Water Footprint Considerations in Biogas-Based Bioenergy Generation: A Systematic Review of South African Evidence

Biogas production through anaerobic digestion is increasingly recognised as a strategic renewable energy pathway capable of addressing South Africa’s energy insecurity, organic waste management challenges, and climate mitigation goals. However, the water-intensive nature of anaerobic digestion raises critical sustainability concerns in water-scarce regions. This systematic review critically examines the water footprint of biogas-based bioenergy systems, with a specific focus on South Africa’s water-stressed context, to understand how water availability, feedstock selection, digester configuration, and governance frameworks influence system viability and scalability. This study adopts a systematic literature review (SLR) approach guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology; peer-reviewed literature published between 2010 and 2025 was retrieved from Scopus and Web of Science and synthesised through descriptive analysis and qualitative meta-synthesis. The review integrates blue, green, and greywater footprint concepts to assess water use across diverse biogas pathways, including livestock manure, agricultural residues, food waste, wastewater sludge, and aquatic biomass. Findings indicate that wet digestion systems, dominant in South Africa, are highly sensitive to freshwater availability, particularly where slurry dilution relies on blue water. In contrast, wastewater-integrated, semi-wet, and co-digestion systems substantially reduce freshwater demand while enhancing methane yields and process stability. The reuse of greywater, industrial effluents, and digestate emerges as a key strategy for lowering water footprints and strengthening circular water–energy linkages. Despite strong technical potential, the adoption of water-efficient anaerobic digestion systems remains constrained by fragmented governance, infrastructure deficits, and limited empirical data on dry and low-water digestion technologies. The review concludes that embedding water footprint considerations into bioenergy planning, policy, and system design is essential for the sustainable expansion of biogas in South Africa. Integrated water–energy–waste governance, coupled with targeted technological innovation, is critical to ensuring that biogas development enhances both energy security and water sustainability in water-scarce regions.