Proceedings

The history of anaerobic digester mixing in the USA started shortly after the advent of the separate sludge digestion tanks. Initially digester mixing was not more than gentle stirring. Digester mixing grew in acceptance by the successive introduction of dual rake stirrers, vertical open propeller turbo mixers, unconfined gas mixing, propeller draft tube mixers, pumped recirculation, low speed turbines and confined gas mixing. During the developments of these mixing systems the improvement of biogas production with rapid and more efficient mixing created the high rate digester that is still in use today as a fully heated and mixed process.

Anaerobic digestion mixing was under study for more than fifty years after the initial introduction of digester stirring until it gathered sufficient interest and practical experience for the initial formulation of sizing guidelines and evaluation parameters. However the last 20 years has seen a renewed interest in this part of the digestion process, thus making possible a significant development in the understanding of the benefits and advantages of digester mixing that has created the current robust approach for the evaluation and selection of mixing systems.

KEY WORDS Anaerobic Digestion, CFD, Digester Mixing, Sludge Mixers, Tracer Studies