Thermal hydrolysis provides an option to better connect sewage treatment with modern resource recovery needs. Bill Barber, author of a new book from IWA Publishing, explains the technology and the ongoing work to expand its application.
At the heart of the vast majority of sewage treatment plants are processes whose use has changed little since the late 19th or early 20th centuries. Sewage sludge is the by-product of this treatment. This includes primary sludge, which is generated by settling solids in the incoming wastewater (primary treatment), and secondary – also known as biological or activated – sludge, produced during the breakdown of contaminants in the overflow from the primary sludge settling by bacteria in the presence of oxygen (secondary / biological / activated sludge treatment).
Addition of anaerobic digestion as a subsequent step to produce biogas dates back almost as far. Consequently, most of what comprises wastewater treatment is not designed to accommodate modern drivers of resource recovery, waste minimisation, and reduction of anthropogenic carbon emissions. Furthermore, wastewater infrastructure is built to function over a long lifespan, which makes decisions influential for many years into the future.
The process of thermal hydrolysis provides a way out of this conundrum. It adds a treatment step that fundamentally changes the properties of sludge by heating it to more than 150oC. In doing so, it optimises the use of existing infrastructure, helping wastewater plant owners meet modern drivers while, at the same time, lowering operating costs.
Rather than recover energy from the sludge, it is also possible to recover higher-value materials
Thermal hydrolysis is most effective on the sludge produced during secondary treatment. During aeration in this treatment stage, large quantities of gelatinous material known as extracellular polymeric substances are produced by the bacteria. These substances consist of mixtures of carbohydrates, proteins, nucleic acids, lipids, and humic acids. They give the sludge a colloidal gel-matrix structure, which is highly viscous. This results in mass-transfer limitations during the subsequent anaerobic digestion of the sludge, slowing down the rate at which biogas is produced. Furthermore, the gelatinous material retains water. This limits the potential for water extraction during dewatering, which is needed for efficient use or processing of the end biosolids product.
By destroying these gelatinous materials, thermal hydrolysis improves the performance of both digestion and dewatering. Typically, digestion performance is enhanced by approximately 15-50%, with gas yields ranging from approximately 390-430m3 biogas/tonne dry solids (tDS) processed compared with plants with no pre-treatment. Dewatering improves by up to 10 percentage points. Given the different characteristics of the sludge types, these improvements increase with increasing concentrations of secondary sludge.
Additionally, as the viscosity of the sludge is reduced by thermal hydrolysis, loading rates to digesters can be increased. This results in requirements of 100-140m3 digestion capacity/tDS processed for digesters that are preceded by thermal hydrolysis, compared with 320-400m3/tDS in its absence. New digestion facilities can, therefore, be significantly reduced in size, while existing ones can have their design capacities increased for processing additional sludge or organic waste materials.
Additionally, the temperatures used during thermal hydrolysis exceed those necessary for sterilisation, and this opens up outlets for biosolids and digestate use following digestion.
These benefits are derived by heating the sludge. This requires energy, the quantity of which is dependent on the type and thickness of the sludge being processed, as well as the quantity of heat recycled by the process. The heat needed can be obtained from combustion of the biogas or other energy sources. Potentially, 25% of the energy produced by the biogas is required for heating for a typical mixed sludge. However, almost always, the vast majority of this can be recovered from a combined heat and power (CHP) plant as high-grade waste heat, after which the energy required drops to 1-3% of the biogas production. Energy demands can also be reduced by employing different configurations.
Heating of sludge at the temperatures required for thermal hydrolysis stimulates reactions between proteins and carbohydrates within the sludge that result in the production of melanoidins. These substances are the end products of the Maillard process, which is what causes food to brown when cooked. These products, with their brown hue, are difficult to break down during wastewater treatment and may enter the treated effluent of the plant. Also, carbohydrates caramelise at similar temperatures to those of the Maillard process, and these substances result in similar concerns. Fortunately, the production of all of these materials can be managed and should be accommodated when assessing the potential for thermal hydrolysis. Another factor to consider is that, by enabling higher loading rates and improving anaerobic digestion, more nutrients are released during digestion. Management of these nutrients is also an important part of the decision-making process.
A standardised technology
Despite these issues, thermal hydrolysis has become a standardised technology. There are now more than 130 installations across five continents, ranging in size from less than 10tDS/d to more than 400tDS/d. These plants treat in the region of 3.4 million tonnes of dry material annually, of which the market leader, Cambi, based in Norway, accounts for more than three-quarters of the global installed capacity.
The first facility, in Hamar, Norway, has been operational since 1995 and produces a high-quality biosolids cake from a piston-press at 38% dry solids. Biogas is upgraded to biomethane at the plant.
In Washington, DC, a facility has been operational since late 2014. It is the largest in the world, with an installed capacity of 405tDS/d. The plant is combined with a new anaerobic digestion plant and belt-press dewatering. Additionally, the biogas is used in a third-party contract to generate 8MW of electricity using turbines, and the liquors from dewatering are processed in a deammonification system, which reduces the carbon required for total biological nitrogen removal.
The utility, DC Water, was guided towards the use of thermal hydrolysis by detailed research programmes, having faced escalating costs for a new anaerobic digestion plant. Besides making the project affordable by lowering the new digestion capacity required, the plant has resulted in operational savings from renewable energy combined with a reduction in biosolids leaving the site. Before installation, raw sludge was treated with lime and recycled at a quantity of more than 1,000 wet tonnes/d. Following thermal hydrolysis, production dropped to 400 tonnes/d. Extensive marketing efforts have resulted in large proportions of that material being sold as a high-quality product rather than being a cost liability.
Before this, the largest installation was with the UK’s United Utilities, which installed thermal hydrolysis in 2013 at its digestion facility in Davyhulme, Manchester Ð ironically, where the activated sludge process was originally developed. This was linked to a previous project to install a raw sludge incinerator, which was consequently made redundant, as thermal hydrolysis could increase the capacity at an existing offsite incineration plant connected to Davyhulme via a sludge pipeline. The installation of thermal hydrolysis meant the sludge could be rerouted to Davyhulme, to be digested with indigenous material using existing capacity. This was possible because of the higher loading rates mentioned earlier.
In the case of DC Water, costs were saved by reducing the needs of new digestion capacity. In contrast, at Davyhulme the capacity of an existing plant was doubled. This prevented further expenditure on digestion capacity. In addition to thermal hydrolysis, installation of a centrifuge dewatering facility and co-generation upgrades at the facility enables land recycling of biosolids cake while producing renewable energy. Through its link with the incineration plant, sludge processed through Davyhulme can be recycled as enhanced treated biosolids from there, or sent through the pipeline to the incineration plant, where it can be dewatered and recycled as a standard quality biosolids product, or burnt in the incinerator. The plant has since become the UK’s largest producer of renewable energy from sewage-derived biogas and has won awards for sustainability, in addition to reducing the utility’s carbon impact by 8%. Some of the biogas has since been upgraded to biomethane for grid injection or vehicle use.
More recently, thermal hydrolysis has been used in China, with five installations managing Beijing’s sludge, as well as other installations across the country. As elsewhere, meeting modern drivers and reducing environmental impact have been major factors in decision-making there.
A great deal of research into future applications of thermal hydrolysis is also coming from China. Some of this is related to further enhancing the energy recovery from sewage sludge. Although thermal hydrolysis improves the digestion process, treated biosolids still have a significant quantity of inherent energy in them.
Research since has investigated combining the technology with thermal processes such as pyrolysis, as well as altering the operating parameters towards those used by hydrothermal carbonisation. Rather than recover energy from the sludge, it is also possible to recover higher-value materials. As the reaction conditions required for thermal hydrolysis also concentrate, solubilise and enhance the release of nutrients, there is growing interest in combining the process with those that specialise in nutrient recovery. Yuchuan Environment, based in China, has been pairing thermal hydrolysis with alkali treatment to make a variety of high-end products from the direct dewatering of the hydrolysate. These nitrogen-based materials can be used in a variety of ways, including advanced industrial and agricultural proteins, foaming agents, and fire-extinguishing chemicals.
Work at Queensland University in Australia is focused on commercialising the recovery of melanoidins produced during thermal hydrolysis, with the aim of extracting them as high-value fertilisers. In this way, the chromophores are put to use, rather than seen as a potential liability. The energy used in their recovery is comparable to that used in the standardised destruction methods currently employed.
Studies are also being undertaken on the sterilisation provided by thermal hydrolysis. Data is being collected from facilities in Beijing related specifically to the destruction of antibiotic resistant bacteria and genes, which are generating growing interest in the wider industry. It may be that some form of sterilisation, rather than lower temperature pasteurisation, will become a formal requirement at a treatment works if there is growing potential for disease outbreaks.
A technology supporting future options
Thermal hydrolysis is currently being used around the world to optimise wastewater treatment infrastructure that is largely based on late 19th and early 20th century technology. It already provides a way to help owners meet modern drivers that are shaping the direction of the water sector. It is likely, as we move forward, that the technology will find other applications and will provide a way to keep these facilities aligned as drivers evolve further in future years.
Sludge Thermal Hydrolysis: Application and Potential
New book from IWA Publishing
Thermal hydrolysis changes the properties of sewage sludge. In doing so, it allows wastewater treatment works to become more efficient, enabling the treatment of greater flowrates to higher standards.
This new book, authored by Dr William (Bill) Barber and published by IWA Publishing, is aimed at students and practitioners alike. It describes the development of thermal hydrolysis technology and highlights the design and economics by means of examples. Benefits and challenges related to thermal hydrolysis are also characterised, alongside selected case studies and ideas for future applications.
Sludge Thermal Hydrolysis: Application and Potential. Author: William Barber. ISBN13: 9781789060270. See: https://www.iwapublishing.com/books/9781789060270/sludge-thermal-hydrolysis-application-and-potential
Dr Wiliam (Bill) Barber is Technical Director at Cambi. He was instrumental in the development of Europe’s largest thermal hydrolysis facility.