Unit 2.4: Mixing

Background and Guiding Questions

The primary reason for mixing is to bring fermenting bacteria in contact with volatile solids and methanogens in contact with organic acids. Mixing can be done mechanically or by bubbling biogas through the slurry column. The amount and frequency of mixing is controlled by the operator.


Guiding Questions

  • What methods can be used to mix slurry in a digester?
  • What are the benefits of mixing?


Slurry Stabilization

Mixing exposes microorganisms to the maximum amount of food, lessens temperature stratification in the digester, reduces the volume occupied by settled inorganic material (such as grit), evenly distributes metabolic waste products during the digestion process, and prevents the formation of a floating crust layer (which can slow the percolation of biogas out of the slurry). Mixing creates a homogeneous environment throughout the digester that enables the digester volume to be fully utilized [5]. The benefits of mixing are:

  • To speed up the process of volatile solids breakdown, and;
  • To increase the amount of biogas production.

Mixing can be accomplished by bubbling biogas through the slurry column in the digester or by mechanical means. An explanation of each method follows.


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