The lower H/C and O/C ratios in hydrochar, compared to raw biomass, reflect its transformation into a more carbon-rich material through the hydrothermal carbonization (HTC) process, which is a desirable characteristic for solid fuels due to its correlation with increased calorific value and stability. In this chapter, the premise that sewage sludge (SS), despite its high moisture content, is an exemplary feedstock for Co-HTC, facilitating a sustainable method for enhanced energy recovery and superior hydrochar quality has been explored. Our analysis assesses the impact of feedstock composition alongside HTC process parameters to identify which agricultural solid waste among Corn Cob (CC), Yard Trash (YT), and Rice Husk (RH) can act as a more effective co-substrate of lignocellulosic biomass to generate a hydrochar with Higher Heating Value (HHV). The chapter reveals that strategic adjustments to moisture content, temperature, and residence time can significantly increase energy recovery rates, achieving up to 91%. The synergistic role of the introduction of citric acid (CA) and h-chloric acid (HCl) is discussed to counterbalance the additional energy requirements and accelerate the reaction process. Moreover, the study showcases the promising use of orange peels (OP) as a natural catalyst that lowers the pH and increases the fuel energy density of the hydrochar, providing a cost-effective alternative to conventional additives. The chapter underscores the viability of SS in HTC and its dual advantages: reducing watering issues and enhancing energy value, thus contributing to the advancement of sustainable and efficient renewable energy generation. The optimization of natural catalysts and process conditions for Co-HTC can produce superior biofuel attributes, reduce dependence on conventional additives or fossil fuels, and mark a significant advancement in sustainability through biomass.