Review on research achievements of blackwater anaerobic digestion for enhanced resource recovery

Blackwater (source-separated toilet wastewater) is highly concentrated in both organic carbon and nutrients. A consensus prevails that blackwater is a valuable resource instead of waste. Anaerobic digestion (AD), with the advantages of sanitation, energy recovery, and nutrient conservation, holds great promise as a core technology employed in blackwater management. The difficulties of blackwater AD processes lie in their poor performance and operational stability. Research in this direction has intensified to achieve comprehensive and effective blackwater recovery. This article provides critical and comprehensive insights into research achievements on blackwater AD. The characteristics of blackwater, influential factors, underlying mechanisms, and recent improvement strategies are discussed and concluded. The results showed that the high solid organic and ammonia contents in blackwater contributed most to the hindrances in AD process. Strategies including appropriate pretreatment methods, co-digestion with high-carbon-content wastes, or enrichment of hydrogenotrophic methanogens can significantly improve the methane recovery efficiency. Moreover, the fate of pathogens and pharmaceuticals during AD process and several digestate management technologies are investigated to achieve safe and reliable recycling. Taking these findings together, the recommendations for further development in blackwater AD are proposed. This work will serve as a basis for future research and chart a path toward a new paradigm of blackwater management.