It sounds like you are interested in doing this really challenging project from the Science Buddies website:http://www.sciencebuddies.org/science-f ... p026.shtml
You will need to read through the project guide to learn more about the topic, however, are some general answers to your question that should help you get started.
The amount of current and the change in current are results that you will determine from the specific microbial fuel cell that you make. You will need to measure the results to find out.
Most lab-scale MFC's produce very low current, so look for a very small electrical device such as a small light bulb or fan to run with your fuel cell. Maybe you could charge a cell phone. Or, you can graph your results of measurements made with a multimeter.
A microbial fuel cell depends on electrons that are transferred to the anode electrode by anaerobic bacteria growing as a biofilm on the electrode. The bacteria use the electrode as a terminal electron receptor at the end of respiration for energy production. You will use mud as a source of bacteria as described in the project; waste water contains small carbon molecules like organic acids that these bacteria can use as a source of energy.
If your project is due on the 26th, you will not have time to make a microbial fuel cell if you have not already made one. The bacteria take a few days to start growing.
Instead, you need to pick a really quick project from this website and perhaps save the MFC for next year.
Please post again if you have more questions.