Mechanisms of cementitious material deterioration in biogas digester

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.

Effect of Acetic Acid on Compressive Strength and Geometric Texture of the Surface of C20/25 Class Concrete

Concrete is a highly alkaline material; therefore, the presence of organic acids (acetic, butyric, lactic) from agricultural sewage constitutes a threat to the concrete and the environment. The investigations were aimed at simulating the influence of highly concentrated organic acids on concrete elements of livestock buildings. Cubic samples 100 × 100 × 100 mm of the ordinary concrete were immersed in 10% acetic acid for 270 days. Then, the compressive strength and mass decrement of the samples were determined as well as using SEM images and a 3D-profilograph to profile the surface roughness and depth of changes evoked in the structure of the samples by penetrating acid. The results were compared with those for a control sample (not subjected to the effects of aggressive agents) as well as for samples placed in a neutral and alkaline environment. The compressive strength of the samples stored in the acidic environment was 22.23% lower than that of the control sample, whereas the compressive strength of those samples stored in an alkaline environment were 44.27% higher. Conclusions from these investigations can be of innovative importance in the preparation of environmental impact reports, which are necessary for obtaining permission for the construction and use of livestock buildings, and afterward in the reduction of the impact of these buildings on the environment and surface and underground water resources.

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.

Evaluation of microbial proliferation on cementitious materials exposed to biogas systems

Understanding the interactions between biofilm and cementitious materials in biogas production systems is an essential step toward the development of durable concrete for this expanding sector. Although the action of the liquid phase medium on the material has been the subject of several research studies, the possible impact of the material's properties on biofilm formation and composition has been little investigated, if at all. The aim of this paper is to evaluate the characteristics of the biofilm according to the surface properties of the materials. Four cementitious materials with different chemical and mineralogical compositions, and various topological surface characteristics (pastes of CEM I, CEM III/C and CAC, and CEM I paste treated with oxalic acid) were exposed to the liquid phase of a fermenting biowaste for 10 weeks. The steps of biofilm formation were observed using SEM. Even though all the cementitious material surfaces were intensely colonized at the end of the experiments, the establishment of the biofilm seems to have been delayed on the oxalate-treated CEM I and on CAC coupons. Roughness and surface pH effects were not of prime importance for the biofilm development. The analysis of bacterial population diversity using 16S rDNA sequencing showed a less diversified microbial flora in the biofilm than in the reaction medium.