## 1 Introduction
Digestate processing using the proposed one-step HTL digestate technology has the capacity of being a preferred digestate handling pathway, because the HTL of the digestate will facilitate the sterilization of the digestate stream while also enabling the production of useful product streams via a 'one-step' process. A simplified illustration of the proposed one-step digestate processing technology compared to existing digestate processing technologies is presented in Figure 1.  
Figure 1 shows that digestate handling requires a solid-liquid phase separation stage to enable the separation of the solid and the liquid phases [19]. Having separated the solid and the liquid phases, a portion of the separated liquid fraction is subjected to tertiary treatment via advanced oxidation technologies to sterilize the liquid fraction prior to its disposal in surrounding water bodies [1]. Another portion of the liquid fraction is recirculated to the AD digester to help replenish the microbial population, thereby sustaining the anaerobic degradation process. Finally, some of the solid fractions are stabilized via drying and composting to aid storage of the remaining solid fraction to be directly introduced to agricultural lands [16]. On the other hand, the proposed hydrothermal liquefaction of the digestate will incorporate a single transformation step with product separation achieved simply by exploring the hydrophobicity of biocrude and the surface properties of biochar, such as porosity and particle size of the product streams [20]. The introduction of the HTL technology as a viable resource recovery strategy from biogas digestate residue will also eliminate the need for additional hygiisation treatments of any by-product streams due to the high temperature and high pressure operating conditions imposed during HTL processes [1]. Therefore, the present study hypothesizes that a shift from the traditional complex and costly digestate handling method to an alternative  
methodology that employs a one-step HTL processing of the digestate, will promote the efficient handling of the digestate with the additional benefit of enhanced resource recovery. In summary, in the present study, the preferred substrate mix ratio, on a volatile solid (VS) basis, for enhanced biomethane production from the co-digestion of the WHDS and the SY waste has been investigated. The dependence of biomethane potential on the C/N ratio of the substrate mixtures and an assessment of the suitability of existing kinetic models to accurately describe biomethane production rates from the ACD of the substrates have also been investigated. Finally, the potential of employing HTL technology for digestate sterilization and enhanced resource recovery has been assessed. The novelty of the present study is emphasized by its exploration of the feasibility of an unconventional integrated process incorporating anaerobic co-digestion and HTL technologies as a pathway for enhanced value extraction from organic waste streams. The feasibility assessment has been undertaken via a consideration of the useful products yields of biomethane, biocrude and biochar and the energetics of the proposed integrated process.