Insights into the local interaction mechanisms between fermenting broken maize and various binder materials for anaerobic digester structures.

Concrete structures of anaerobic digestion plants face chemically aggressive conditions due to the contact with the complex liquid fraction of the fermenting biowaste. This paper aims to determine the biogeochemical dynamic interaction phenomena at play between the biowaste and cementitious matrices at the local scale, and to identify durable binders in such environments. Binder materials likely to show increased durability - slag and calcium aluminate cement, and a metakaolin-based alkali-activated geopolymer - and a reference Portland cement were inserted into sealed bioeactors during 5 cycles (245 days) of broken maize anaerobic digestion. Cementitious pastes suffered chemical and mineralogical alteration related mainly to carbonation and leaching. However, they had no negative impact on the bioprocess in terms of pH, metabolic evolution of volatile fatty acids and NH4+, planktonic microbial community composition or CH4 production. In all reactors, the microbial community was able to perform the anaerobic digestion successfully. The MKAA was only slightly altered in its outermost layer. Its presence in the biowaste induced lower NH4+ concentrations, a slightly higher pH and a marked shift in the microbial community, but CH4 total production was not affected. Substantial enrichment of acid forming bacteria, especially members of the genus Clostridium, was observed in the biofilm formed on all materials.

Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms.

In order to promote the development of the biogas industry, solutions are needed to improve concrete structures durability in this environment. This multiphysics study aims to analyse the multiphases interactions between the liquid phase of an anaerobic digestion system and cementitious matrices, focusing on (i) the impacts of the binder nature on the anaerobic digestion process at local scale, and (ii) the deterioration mechanisms of the materials. Cementitious pastes made of slag cement (CEM III), innovative metakaolin-based alkali-activated material (MKAA), with compositions presumed to resist chemically aggressive media, and a reference binder, ordinary Portland cement (CEM I), were tested by immersion in inoculated cattle manure in bioreactors for a long period of five digestion cycles. For the first time it was shown that the digestion process was disturbed in the short term by the presence of the materials that increased the pH of the liquid phase and slowed the acids consumption, with much more impact of the MKAA. However, the final total production of biogas was similar in all bioreactors. Material analyses showed that, in this moderately aggressive medium, the biodeterioration of the CEM I and CEM III pastes mainly led to cement matrix leaching (decalcification) and carbonation. MKAA showed a good behaviour with very low degraded depths. In addition, the material was found to have interesting ammonium adsorption properties in the chemical conditions (notably the pH range) of anaerobic digestion.