The effects of agitation in anaerobic biodigesters operating with substrates from swine manure and rice husk

The use of agitation in anaerobic biodigesters affects biogas generation, but there is still no consensus on what is the best mechanism and the most appropriate intensity. In addition, there are many studies that do not provide necessary information to reproduce scale-up agitation. Qualitative terms such as "manually", "vigorously" and "minimally" are usually used to describe it. Based on this context, the objective of this work was to propose and evaluate a mixing model, which uses data from fluid and biodigesters characteristics, in order to calculate the time required for the system homogenization. This work combined Computational Fluid Dynamics (CFD) simulation with experimental anaerobic biodigestion tests. The experimental tests performed the codigestion with a mixture of swine manure and rice husk, as substrate. From the CFD analysis, specially, from velocity profile it was possible to identify a similar mixing pattern from the substrate and water, and dead zones on the biodigester base (near the sludge). From heat flux analysis it could be seen the heat distribution was not uniform in the biodigester and some regions presented temperatures below 30 degrees C. According to the experimental results, the biodigester that used intermittent agitation showed a better performance of the codigestion process compared with the biodigester without agitation. Results were better for biogas production, process stability and volatile solids removal. The proposed model, based on mixing time, can be used as reference for the anaerobic biodigestion of fluids with low solids content.