As freely available but not yet commercially acquired biomass resource, water care material (WCM) is generated seasonally in the periodic maintenance of surface water bodies and consists of mainly aquatic and/or rural-associated biomass of the water body profile, as well as wood, soil substrate, water or other possible impurities. In addition to other recovery options, such as composting or utilization in biogas production, hydrothermal carbonization (HTC) was selected as a thermochemical process because it is suitable for converting biomass with a high content of carbon into high-quality combustibles. The biomass sample used in this investigation was obtained during a single sampling event from a small stream in the North German lowlands. The material was pretreated by shredding it to a particle size of <0.12 mm. Through a 5 L stirred reactor, hydrothermal treatments were performed under low temperature conditions (200, 220 and 240 °C), residence times (120, 180, 240 min) and solid dry matter of the sample content: 6%. Solid phase was evaluated in terms of calorific value and proximate and ultimate analysis. The results suggested that the hydrothermal carbonization of WCM gave a high heating value of 23.84 MJ/kg for its char after being dried for 24 h at 105 degrees. At the same time, biochar can be used in agriculture to improve soil properties. To understand to what extent the product is suitable for soil amendment, the surface and the nutrient content of the resulting hydrochar were analyzed in detail. As the initial material is rich in fiber contents, process temperatures up to 240 °C have a huge impact on effective particle size. Furthermore, the analysis of selected nutrients, minerals and heavy metals shows the suitability of the produced hydrochar for soils in accordance with current legislation.

Hydrothermal carbonization (HTC) converts wet biomass such as sewage sludge (SS) into hydrochar and also produces highly contaminated process water (PW) that requires further treatment. This study evaluated the aerobic biological treatment of diluted PW in a laboratory-scale mesophilic trickling filter operated in repeated-batch mode with intermittent recirculation under sequentially increasing organic loading conditions. SS was hydrothermally carbonized at 180, 200, and 220 °C for 1 h. The resulting PW samples were chemically characterized and assessed for aerobic biodegradability. PW generated at 220 °C was selected for treatment in the trickling filter. After reactor start-up and biofilm adaptation, the main 75-day experiment was conducted at 30 °C, with the recirculation-based organic loading rate (OLR) increased sequentially in six stages. TOC removal averaged 78.00 ± 3.39%, while VFA removal averaged 76.42 ± 4.47%. NH₄-N removal averaged 97.95 ± 4.14% during the first five loading stages but decreased to 89.53% at the highest OLR. Under the investigated conditions, approximately 30–33 kg COD m⁻³ d⁻¹ represented the highest recirculation-based loading range at which carbon, VFA, and NH₄-N removal remained jointly stable. Qualitative GC-MS screening indicated transformation or removal of part of the detectable organic fraction, while 16S rRNA gene amplicon sequencing showed changes in microbial community composition during high-load operation. Overall, the laboratory-scale trickling filter effectively treated diluted SS-derived HTC-PW. However, substantial PW dilution, the repeated-batch operating mode, and residual poorly biodegradable organics limit direct extrapolation to full-scale treatment of raw PW.