Thermal Hydrolysis Configurations and Their Unique Strengths

There are three main thermal hydrolysis(THP) configurations utilities can use when installing the technology in their sludge line, each with its unique advantages. Of these, the most common is where THP is used as a pretreatment method for digestion, but changing regulations and trends in the wastewater industry could soon see greater adoption of the two other configurations. Curious about the merits of each?  Read on to learn more.

When it comes to the thermal hydrolysis process (THP), the word "configuration”, as with any highly technical system, can be understood in many ways. At Cambi, a THP configuration implies the placement of the system with respect to anaerobic digestion. Simply put, it's where the THP is “slotted” into a sludge line.

All Cambi THP configurations can achieve at least two benefits for a utility or plant, regardless of which configuration they choose. These are biosolids or final sludge volume reduction and increased biogas production. The choice of the configuration not only determines the level of attainment of these two benefits but the presence of other select benefits, most notably, the hygienisation of the biosolids or sludge cake and the increase in digestion capacity.

Cambi’s Main THP Configurations

The three CambiTHP configurations are as follows: THP before anaerobic digestion, THP after anaerobic digestion, and THP between digesters. An application of the first configuration that is sometimes cited as a fourth configuration is the "WAS-only application". All three configurations, with the WAS-only application, are discussed below.

THP Before Anaerobic Digestion

Sometimes called "Full Cambi", "Full THP", "Pre-AD THP", or "Classic THP", this configuration is the most common placement of the thermal hydrolysis system in a sludge line or sludge treatment plant.

When using Cambi as pretreatment for digestion, all sludge passes through the THP system prior to being digested. The digester significantly benefits from thermally hydrolyzed sludge, which is easy to break down.

This configuration's main benefits are the following:

• Digester loading rates improve, with the digestion capacity required going down to a third or half of conventional digestion’s required capacity. It has helped many wastewater utilities increase digester capacity without needing to build additional digesters and work within certain space constraints.
• Biogas is increased by 20 to 50%. The Xiaohongmen plant, for example, reported a 43% increase in biogas after installing THP.  
• Dewatering is improved, typically halving biosolids volumes and therefore lowering transport costs.
• The biosolids produced are considered either Class A Exceptional Quality (US EPA's Biosolids rule) or Enhanced (EU regulations). Considering local regulations, this means that they can be used in agriculture or other applications as a biofertilizer or soil product.

Among all other options, the THP before AD configuration results in the greatest digestion capacity increase while ensuring that the biosolids or final sludge cake is pathogen-free. However, it also requires the most energy. Though it can achieve impressive results in biosolids volume reduction, it is not the most efficient configuration when it comes to this benefit. This is due to extracellular materials forming in the digester after thermal hydrolysis, making dewatering less effective.

Almost all existing Cambi THP systems use this configuration, which is why THP is often referred to as a "pretreatment method." Some examples of plants using this setup include Beijing Drainage Group's Gaobeidian plant, Australian Water Corporation's St. Mary's plant in Sydney, and the HIAS plant in Norway. 

An application of the first or classic THP setup, which is so common that it's typically referred to as another THP configuration, is what's called the waste activated sludge or WAS-only configuration.

THP Before Anaerobic Digestion: WAS-Only

The WAS-only configuration, sometimes called "SAS-only" (surplus activated sludge) in the UK market, has become commonplace because THP has been shown to reap the most benefits when used on this type of sludge, which has a lot of extracellular material or proteins. In this scenario, THP is applied to waste activated sludge only. The configuration can be used by plants that either only process WAS (meaning the plant does not produce primary sludge) or those that can process WAS separately from primary sludge (PS). In the latter case, once the WAS is thermally hydrolyzed, it's mixed with primary sludge before digestion.   

As of 2024, about a quarter of all Cambi thermal hydrolysis plants use the WAS-only application of the pre-AD configuration – a testament to its benefits:

• This WAS-only application is one of the most energy-efficient THP configurations. The use of THP is optimized as it is most effective on WAS.
  
  
• In cases where primary and waste activated sludge are mixed before digestion, the thermally hydrolyzed WAS can be used to heat PS to the required temperature for digestion. A cooler, therefore, is not needed for the WAS, saving further on the utility’s expenses. However, this typically requires some additional monitoring. 
  
  
• The facilities can invest in a smaller Cambi system while gaining many of THP’s desired effects. However, if the unhydrolyzed primary sludge is mixed in with the WAS after hydrolysis, it will not achieve pathogen-free biosolids.
  

Plants that use this application of the pre-AD THP configuration include the EYDAP’s Psyttalia plant servicing Athens, Aguas Andinas’ Mapocho plant in Santiago de Chile, and Singapore Public Utilities Board’s Jurong plant.

THP After Anaerobic Digestion 

Thermal hydrolysis after anaerobic digestion, often called “Post-AD THP” or “post-digestion THP” and trademarked as “SolidStream” for some markets, utilizes THP technology after digestion and before final dewatering.

• Compared to all other THP configurations, Post-AD THP reduces biosolids volumes and increases biosolids dewaterability the most. This makes the configuration especially attractive to plants incinerating or drying their sludge since drier, energy-rich feedstock requires less fuel for the thermal process. 
  
  
• The biosolids in this setup are of superior quality and pathogen-free, and meet enhanced sludge standards. In the US, they cannot yet be classified as Class A according to the US EPA Biosolids Rule due to some technicalities. This is likely to not present any issue with other regulations.  
  
  
• Another important benefit of this setup is that it results in the highest biogas production among all configurations, up to around 50%. This is because the energy-rich process water from final dewatering is funneled back to the digesters.

Utilities must note, however, that since the sludge is hydrolyzed after digestion, THP will not positively affect digester capacity. 

The Geiselbullach plant in Germany is an example of a Cambi plant using this configuration, and it has shown impressive results with the setup. Aquafin's Schijnpoort facility in Belgium is another that is set to be operational in 2024, while Veas' facility in Norway will be commissioned in 2026. 

THP in Between Digesters, Commercially Called I-THP

Also called intermediate THP, I-THP is an emerging configuration, similar to post-digestion THP. This setup achieves somewhat of a balance of the benefits between THP pre-AD THP and post-AD THP. The benefits of this configuration include:

• It increases biogas production and maximizes biosolids reduction at rates higher than what can be achieved by Pre-AD THP but not quite as high as what Post-digestion THP can manage.
  
  
• The biosolids it produces still qualify as Class A under the US EPA Biosolids rule and as enhanced sludge by other regulations.
  
  
• This is a more energy efficient system than Classic THP due to the smaller sludge volumes to be treated after the first digestion stage.  Additionally, the initial digestion phase processes the more easily degradable sludge components, leaving the THP system to handle the harder-to-digest material, thereby optimizing its use. 
  
  
• The THP system treats a smaller volume than Pre-AD THP and will therefore require a smaller THP system, translating to less capital expenditure.

This setup's disadvantage is that it is not a good solution for increasing digestion capacity, as only a portion of the digesters benefit from the hydrolyzed sludge.

A plant using this THP configuration is the Południe wastewater treatment plant owned by the Municipal Water and Sewage Company of the Capital City of Warsaw or MPWiK in Poland.

Comparing the Various THP Configurations

It can be difficult to compare the configurations side by side, but a simple visualization is attempted below, focusing only on the common project drivers: the need to increase digestion capacity, the potential to increase biogas, the dewaterability and sludge reduction capacity, and the requirements for enhanced or pathogen-free sludge. A green circle indicates that the configuration can achieve the benefit, and the quantity or fullness of the circle indicates to what extent. The red circle indicates that the configuration either does not achieve the benefit or that it is not a good fit for that particular driver or factor.

  A visual comparison of Cambi THP configurations from the webinar “Where to install thermal hydrolysis in a wastewater treatment plant” (Bill Barber, Cambi Academy, 2019).

For the WAS-only application, pathogen-free biosolids can be achieved if the facility only processes WAS and does not have primary sludge.  

From the above, you can see that each configuration has its own strengths and that depending on a utility's priorities, one configuration will outperform the others. Thermal hydrolysis before digestion is best suited for plants needing to increase digestion capacity but is also the most energy-intensive, with the WAS-only application of this configuration being an energy and cost-efficient option, though it may be a bit more operationally complex. Post-digestion THP or SolidStream configuration offers enhanced performance but requires sufficient digestion capacity and may not fully meet Class A biosolids requirements. I-THP is a promising configuration that tries to balance these benefits, providing improved results over Classic THP but, again, is suited for plants with ample digestion capacity.  

A few other factors not fully discussed in this article also differ among the configurations, namely:

• the need for specific ancillary equipment
• parasitic energy demand
• consideration for return liquors
• operational complexity
• redundancy

These should also be part of a plant's consideration of its THP configuration.

The Right Configuration for the Right Benefits

Where you place a THP system in a sludge line has significantly different benefits for plants or utilities. Though most thermal hydrolysis plants today use THP before anaerobic digestion, that is not to say that this will be the dominating configuration in the years to come.

In some countries around the world, land application of biosolids or treated sludge is still the most attractive endpoint for the material as it recycles nutrients back into the soil and is often the lowest-cost option. However, others are showing increasing consideration for sludge incineration or other processes like pyrolysis, gasification, and hydrothermal carbonization. This is primarily due to concerns surrounding per- and polyfluoroalkyl substances and their related compounds, microplastics, and other organic micropollutants. The apprehensions around these materials could increase the adoption of other THP configurations that better suit thermal methods, like WAS-only, Post-digestion THP and I-THP.

Want to stay updated with trends in sludge treatment related to advanced digestion? Sign up for Cambi Academy to gain access to regular webinars!

This article was updated on 25 September 2024.