Advanced thermal treatments (ATTs) of digestate post-anaerobic digestion (AD) are pivotal to reducing reliance on land application and improving resource recovery from sewage sludge, thereby supporting UK net-zero and resource-efficiency ambitions. Hydrothermal oxidation (HTO), a form of ATT, offers a viable alternative to traditional sludge-to-land application by significantly reducing digestate volume and converting inert organics into more recoverable forms, thereby facilitating the valorisation of digestates. In this study, a pilot-scale HTO treating digestate at 250°C and 5 bar for 30 minutes achieved 70-80% COD destruction. Remaining organics were largely solubilised into degradable compounds, such as volatile fatty acids (VFAs), primarily acetic acid. Post-HTO, ammonia concentrations rose from 1500-2000 mg/L to 2500-3500 mg/L, which potentially inhibits biological processes, while elevated temperatures may promote the formation of recalcitrant Maillard reaction products. This study evaluates the treatability of HTO effluent via anaerobic (Upflow Anaerobic Sludge Blanket, UASB) and aerobic (Activated Sludge Process, ASP) routes, and the risk of recalcitrant formation during HTO processing. Preliminary batch aerobic tests with acclimatised inoculum showed 90% biodegradability as COD, with over half of the COD reduced within 24 hours. Initial UASB batch results showed less than 40% COD degradation over 33 days, with a six-day lag phase. Preliminary studies on acclimatising anaerobic granules to HTO effluent using short batch tests indicate potential to further maximise biogas recovery and reduce lag phase. Analysis of recalcitrant formation during HTO is currently ongoing. This research offers a basis for assessing treatability and valorisation strategies for HTO effluent.