The project “Energetic sector coupling between water and energy sectors through heat‑value oriented wastewater treatment” was carried out at the Technical University of Dresden’s Institute for Urban and Industrial Water Management from 1 September 2022 to 31 July 2024. It was funded by the German Federal Environment Foundation with a grant of €124 360 and was part of the DRESDEN‑concept network. The project team was led by Dr. Ing. Thomas Schalk, with M.Sc. Julius Böckmann and Dr. Ing. Markus Ahnert as co‑researchers. The study aimed to reduce the loss of calorific value in primary sludge by intensifying pre‑treatment with carbon‑based adsorbents such as plant bio‑char or pyrolysis coke in combination with a flocculant, thereby improving dewatering and enabling direct thermal utilisation of the sludge.

Experimental work was organised in a series of laboratory and pilot‑scale tests. Adsorption trials with wastewater (series V1, V2) examined the removal of dissolved carbonaceous suspended solids (CSB). Results showed that activated carbon (Kohle P) eliminated dissolved CSB by adsorption, but had no effect on particulate CSB. When a flocculant was added, it competed with dissolved CSB for adsorption sites, reducing the specific CSB adsorption of the carbon. The specific elimination of dissolved CSB was quantified as 5 mg CSB per mg of flocculant and 5 mg CSB per gram of total residue when both flocculant and carbon were present. Iron(III) chloride proved superior to polyaluminium chloride under the tested conditions, yielding lower sludge volume, lower specific costs, and better dewatering characteristics. A contact time of five minutes with activated carbon achieved an 80 % elimination of dissolved CSB. Adsorption of ammonium nitrogen and phosphate by activated carbon was below the detection limit, indicating negligible removal of these nutrients.

Settling experiments (series V6–V8) and a long‑term series V8 evaluated sludge properties. Flocculants increased both sludge volume and solids content, while the addition of activated carbon raised the solids fraction. Both activated carbon and iron(III) chloride improved dewatering, as evidenced by reduced specific gravity and higher dry solids yield. The calorific value of the organic matter increased with flocculant addition, and the increase due to activated carbon was attributable solely to the carbon itself. Odour tests (series V11) revealed that activated carbon and excess carbon from the fourth recycling gas system (RGS) mitigated odour emissions in the dried sludge. Sludge samples with more than 20 % carbon content were often odourless or even produced a positively rated scent.

The study also included anaerobic batch tests (series V9) to assess potential biogas production, although the project’s focus was on direct thermal utilisation rather than biogas generation. Olfactory and sieving tests (series V10, V12) confirmed the practical applicability of the proposed pre‑treatment scheme.

In summary, the project demonstrated that integrating carbon‑based adsorbents with flocculants can significantly reduce the loss of calorific value in primary sludge, improve dewatering, and lower odour emissions, thereby facilitating the direct thermal utilisation of sludge. The findings provide a technical basis for a sector‑crossing energy generation model that couples wastewater treatment with thermal energy production, aligning with the objectives of the German Federal Environment Foundation and the DRESDEN‑concept network.