Ask away, and answer here as well!

A new question? What is the simplest (and most appropriate) way to explain to pupils (age 15-16) about the advatgages and disadvantages of different designs of biogas generators? Should it be broken down into solid and liquid or batch and continuous or .....? Basically I think I am asking how should they be split up at the highest level, if that makes sense?


Top level - type of waste - solid (MSW etc) or liquid ("conventional" digestion).
Both of the above can be "batch" (filled, processed, emptied and refilled), fed intermittently or "continuous" (fed a small proportion of the working volume and discharged at the same time).
Then you start getting detailed as you have "Continuous Flow Stirred Tank", "Plug Flow", "Upflow Anaerobic Sludge Blanket", "Packed Bed", "Anaerobic Filter" and other designs of liquid digesters (some of which won't handle suspended solids) and a range of options for solid digesters as well. There is a "Handbook of Biogas Utilization" that has a good summary of types/applications.

What can Biogas be used for?

Biogas is best used directly for cooking/heating. It can also be used to run gas lamps and absorption refridgeration. As electricity generation uses about 5 times the energy actually contained in the electricity produced it is one of my last uses for biogas (but it is a bit better if you can also use the waste heat). As methane (the fuel in biogas) is VERY hard to compress (it won't liquify room temperatures, so you need cylinders like oxygen/nitrogen bottles rather than LPG tanks) it is my lowest preference use, as so much biogas (or equivalent energy from some other form!) must be used to provide some "useful" fuel.

Is it possible to have anaerobically composting, methane producing biogas systems in regions of extreme cold (i.e. temperatures down to -30C)?

Yes, but the bacteria need at least 10 C to function. Solid digestion with an insulated cover may be a good option, as the self heating may keep the core going until thaw.

Are there any specific bacteria that produce this methane? Scientific names perhaps?

There are a number of "methanogens" - some that use acetate as their food source (giving methane as a byproduct) and some that convert carbon dioxide to methane. Then you need "acetogens" to provide the acetate and other bacteria to hydrolise complex organic molecules into successively smaller molecules until acetogens can use them - a very symbiotic system as one stage's waste is the substrate for another stage. As an engineer I don't know many names (and the system still works!).

Respected,sir my name is girish i want to do a project of biogas and i want to ask a question abought biogas if we use biogas in the vechiles beacause it is a naturalgas.I have asked my teacher she told me that we have to reduce the chemicalcompounds of biogas.if you want replay to me you can send this message to my e-mail.

My e-mail is
I always say that compressing biogas for transport use is a waste of energy - gaseous fuels are best used in stationary applications where variable volume (simpler!) storage is possible. 30/1/2008

What are costs for building a digestor in Tanzania?
I have seen so many lists of needed material for building a digestor - but what is the price of the particular construction particles? Can somebody, please, make an estimatation of prices in today Tanzania (or generally Sub-Saharan Africa) of the following particles:

  1. Cubic meters of sand
  2. Meter of a strong, yet flexible, plastic sheet with at least 2.8 meters in width
  3. Meter of 3" PVC tubing
  4. 50-kilogram sacks of cement
  5. Blocks of cement measuring 12 cm X 20 cm X 40 cm
  6. 1/2" PVC tubing sufficient enough to make a rectangular frame with a circumference of 16.6 meters
  7. Meter of thin rope
  8. Gallon jug

Regards, Miroslav.

G'day Miroslav,

Local people would have to supply the costs, as they will probably vary from region to region depending on source of materials, transport costs and market competition (a monopoly can charge what it likes but competition will drive the price down).

Happy digesting,
Paul Harris