Sludge recycling by hydrothermal carbonization with TerraNova® Ultra technology

The TerraNova® Ultra technology extracts nutrients namely phosphorus and nitrogen, that can also cause eutrophication of water bodies. It also produces a charcoal-like fuel and boosts biogas production from sewage sludge in wastewater treatment plants. Sewage sludge is utilized in an innovative way - for climate and environmental protection. Through the TerraNova® Ultra process sewage sludge is carbonized into a coal - similar to lignite coal. But unlike that the "sewage coal" is climate friendly since it is produced from regenerative sewage sludge.

In the process the dewatered sewage sludge (dry matter content of 5-30%) is conveyed into the input heat exchanger by means of a high-pressure sludge pump. The preheated sewage sludge is carbonized in an agitated reactor under the addition of catalyst for 5 hours at a temperature level of around 180°C. The reactor is externally heated by thermo oil. The heat source can be, for example, the exhaust heat of a Combined Heat and Power Plant (CHP). The resulting coal slurry is cooled down within the output heat exchanger. The extracted heat is fed back to the input heat exchanger by means of a separate thermo oil circuit.

In a dewatering unit, for example a fully automated chamber filter press, the coal slurry is dewatered to a dry matter content of 65-70%. The ultra-dewatered sewage coal can be utilized for energy production. The volume is reduced to less than 1/3 compared to the original volume, also minimizing transportation costs.

The filtrate from dewatering contains a large part of the sewage sludge nitrogen and phosphorous in plant available form and can either be directly used as liquid fertilizer or in an additional process for recovery of P or N. Since its remaining carbon content is easily bio-degradable, the biogas production of the sewage sludge digestion can be increased by 15% if the filtrate is fed back into the digestors.

P recovery steps in addition to the process below consist of the following:

  • An acidification step that decreases the pH level of the coal slurry below pH 2. During the acidification, the chemically bound phosphorus (from chemical P precipitation during wastewater treatment) is leached from the Fe- or Al-components into the liquid phase, resulting in the transfer of abour 80% of the phosphorus bound in the sewage sludge.
  • A solid dosing step that adds Calcium-Silicate-Hydrate granulates (CSH) to the filtrate, which adsorbe the dissolved phosphorus. The CSH granules are produced from the slurry dewatering in the chamber filter press.
  • A separation step for the phosphorous product, consisting of amorphous P-components, hydroxylapatite and struvite, with good plant bioavailability, reaching up to 80% (compared to superphosphate).

Results and next steps

TerraNova® Ultra is installed on sewage treatment plants that require most energy sufficient sludge drying – for example by available waste heat. It is also suitable for the stabilisation of sludge, offering an alternative to anaerobic sludge digestion or composting. Phosphorous recovery can be added as an option to the process.

The TerraNova® Ultra technology will therefore:

  • apply Hydrothermal Carbonization to the sludge;
  • reduce the sludge amount by smart sludge drying;
  • consume 80% less energy than conventional thermal dryers;
  • allow for Phosphorous recovery.

TerraNova® Ultra can be integrated into any sewage treatment system with little cost and effort and without interfering with the treatment process.

The Hydrothermal Carbonization technology of TerraNova® Ultra runs under the exclusion of air at approx. 180° C and under a pressure of more than 20 bar. This process ensures that the hazardous substances in the sewage sludge are mostly destroyed or can be safely disposed in the next processes.

  • Inorganic hazardous components, such as heavy metals, are bound to the sewage sludge coal and could be safely further disposed through incineration;
  • Organic hazardous components, such as pharmaceutical residues, are largely chemically destroyed and lose their toxicity;
  • Pathogens are completely killed of as the sewage sludge coal is sterlised under high temperatures;
  • The synthesis of dioxins or furans is impossible with the TerraNova®Ultra process.

In addition, toxic waste has long been an unused source of nutrients such as nitrogen and phosphorous, which are both valuable fertilizers. For the first time, a large portion of these nutrients can now be recovered economically through the TerraNova®Ultra method, either as a topsoil-forming organic fertilizer or as a basic material for the fertilizer industry. In view of decreasing natural resources, the growing demand and rising prices, phosphate reutilization is of particular importance.

Within TerraNova® Ultra, phosphorous is leached out from the sewage sludge and recovered by adsorption to natural minerals. The resulting product is a granulate phosphate fertilizer that is already in complience with potential upcoming legal regulations. The very moderate amount of chemicals as well as the easy one-step process allows for low production cost of the recycled phosphorous.

Smart sludge drying to make the green energy of sewage sludge

TerraNova® Ultra runs with minimum emissions of greenhouse gases and the resulting coal is categorized as a renewable fuel, because only biogenic CO2 is released when it is burned. Therefore, during combustion of the coal obtained through the TerraNova® Ultra method, recyclers can count on a CO2 credit. The fuel value of the biochar is comparable to the German lignite coal, while preventing the CO2 emission quota from rising. 

If a city with 100,000 inhabitants, for example, implements the TerraNova Ultra® method in its sewage treatment plant, annual emissions of CO2 are lowered by 1,220 tons* – which is more than one square kilometer of forest can extract from the atmosphere.

*Substitute for lignite coal in, for example, cement plants: 0.36 tons of CO2 per megawatt hour; the CO2 debit of the TerraNova® Ultra method is already taken into account)

Potential applications in the BSR

The process is designed for middle sized and large sewage treatment plants. TerraNova® Ultra is currently being tested at the Norrsundet Slamförädling AB pilot plant in Gävle, around 175 km outside Stockholm.

The TerraNova Ultra® method has proven itself through numerous scientific studies and many years of continuous operation on many sewage sludge treatment plants of varying size. Its importance as an innovative way to reutilize sewage sludge is also recognized today internationally, where operation of TerraNova energy systems is already agreed upon or planned.The first industrial size TerraNova® Ultra plant is being operated in Jining/China. The TerraNova Energy is also one of the three winners of the BONUS RETURN project.

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