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Following our visit to the Dubai manure-to-biochar project, we had the pleasure of visiting a sewage sludge-to-biochar facility in Nordhausen, Germany. The plant, delivered by Next Generation Elements Gmbh (NGE), is part of a Nordhausen University research project called CarboMass. The project "aims to achieve optimisation in technical development and the expansion of inter-municipal cooperation in sewage sludge processing.” Using innovative approaches and new technical developments, the project transforms previously unvalorised sewage sludge into valuable biochar.
The Nordhausen project is situated at a local wastewater treatment facility, which processes effluent from a population of up to 96,000 people. The installed pyrolysis reactor, supplied by Next Generation Elements GmbH, receives sewage sludge that has undergone initial processing to reduce the sludge's moisture content and the overall presence of potential contaminants. Prior to entering the pyrolysis reactor, the sludge's high moisture content—up to 80%— is further reduced.
The drying process is conducted in two distinct stages. First, the sludge is dried using a belt press, reducing its moisture content from approximately 80% to 20–30%. In the second stage, the partially dried feedstock (see image 2) undergoes further processing in an enclosed, automated screw conveyor dryer, immediately before entering the reactor. This ensures the material meets the stringent moisture requirements to be compatible with the installed Pyrodry equipment.
The final pre-treatment step, pelletisation, is critical for ensuring the dried sludge achieves a uniform and consistent structure. Standardising the feedstock reduces fines and ensures predictable heating and reaction rates during pyrolysis. Inside the reactor, the feedstock is processed at temperatures of 900–1000°C for 45–60 minutes. The feedstock composition results in a high conversion rate, wherein 50 kg of feedstock is converted into 30 kg of biochar every hour.
The high conversion rate achieved in here highlights the importance of addressing the challenges associated with sludge as a feedstock. Sludge requires careful handling due to its high ash content, variable organic composition, and potential contaminants. Pre-treatment steps such as drying and pelletisation are essential to overcome some of these challenges, and further helps mitigate operational issues, such as equipment wear and tear, while also allowing for a consistent and predictable parameters to be used for the conversion process.
Currently, the application of biochar is restricted under German legislation, which prohibits the use of biochar derived from sewage sludge in soil-based applications. However, a special exemption was granted for biochar produced under the CarboMass initiative, allowing its use in two soil-based research projects across a limited area.
The first project involves using the biochar to assess its effectiveness in soil decontamination and remediation efforts, with applications at nearby slag heaps left behind by previous gypsum mining activities. A smaller portion of the biochar is dedicated to on-site studies examining the growth of naturally occurring flora under varying conditions.
We observed several key takeaways and notable differences between the two projects, reflecting the diverse challenges and solutions tailored to each context.
Although both projects utilised manure-based feedstock, the significant differences in composition necessitated vastly different pre-treatment processes. Sewage sludge, being more moist and containing higher levels of contaminants, required extensive pre-treatment before pyrolysis. By contrast, camel manure, which was naturally drier, needed less moisture adjustment but still required additional steps to remove contaminants such as rocks and sand.
In Nordhausen, risks associated with contaminants like sand were largely mitigated during earlier phases of the wastewater treatment process. However, in scenarios where sand contamination remains a concern, specialised equipment such as tumble dryers or sand recovery basins would typically be required. These measures are vital for minimising abrasion risks and prolonging the operational lifespan of the equipment.
Another significant difference was the necessity of pelletisation for dried sewage sludge due to its fine and brittle nature. Pelletisation was essential for ensuring consistent pyrolysis process parameters and biochar quality, while also improving storage and handling efficiency. However, this process introduced challenges, particularly the need to control moisture content and sand contamination, which could lead to excessive wear on machinery. Effectively managing these risks requires tailored strategies, such as implementing advanced filtration or separation techniques during earlier sludge processing stages.
The physical design and setup of the facilities further underscored their contrasting approaches. Dubai’s Carbo-Force plant employed a modular, plug-and-play design, delivered in shipping containers for rapid and straightforward deployment. In contrast, the Pyrodry facility near Nordhausen featured a more traditional open-layout design, assembled on-site to suit its specific requirements.
Dubai’s plant, however, faced additional challenges due to the local environment. For instance, air conditioning systems were later installed to cool control cabinets, addressing operational demands imposed by the region's extreme heat. These differences demonstrate the adaptability of pyrolysis technology to accommodate diverse geographic, climatic, and logistical conditions.
Operational strategies further distinguished the two projects. Dubai’s facility relied on a full-time team of operators to oversee daily activities and ensure immediate responses to any issues. By comparison, the Pyrodry plant by Next Generation Elements is fully automated, requiring minimal supervision. Minor operational tasks were handled by trained personnel from the adjacent wastewater treatment plant, with technicians on call to address any significant issues.
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