Biogas Digesters

Similar to a septic tank, a digester treats waste with anaerobic bacteria but includes a methane gas collection system.

Treatment Level:  Primary
Relative Cost:
Amount of Maintenance Interaction:  high
Beneficial Byproduct:  some –  lots (gas and nutrient rich slurry)


  • Generate gas to supplement energy needs
  • “Digestate” slurry can be used as fertilizer (preferably fruit trees) or fish food
  • Ease strain on local natural fuel resources


  • 1 family may not generate enough without animal waste as well
  • Needs daily interaction to ‘feed’ the system
  • If animal waste is needed, must either co-locate toilet, sty & digester or manually collect & load the waste
  • Needs regular supply of water
  • Need training & time to maintain temperature, fermentation times & feedstock material

Operation & Maintenance

  • Add manure every 1 – 2 days
  • Assuming digester is kept active, clean out digester every 2 years.  Otherwise, clean it out when digester has been inactive
  • Replace tubing between digester & storage/use every 2 years

Blackwater vs Graywater?

  • The process begins with a slurry, so it’s best for combined wastewater; otherwise, liquid must be added

Secondary Treatment Options

  • Slurry:  Aquaculture, treatment wetlands, drying beds, or land application

Additional Notes:

  • Need to locate in an area with easy access for inspection & maintenance
  • Watch what you flush!  Minimize bleaches & chlorine, don’t use caustic products or drain cleaners as these things can kill the beneficial bacteria in the digester
  • Typical uses are methane cook stoves, burners, and lamps.

Farm Scale Example (