Science Buddies: "Ask an Expert"

Hello

My daughter is working on the science fair project for Trash to biogas, however, she has switched out the cow manure for guinea pig manure.

She is at the end of the experiment and had interesting results that she is trying to understand.

She had a control of just guinea pig poop.
She had guinea pig poop with pine chips (bedding material).
She had guinea pig poop with carrot peelings.

All aspects followed the experiment for set up - distilled water, balloons, etc.

The pine chip bottles never produced any gas.
The manure only bottles produce gas after day 1 through day 3, but then the balloons shrunk and would get sucked into the bottle.
The carrot peelings and manure produce gas (the most gas) day 1-3, and then held the gas in the balloon for 14 days (length of experiment).

She repeated the experiment at a higher temperature and without the pine chips (just manure only and carrot peels plus manure). The results were the same: gas production day 1-3, and then manure only sucked into bottle while carrot peel held gas in balloon.

Question - what is happening to make the balloons get sucked into the bottle for the manure only? She has done a fair amount of research, and it seems like it produced the gas at first, and then starts another reaction that depletes the gas in the balloon and sucks it as a vacuum? Can the methane and CO2 that is produced get re-absorbed in the slurry?

I think she has some great results, but she really wants to be able to explain the vacuum.

Moderator note: I'm cross-posting this to the Physical Sciences forum in case those experts want to comment!

Hi sherrillm,

I'm assuming that you're working on the project described here:

https://www.sciencebuddies.org/science- ... gy#summary

I must say that this topic is outside of my expertise. But I agree that the gas initially produced in the manure-only test either dissolved in the slurry or reacted.

Both methane and carbon dioxide are soluble in water. It's possible that the manure-only test produced gases until the available nutrients ran out. Afterward some of the gases may have dissolved and caused the balloon to deflate.

Bacteria that can convert methane to methanol (liquid at room temperature) are known and can be found in soil. For example, see the article at the following link:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC106402/

You might try an online search to find out if similar bacteria are found in manure.

I hope this helps. Please ask again if you have more questions.

A. Norman