Anaerobic co-digestion of macroalgal biomass with cattle manure under high salinity conditions

Anaerobic fermentation of various organic wastes to produce valuable intermediate products and generate a mixture of combustible gases has been researched actively in the past decades. For one, marine biomass as an abundant organic residue of water-locked countries has recently gained increasing popularity.

However, such biomass is known to contain high levels of salt, which can be detrimental to the machinery and affects the microbial activity involved in the process. To evaluate the latter, a comprehensive study of sodium chloride and marine salt as potential inhibitors of the anaerobic digestion process is hereby presented.

The hypothesis was tested in three major steps. In the first step, batch toxicity assays showed that marine salt is already inhibitory to the anaerobic digestion microbiome, at a half-maximal inhibitory concentration of 2.8 g-Na L-1, compared to synthetic sodium that was inhibitory only at a concentration of 10.1 g-Na L-1.

Next, experiments with a continuous reactor showed that the gradual addition of salt to a reactor leads to irreversible inhibition, while a similar reactor that is also spiked with salt might be recoverable to some extent. Finally, a bioconversion model, which for the first time was extended with a dynamic salt inhibition module, produced simulations that agreed well with the results of the continuous experiments, suggesting the presence of agile methanogenic archaea that can temporarily buffer the negative effect of increasing salt concentrations.

The findings of this work may provide inputs to future anaerobic fermentation experiments with salts and could benefit from detailed microbial analyses.