Let me quote from the SCIENCE BUDDIES web page:
After having thoroughly researched a topic, you should have some prediction about what you think will happen in your experiment. This educated guess concerning the outcome is called your hypothesis."
Look under "Your Project" , "How to Do a Science Fair Project"
http://www.sciencebuddies.org/mentoring ... opic.shtml
Items 2 and 5 are relevant to your question.
Think about what parts of a biogas digester can be varied in a controlled experiment (this would be your dependent variable(s) ). You can try to predict what will happen to the amount of gas produced by changing one of the dependent variables. i.e. Can you change something that makes the amount of gas output increase or decrease? If you double the size, do you get twice as much gas?