Solid Manure treatment in dry anaerobic batch digester: process performance and startup optimization

International audience; Dry anaerobic digester operated in batch mode with liquid recirculation by sprinkling of the leachate on the static solid phase is a process well-suited for treating dry farm waste. At start-up, the recycling of liquid fraction (leachate) and/or solid fraction (digestate) from a previous batch into the new one is a key factor for a better industrial management when looking for a balance between energy production, substrate biodegradability and the initial investment. In this work, six types of manure of four different origins (sheep, goat, horse and cow) were compared. The results showed that, independently of their origin, all types of manure were rather easily digested in a dry anaerobic batch system. Indeed, after 60 days of digestion, at a S0/X0 ratio of 10 and with a leachate recirculation rate of 40 L/kgTS/day, the average volatile solid degradation was 50% and methane production ranged between 152 and 190 NmL CH4/g VS or between 32 and 59 NmL CH4/g of raw mass. After 60 days, the BMP average expression was 78 %. The influence on the degradation kinetics of the recycling at the start-up of a new batch of leachate or digestate was also investigated. The performances of three systems were compared. The first one was filled-up with only cow manure and tap water; in the second one, some digestate from a previous batch was added and; in the third one, some leachate was added. Results showed that leachate recycling is the best choice in terms of volumetric methane production, whereas digestate recycling improved the degradation kinetics and VS degradation. The batch without inoculation showed that an additional inoculation is not absolutely necessary for manure. Indeed, the simple use of water did not lead to a start-up failure but induced slightly lower global performances.