Conflicting Reports on Mixing in Digesters

The literature contains conflicting information regarding the mixing intensity and duration that is necessary in digesters to treat a waste with a high concentration of VS. In general, workers agree that mixing is necessary to enhance substrate contact with the microbial com­munity, improve pH and temperature uniformity, prevent stratification and scum accumula­tion, facilitate the removal of biogas, and aid in particle size reduction (Stafford 1981) . However, mixing can be performed intermittently or continuously at different intensities (Hansen et al. 1999; Karim et al. 2005). On the one hand, Dague et al. (1970) showed that intermittent mixing resulted in an increased biogas production and increased COD and VS reduction efficiencies due to enhanced bioflocculation (settling) compared with continuous mixing. In addition, Stroot et al. (2001) and McMahon et al. (2001) reported that minimally mixed digesters demonstrated a much more stable operation than digesters that were continu­ously and vigorously mixed. They postulated that vigorous, continuous mixing inhibited relationships between syntrophic bacteria and their methanogenic partners, possibly by dis­rupting the spatial juxtaposition between these organisms. On the other hand, Lanting (2003) and Muller et al. (2007) showed that high mixing intensities results in particle size reduction and diffusion limitation reduction, which increased processing capacity for a digester treating waste-activated sludge. The choice in mixing intensity and duration is important, because continuous and vigorous mixing may significantly increase the necessary energy (i. e., elec­tricity) input for the digestion system, and thus reduce the net energy balance ratio. To shed light on the advantages and disadvantages of mixing intensity and duration for animal waste digestion, we conducted two different studies: one with swine waste in high))ate ASBRs,

which is described in the Mixing in a High-Rate System Treating Swine Waste section (Angenent et al. 2001- , and the other with dairy waste in the low-rate completely stirred anaerobic digester, which is described in the Mixing in a Low-Rate System Treating Dairy Waste section (Hoffmann et al. 2008- . We found that low-rate and high-rate reactors are fundamentally different in regard to mixing requirements, and that the use of different reactor types and configurations may explain the discrepancies in the digester literature in regard to mixing characteristics (Hoffmann et al. 2008).

Mixing in a High-Rate System Treating Swine Waste

We operated two lab-scale ASBR systems in parallel and the mixing duration and intensity were changed in one bioreactor from gentle, intermittent mixing to gentle, continuous mixing to vigorous, continuous mixing, while the other bioreactor (control) was operated throughout the experiment with gentle, intermittent mixing (Angenent et al. 2001). The substrate for this study was diluted swine waste (20 gVS/L) at a volumetric loading rate of ~4gVS/L/day. The inoculum of the ASBRs was from previously operated ASBRs that contained well-acclimated biomass with excellent settling characteristics (Angenent et al. 2002a) , and therefore the start-up period was less than 10 days (Figure 4.5A).