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.