New insights into co-digestion of activated sludge and food waste: Biogas versus biofertilizer

•Two approaches for co-digestion of sludge and food waste were developed.•Co-digestion with production of methane and fertilizer is more economically viable.•62.5% of TS reduction in the co-digestion with production of methane and fertilizer.•Fungal mash was highly efficiency in hydrolysis of activated sludge and food waste. This study explored two holistic approaches for co-digestion of activated sludge and food waste. In Approach 1, mixed activated sludge and food waste were first hydrolyzed with fungal mash, and produced hydrolysate without separation was directly subject to anaerobic digestion. In Approach 2, solid generated after hydrolysis of food waste by fungal mash was directly converted to biofertilizer, while separated liquid with high soluble COD concentration was further co-digested with activated sludge for biomethane production. Although the potential energy produced from Approach 1 was about 1.8-time higher than that from Approach 2, the total economic revenue generated from Approach 2 was about 1.9-fold of that from Approach 1 due to high market value of biofertilizer. It is expected that this study may lead to a paradigm shift in biosolid management towards environmental and economic sustainability.