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Optimizing the composting parameters as a post-treatment of digestates of organic fraction of municipal solid waste

Optimisation des paramètres de compostage comme un post-traitement des digestats de méthanisation de la fraction organique des ordures ménagères

Abstract : There are increasing concerns about the management of digestates since the application of anaerobic digestion has grown spectacularly in recent years. Despite their good fertilizing properties, the digestates are often incompletely stabilized and concentrated in ammonium. Moreover, some pathogens can survive anaerobic digestion and complicate digestates valorization. Considering the drawbacks of direct utilization of digestates in agriculture, we studied the composting as a post-treatment of a typical digestate, the digestate of organic fraction of municipal solid waste. Our work was based on an experimental design of the composting practical parameters including the aeration rate, the type and size of bulking agent, the mixing ratio of bulking agent with digestates and the recycling of mature compost and used bulking agent. We evaluated the stabilization of the compost produced, the ammonia and nitrous oxide emissions during composting and back to the contents the self-heating abili ty of composting pile in function of these parameters. We showed that changing from an inert bulking agent (plastic pall ring) to an active one (wood chips) helped to stabilize the digestates and reduced by 70-fold and four-fold ammonia and nitrous oxide emissions, respectively. Increasing the mixing ratio from two to four doubled the oxygen consumption of the compost and reduced 50-fold nitrous oxide emissions. The recycling of mature compost reduced ammonia emissions from 0.25 to 0.08 g nitrogen per kg organic matter but increased nitrous oxide emissions from 2 to 5 g nitrogen per kg organic matter. This work discusses the mechanisms of our observations and proposes optimized parameters for the composting of digestates.