Fly ash from coal combustion as improver of anaerobic digestion: A review

The increasing production of fly ash from coal combustion has become a problem due to the environmental risks involved in its disposal. These wastes have a porous structure and contain several transition metal oxides that are active components of many catalytic systems. This document presents the possibility of using fly ash as an additive to improve anaerobic digestion processes. To this end, the effect of fly ash on methane production and the biodegradation of different substrates is reviewed. Moreover, this review gives emphasis on possible mechanisms responsible for improving anaerobic digestion by adding fly ash. In this context, the importance of trace elements for methanogenic archaea, as well as the techniques studied to determine the availability of trace elements for microorganisms are addressed. The importance of the metallic oxides contained in the fly ash on the direct interspecies electron transfer and the possibility that the fly ash can act as a support for the formation of biofilms is reviewed. In addition, in this review, the possible effects that can occur in the digestate by adding metals in the anaerobic process are presented, and the mathematical models that incorporate the effect of fly ash on anaerobic digestion. Finally, this manuscript discusses the limitations on the use of fly ash on an industrial scale and the outlook for its continued use.