Co-Digestion-Based Circular Bio-Economy to Improve Biomethane Generation and Production of Nutrient-Enriched Digestate in Bangladesh

This study aimed to find the optimum ratio for biogas production from ACoD of CD and MC, forecasting the ACoD process, and investigating the contribution of ACoD of CD and MC in the circularity of bio-economy. The livestock sector plays a significant role in the development of a country by meeting the demand of nutrients and facilitating improved livelihood. In Bangladesh, the annual production of cows is nearly 29.45 million, resulting in around 12.3 million tons of cow dung annually. Managing such a huge amount of cow dung has become a great concern, with some indigenous cow dung management practices including burning, cleaning material, and insect repellent. However, these practices cause the emission of greenhouse gases (GHGs), organic substances, and particulate materials. Anaerobic digestion is a green energy generation process that facilitates biogas production from cow dung along with quality digestate. However, mono-digestion of cow dung results in lower gas yield due to nutrient imbalance, presence of significant non-biodegradable materials on dairy manure composition, and poor microbial activity. ACoD of cow dung with another carbon-enriched substrate may increase biogas yield through nutrient balancing and enhancing microbial activity. ACoD is a technique in which microbial breakdown of two or more substrates occurs simultaneously in the absence of oxygen when substrates are mixed to augment the digestion process and improve gas yield. The study used a batch study for 99 days in an incubator maintaining 35 ± 1 ◦C temperature for seven different test groups of CD and MC (100:0, 90:10, 70:30, 50:50, 30:70, 10:90, and 0:100). The highest biogas production (356.6 ± 21.2 mL/gVS) was at 50:50 ratio with 138.05% and 32.02% increments compared to the digestion of CD and MC alone, respectively. Kinetic modeling showed the best fit using a Logistic model to evaluate ACoD of CD and MC mathematically. ACoD of available CD and MC in Bangladesh could produce 716.63 GWh/yr electricity for consumption and a large volume of nitrogen-enriched fertilizer to use in nitrogen-deficit soil. The study also investigated the contribution of ACoD of CD and MC in the circularity of bio-economy based on the proposed approach. The results showed that ACoD of CD and MC can promote circular bio-economy by using waste from agricultural production systems as the input material for bio-energy production systems and fulfill sustainable development goal 7 (Affordable and Clean Energy). The study concluded that ACoD of CD and MC can be a viable option for biogas production and nutrient-enriched fertilizer production, contributing to the circularity of bio-economy in Bangladesh.