Design Features: Parasitic Load

Design Features: Parasitic Load

Parasitic Load

In our example parasitic load is the energy required in the anaerobic digestion process that is not contributing to the net electric yield. If operating a blower or mixer is required to make the digester work, that energy cost should be deducted from the energy the digester creates to obtain the net yield. Typically the greatest parasitic load in a digester would be generated by blowers and mixers.

Continuous Gas Blower

Typical continuous gas blower type mixer

Intermittent Submersible

Typical intermittent submersible mixer


Let’s look at two systems; a typical partially mixed plug flow which is mixed by continuously operating blowers and a typical complete mix system by Northern Bio-gas using intermittent mixers.

  • Typical Mixed Plug Flow: 1,000,000 gallon tank, 1 -25 Hp blowers run continuously for mixing at 7 cents KWH annual operating cost = $11,430.00
  • Complete Mix: 1,000,000 gallon tank, 3 – 20 Hp mixers running ten minutes and hour at 7 cents KWH annual operating costs = $4,572.00

As you can see parasitic load has a major influence to the bottom line and should be evaluated closely before selecting an anaerobic digestion system.