SolidStream: Post-AD THP and Its Potential Unlocked

The SolidStream configuration, a thermal hydrolysis process (THP) applied after anaerobic digestion, results in enhanced performance on many fronts. Interest in this format is growing especially among utilities wishing to maximise biosolids volume reduction. Deep-dive into this layout's strengths and challenges and see the promising capabilities of this THP offering.

SolidStream 101

The configuration placing thermal hydrolysis technology after digestion is known by many names and abbreviations. Cambi uses the name SolidStream™, also a trademark for the system, but the configuration is also referred to as "Post-AD THP," "Post-digestion THP," or "PAD-THP,” among other variations.

As the various names suggest, this setup slots THP systems after anaerobic digestion and before final dewatering. The configuration outperforms all others in three areas: biogas production, biosolids or final sludge cake reduction, and energy efficiency. However, utilities should be aware that since sludge is hydrolysed after digestion, THP cannot improve digester capacity in this setup. The biosolids in this setup are also sterilised, of high quality, and considered an enhanced product under many regulations.

The following sections will discuss how this setup delivers such results and other factors to consider when choosing this configuration.

Superior Energy Balance Due to Liquor Recycling

How exactly does it result in the highest renewable energy output out of the three thermal hydrolysis configurations when it occurs after digestion? The answer lies in the centrate from final dewatering.

After digestion and thermal hydrolysis, the treated sludge enters the final dewatering stage. Because the sludge has just been hydrolysed, the suspended organic solids in the material are solubilised before entering final dewatering. A significant portion of these solubilised contents are separated from the biosolids via the dewatering equipment. This separated liquid or “high-heat, high-Chemical Oxygen Demand or COD centrate” is piped back to the digesters, where it has a conversion rate of about 80%. Additionally, the heat from the THP system is also recycled for digester heating, contributing to the plant’s energy efficiency.

The energy balance of the SolidStream configuration for a typical sludge plant is estimated to result in about an additional 2.8 megawatts of electricity per 100 tonnes of dry solids when compared to conventional digestion. Though a portion of this will be used by the Cambi system for steam and dewatering, the overall surplus is still estimated at about 1.9 megawatts for a typical setup.

Utilities must note, however, that the increase in biogas production typically leads to higher ammonia levels in the final effluent.

Sludge Incineration and SolidStream: A Great Match Due to Optimal Dewaterability

In addition to its positive contribution to a plant’s energy profile, SolidStream also achieves the best sludge dewaterability outcome among configurations. This is because the hydrolysed sludge immediately goes to dewatering and skips the formation of what’s called EPS or extracellular polymeric substances.

Extracellular polymeric substances naturally occur in sewage sludge prior to any treatment but can also form during digestion. When EPS undergo “steam-explosion” inside the THP system, the viscosity of the material changes. This process releases water previously trapped within the EPS, allowing it to be more easily separated from solid components. When SolidStream THP breaks down the EPS within the sludge and sends it directly to dewatering, it prevents EPS from reforming and, therefore, reducing dewaterability, as would be the case for Pre-AD THP. The direct dewatering of the hydrolysed sludge also improves dewatering properties as the sludge is still of a high temperature, which helps lower viscosity. These conditions help SolidStream outperform the other configurations in terms of dewaterability.

One scientific treated 32 samples using SolidStream to study its effects on dewaterability and moisture distribution in digestates revealed that, on average, it increased the dry solids (DS) concentration of the sludges by 87%. As for the biosolids reduction expected from the configuration, this is highly dependent on a plant’s sludge characteristics. Cambi's own testing in 2018 for the UK market noted a total sludge cake reduction of 55% to 65%.

For utilities drying and/or incinerating their sludge, SolidStream typically reduces the amount of water to be evaporated from the sludge to one-third, saving on energy and potentially the need for additional assets. Several plants across the globe today already use the Pre-AD THP configuration for drying and incineration, but the SolidStream configuration is an emerging setup with several large-scale projects underway. The performance of Post-AD THP has been demonstrated at a plant in Germany with remarkable results, while plants in Belgium and Norway are soon to also harness the configuration’s dewatering capabilities.

A Case for SolidStream: AmperVerband in Germany

Left to right: Cambi SolidStream THP modules during installation at the Geiselbullach plant, an aerial view of the plant (Google Earth)

AmperVerband’s Geiselbullach plant in Germany services several municipalities on the outskirts of Munich. The site processes about 4,400 tonnes of dry solids per year. The sludge cake produced is transported over a long distance for incineration, which incurs high gate fees.

In an effort to reduce these expenses, the plant initially applied THP to its waste activated sludge in 2007, which represented about 40% of the total sludge processed by the plant. The results with the WAS-only configuration were positive, but even more so when the client decided to try the Post-AD THP or SolidStream configuration years later.

Cambi’s SolidStream configuration was commissioned in 2015. The plant was revisited in 2016-2017, and the values from a case study showed that Post-AD THP helped the plant achieve the following:

• About a 40-50% increase in biogas production versus standard digestion
• 70-75% volatile solids (VS) reduction
• Dewaterability of about 38-42% compared to 19-22% in conventional digestion
• An average of 60% biosolids or final sludge cake reduction, alone resulting in an annual cost saving of €515,000. A video of the high-dryness cake is shown below.
• The wet biosolids cake’s energy values increased significantly, compared to conventional wet cake. As wet cake is what’s fed into the incinerator, the energy balance of the incinerator improved.

As expected with using THP after digestion, ammonia levels in the final effluent increased but were well within the site’s discharge limits.

Biosolids treated with the Post-AD THP configuration at AmperVerband's Geiselbullach plant in Germany

When Is SolidStream Right for a Utility?

Ultimately, deciding which THP configuration to implement should be based on a thorough analysis of the treatment plant's goals. However, plants that tick all the boxes below may find THP after digestion to be an attractive option :

• The main driver for the plant’s adoption of thermal hydrolysis or advanced digestion is to achieve maximum biosolids or sludge cake reduction alongside enhanced energy efficiency. The wastewater facility or utility in question typically has high disposal and energy expenses.
• The plant must have sufficient digestion capacity, as the configuration does not impact it.
• Biosolids may be intended for incineration or land application, but if it’s the latter, reuse must comply with local regulations. In the United States, biosolids from this configuration have not yet been classified as Class A under US EPA standards.

In the coming years, two new facilities will further demonstrate the capabilities of the SolidStream configuration. Schijnpoort’s THP facility, owned by Aquafin in Belgium, will soon be operational, with the final sludge cake sent to incineration. Norway’s largest wastewater facility run by Veas will use THP after digestion by 2026, with the intent to land apply the resulting biosolids.

Want to learn more about THP after digestion? Sign up for an upcoming Cambi Academy webinar on SolidStream to be given by Cambi’s Chief Engineer, Hans Rasmus Holte.