Measurement of diffusion coefficients in dry anaerobic digestion media

Dry anaerobic digestion of solid wastes is now commonly used at the industrial scale. Dry solid content in waste material is usually higher than 15%. w/w in these processes most industrial technologies in dry modes are not mixed, or sometimes intermittently mixed. In these situations convective transfer is negligible and diffusive transfer predominates and controls the mobility of soluble substrates within the digestion media. However, no data specifically related to diffusive phenomena in dry anaerobic digestion media (digestate) can be found in the literature. In the present study, an innovative experimental method was developed to determine diffusion coefficients experimentally. This method was based on the use of a specifically designed diffusion cell where iodide was used as a soluble tracer. The influence of total solid (TS) content on diffusion coefficients in digestate material was then investigated. Experimental results showed a strong influence of TS content. It was found that the effective diffusion coefficient (Deff) decreased drastically when TS content increased, with numerical values 50 to 185 times smaller at 8% TS and 25% TS, respectively than the reference value in water. This observation was probably due to the huge increase of viscosity within this range of dry solid content.

► A method for diffusion measurement in dry anaerobic digestion is presented. ► Effective diffusion decreases between 0 and 25% of total solid content. ► The diffusivity is about 100 times smaller than in pure water at 15% solids.