Anaerobic digestion is the process of breaking down the volatile organic solids in sludge, producing useful byproducts that include methane and high-nutrient fertilizer. Sludge digestion has been practiced for more than a century in some parts of Europe, but modern practices have brought about significant improvements in the process. Most importantly, the optimal conditions for proper digestion have been identified. These include two major components:
  • effective temperature control and uniformity throughout the digester

  • complete mixing of the entire digester with the minimization of dead zones
Temperature control is critical because the bacteria that generate methane from the sludge are extremely sensitive to heat. Methane-producing bacteria should be kept between 90 and 95F for optimal growth and digestion. The methane-producing bacteria live in a sensitive balance with the acid-forming bacteria that break organic solids down into liquid organic acids. The acid-forming bacteria are hardier than methane-producing bacteria, so the methane-producing bacteria act as the limiting factor within the digestion process. If the methane-producing bacteria can't keep up with the acids generated by the acid-forming bacteria, the digester will fall out of balance and begin producing less than optimal results.

Thorough mixing of the digester contents is absolutely necessary for effective performance. Where "dead zones" of poor mixing are allowed to form, the methane-forming bacteria can become overwhelmed by the generation of liquid organic acids by the acid-formers and the process can come to a halt. Improper mixing can also mean that new sludges introduced to the digester can fail to become integrated with the existing process, leading to inefficient digestion.

Proper mixing is directly related to proper heating because temperature variations of as little as 1F within the digester can diminish performance noticeably. A good mixing system will maintain uniform volatile solids distribution throughout the digester, with variations limited to +/- 10% from the mean.

A well-operated, properly-mixed, and evenly-heated digester can produce 50% more useful methane and destroy half again as many of the volatile organic solids present in the sludge as a poorly-operated digester. Thus, the proper design and loading of an anaerobic digester is crucial.

Contact us for design assistance by using our digester quote request page and we'll be happy to assist you.

last revised 10.7.2005