Hydrothermal carbonization (HTC) has developed considerably over the last 15 years and offers a viable alternative for the utilization of municipal and industrial organic waste such as sewage sludge. However, the technology has yet to establish itself as a valorization process for waste biomasses (2024) and is not yet a recognized state of the art. Nevertheless, the HTC technology could gain greater relevance in the future, especially as an alternative valorization pathway for sewage sludge. During HTC, significant amounts of HTC process water (PW) are produced as a byproduct. The process water is inorganically and organically polluted and has to be treated, as it would be a burden on water bodies and thus on the environment if left untreated. In the EU and specifically Germany, industrial wastewater producers like HTC-plant operators are obliged to treat their industrial wastewater before discharging it into the environment. In addition to a large amount of PW and its treatment to the required limits, the organic load and possible persistent and toxic substances pose major challenges for plant operators. Many proven processes from industrial wastewater treatment were transferred for the treatment of PW. Treatment of the PW in a manner that is industrially viable, economically viable, and efficient is crucial for the effective commercialization of HTC technology. In this, the challenges and opportunities of PW composition, management, and treatment, including legal aspects, are mainly discussed. Therefore, the legal framework in the European Union and specifically for Germany will be elaborated. Furthermore, different treatment pathways are also highlighted.

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H₂SO₄) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H₂SO₄, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H₂SO₄ in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar.

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.

As the result of new regulation from the German Sewage Sludge Ordinance (AbfKlärV 2017) and the future obligation to recover phosphorus, thermal treatment (mono-incineration) has become increasingly popular, whereas land-based utilization has decreased. Germany has produced 1.71 million metric tons (DM) of sewage sludge in the year 2021. Sewage sludge contains important nutrients such as phosphorus but also heavy metals and organic pollutants making the direct utilization of sewage sludge in agriculture controversial. Rural areas in particular have benefited from land-based sewage sludge utilization however the future ban on direct land-based utilization is forcing them to find alternative solutions for sewage sludge treatment and management. Hydrothermal carbonization (HTC) has developed considerably over the last 15 years and offers a viable alternative for the utilization of municipal and industrial organic waste such as sewage sludge. The process takes place in an aqueous environment without the need for pre-drying sewage sludge and thereby facilitating direct processing. HTC is especially suitable in combination with the recovery of nutrients like phosphorus. Technologies to recover this essential resource are important because phosphorus is an element that cannot be substituted and is therefore essential. HTC could make a significant contribution to sewage sludge management in combination with phosphorus recovery. However, the technology has yet to establish itself as a sewage sludge valorization process (2023) and is not yet a recognized state-of-the-art. Nevertheless, the HTC technology could gain greater relevance in the future, especially as an alternative valorization pathway for sewage sludge in rural areas of Germany.