Designing a Anaerobic Digestion Mixing System:
Anaerobic Digestion or Anaerobic Degradation is a process where microorganisms break down biodegradable material in the absence of oxygen to process waste and release energy (methane). The anaerobic processes can be managed in a "digester" (an airtight tank) or a covered lagoon (a pond used to store animal waste) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. In very large systems, the biogas production is generally captured and then recycled for use in for other unit operations within a treatment plant.
The methane gas that evolves from a digester has been used to generate electricity. This electricity has been used for general plant operation or to supplement the energy requirements for blowers used for diffusers used in aeration. The methane has also been used to heat the facilities or to warm the contents of the digester itself, which generally results in higher gas production.
There are numerous types of anaerobic digesters that are available. Some of these anaerobic digester use top entering mixers. Defining the mixing requirement for an anaerobic digesters of a closed tank system will be dependent upon the process requirements of the system used and the type of containment vessel used.
Viscosity studies of various sludge's indicate that they are special pseudo-plastic shear thinning non-Newtonian slurries. This sizing and mixer selection procedure is based upon general assumptions about sludge characteristics. Viscometer readings that determining shear rate vs. viscosity, which are typically used for flow evaluation (tank turnovers), should be somewhat tempered as sludge has very unusual visco-elastic characteristics. In short, thickened sludge does not act like general comparable pseudo-plastic fluids.