost-effective digesters for new installations.

Key Advanced Anaerobic Digestion Technologies

With the continuous advancement of anaerobic digestion technologies, many products and systems in the industry claim to achieve advanced anaerobic digestion. This article focuses on technologies implemented at municipal wastewater treatment plants and recognized by academic institutions, though others may also be referenced. These AAD technologies are categorized as either modifications to the digestion process or separate sludge treatment processes that operate in conjunction with digestion.

Advanced digestion technologies that modify the traditional anaerobic digestion processes include thermophilic anaerobic digestion, temperature-phased digestion, staged digestion, and acid/gas-phased digestion.

Thermophilic Anaerobic Digestion (TAD)

Anaerobic digestion can occur at temperatures above approximately 10°C, but the process slows significantly at lower temperatures. Research has identified two temperature ranges where biogas production is most efficient, each supporting distinct microbial communities. Of these two, mesophilic anaerobic digestion or MAD has become the industry standard.

Mesophilic digestion treats sludge at around 35-40°C, and is commonplace because of its ease of use and process stability. The other range is that of thermophilic anaerobic digestion or TAD, which functions at elevated temperatures of about 50-60°C and is considered a form of advanced digestion because it offers enhanced performance benefits.

Sludge treatment plants widely employ mesophilic anaerobic digestion (MAD), which operates at temperatures of approximately 35-40°C, due to its operational simplicity and process stability. In comparison, thermophilic anaerobic digestion (TAD) operates at higher temperatures of about 50-60°C and is classified as an advanced digestion method, offering enhanced performance benefits.

Compared to MAD, thermophilic digestion yields increased pathogen destruction and biogas production, as well as reduced biosolids volumes. Utilities, however, may struggle with the disadvantages of this process, which include decreased process stability, a sludge cake with a stronger odour, and increased energy requirements versus MAD. The process may require batch processing (applying TAD to sludge going through a series of digesters) to achieve enhanced sludge or Class A biosolids.

Compared to MAD, thermophilic anaerobic digestion (TAD) offers benefits such as higher pathogen destruction, increased biogas production, and reduced biosolids volume. However, utilities may face challenges with this process, including lower process stability, stronger odors in the sludge cake, and higher energy demands compared to MAD. Additionally, achieving enhanced sludge quality or Class A biosolids often requires batch processing, where sludge passes through a series of digesters.

Staged Digestion, Digestion in Series, or Sequential Digestion

Both mesophilic and thermophilic digestion can be performed in multiple reactors arranged in series, a configuration known as staged digestion. This approach enhances higher biogas yields, pathogen reduction, and volume reduction compared to single-stage treatment. However, it requires greater digestion volume or capacity, posing a challenge for facilities with limited land availability.

Temperature-Phased Digestion (TPAD)

A type of staged digestion, this process combines both thermophilic and mesophilic digestion stages, leveraging the advantages of both temperature ranges. Typically, the thermophilic stage precedes the mesophilic stage, enhancing pathogen reduction and hydrolysis step within the anaerobic digestion process. This is followed by the mesophilic stage, which stabilises biogas production and improves process efficiency and stability. Placing TAD before MAD also helps mitigate the stronger odour associated with thermophilic digestion. Due to the great need for digestion volume , TPAD is best suited for treatment plants with large spatial footprints and enough digesters to accommodate the process.

Acid/Gas-Phased Digestion

In conventional AD, all four steps—hydrolysis, acidogenesis, acetogenesis, and methanogenesis—occur within a single tank or digester. A form of staged digestion, this system separates the initial three steps from the final methanogenesis stage into distinct reactors, creating optimised conditions for each microbial community. This separation enhances process stability and increases biogas production. By implementing phased digestion, facilities can achieve better control over pH levels and retention times in each phase. However, like other staged systems, this approach requires additional space for multiple reactors. Acid/gas-phased digestion, also known as acid-phase or two-phase digestion, exemplifies this technique.

The next group of advanced anaerobic digestion technologies involve additions to the digestion process rather than a modification of it. Among these, treatment technologies that enhance hydrolysis—the first and rate-limiting step of AD—are the most widely recognised. As the slowest and most critical phase, hydrolysis determines the overall efficiency and speed of organic matter breakdown. These technologies are typically used alongside conventional mesophilic digestion but can also complement the digestion-modifying technologies mentioned earlier.

Thermal Hydrolysis

Also known as the thermal hydrolysis process or THP, this involves subjecting organic feedstock to high temperatures (typically between 160°C and 180°C) and pressures (approximately 6–10 bar) for a short duration, followed by rapid decompression. This process breaks down the complex organic structures in sludge, making them more accessible to microbial degradation during subsequent anaerobic digestion.

This technology enhances biogas yields, reduces digestion time, and improves the digestate's dewaterability. Additionally, it effectively destroys pathogens, resulting in a final sludgethat is utilised worldwide as a soil product or as feedstock for incineration.  

Thermal hydrolysis is arguably the most recognised advanced digestion technology in this group, with more than a hundred installations worldwide. It is used in various configurations, placing the process before, after, or in between digesters, each setup offering unique benefits.Several plants apply the process exclusively on their waste-activated sludge, where it has proven to be particularly effective. Cambi, which developed this technology, has delivered nearly 90 projects as of 2024.

Chemical Hydrolysis

This treatment involves adding chemicals, such as acids or alkalis, to solubilise complex organic matter, making it more accessible for anaerobic microorganisms. This pretreatment can accelerate digestion rates and biogas production. However, careful control is necessary to prevent inhibitory effects on microbial communities. As of 2023, this technology has been implemented in at least two full-scale installations in the United States.

Mechanical Hydrolysis

This technique employs mechanical forces, such as high-pressure homogenisation or ultrasonic disintegration, to break down cell walls and facilitate faster and more complete anaerobic digestion. Mechanical pretreatment methods are often used in conjunction with other hydrolysis processes to enhance sludge digestibility. However, these methods alone typically do not achieve significant pathogen reduction. As of 2024, this type of hydrolysis has been implemented in only a limited number of full-scale installations.

Considerations and Challenges with AAD

While advanced anaerobic digestion technologies offer numerous advantages, they also face notable challenges as an area of scientific development. Perceived risks are common, particularly for newer technologies with limited long-term reference projects. Despite anaerobic digestion’s century-long history, its adoption remains limited in many regions, particularly in developing and underdeveloped countries. This means that the application of advanced digestion technologies is still in its early stage, with existing AD plants testing out the existing AAD methods and products on varying types of sludge. Countries like the United Kingdom, the United States, China, and Norway are some of the countries producing the most research in this field. The newest findings are typically presented at key wastewater and biosolids conferences.

The Future of Advanced Anaerobic Digestion

Anaerobic digestion allows treatment plants to stabilise sludge, recover energy through biogas production, and comply with sustainability goals. Advanced applications can refine the process, making it more efficient in terms of energy and cost while enabling the production of biosolids that meet the most stringent standards.

As the global focus on sustainable waste management and renewable energy sources continues, the adoption, testing, and further development of these advanced technologies will help utilities get more out of the anaerobic digestion process. This progress may drive broader adoption of AD or help both new and ageing facilities meet their environmental and energy goals. As sludge treatment continues to evolve rapidly, it will be interesting to see how advancements in existing technologies, the emergence of new innovations, or novel technology pairings shape the future of the process.

Curious about how world-leading utilities achieve advanced anaerobic digestion with thermal hydrolysis? Visit our customer stories.