The increasing global demand for energy and the environmental effects of fossil fuel usage necessitate the exploration of sustainable alternatives, with a focus on biomass energy. Waste-activated sludge (WAS), a by-product of wastewater treatment plants (WWTPs), emerges as a promising biomass source for bioenergy production, specifically biomethane. As a renewable energy option, biomethane has gained importance for its environmentally friendly attributes. This chapter explores the diverse characteristics of WAS, emphasizing its multifaceted properties and potential for sustainable biomethane generation. Differences between municipal WAS and industrial WAS are highlighted, acknowledging the variability in WAS attributes based on its source. A comprehensive analysis of the physical, chemical, and biological characteristics of WAS is presented, laying the scientific groundwork for understanding its biomethane potential. Beyond its energy implications, the chapter underscores the broader impact of WAS on efficient waste management practices. The generation of biomethane from WAS is positioned not only as a renewable energy source but also as a solution to the economic and environmental challenges faced by WWTPs. This study contributes to the understanding of WAS as a promising bioenergy reservoir and highlights its multifaceted role in achieving sustainable development goals (SDG).