The ongoing increase in global population has intensified demand for natural resources, leading to freshwater scarcity and environmental degradation from human activities such as agriculture, fishing, and energy exploration. Practices such as deforestation have further destabilised ecosystems. This chapter highlights wastewater sludge, rich in organic matter, as a promising alternative energy source for biogasBiogas production via anaerobic digestionAnaerobic digestion (AD). To maximise both the yield and quality of biogasBiogas, effective wastewater and sludge valorisation is essential, enabling its use as a renewable energy source to replace fossil fuels and support the achievement of the Sustainable DevelopmentSustainable development GoalsSustainable Development Goals (SDGs) (SDGs) campaign towards the attainment of clean water and affordable energy. Moreover, this chapter examines various valorisation pathways, discussing their benefits and limitations to ensure sustainable conversion of waste into energy while also promoting nutrient recoveryNutrient recovery.